Transcript Molecular-3

Lecture 3
Patterns of Single-Gene
Inheritance
Autosomal Dominant Inheritance

More than half of all mendelian disorders
are inherited as AD traits.
 The incidence of some autosomal dominant
disorders is high, e.g., familial
hypercholesterolemia, myotonic
dystrophy, Huntington disease,
neurofibromatosis, and polycystic kidney
disease.

AD disorders are individually much less
common, in aggregate their total incidence
is appreciable.
 The burden of autosomal dominant
disorders is increased because of their
hereditary nature; they become problems for
whole kindreds, often through many
generations.
 In some cases, the burden is compounded
by social difficulties resulting from physical
or mental disability.

The risk and severity of dominantly inherited
disease in the offspring depend on whether one or
both parents are affected and whether the trait is
strictly dominant or incompletely dominant.
 Denoting D as the mutant allele and d as the
normal allele, matings that produce children with
an autosomal dominant disease can be between
two heterozygotes (D/d) for the mutation or, more
frequently, between a heterozygote for the
mutation (D/d) and a homozygote for a normal
allele (d/d):
Parental Mating
Offspring
Risk to Offspring
Affected by unaffected
D/d ×d/d
1/2 D/d, 1/2 d/d
1/2 affected
1/2 unaffected
Affected by affected D/d
×D/d
1/4 D/D, 1/2 D/d, 1/4
d/d
If strictly dominant:
3/4 affected
1/4 unaffected
If incompletely dominant:
1/2 affected similarly to the
parents
1/4 affected more severely
than the parents
1/4 unaffected

Offspring of D/d x d/d are approximately
50% D/d and 50% d/d.
 Each pregnancy is an independent event,
not governed by the outcome of previous
pregnancies.
 Thus, within a family, the distribution of
affected and unaffected children may be
quite different from the theoretical expected
ratio of 1:1, especially if the sibship is
small.
A pedigree showing typical inheritance of a form of
progressive sensorineural deafness (DFNA1) inherited as
an autosomal dominant trait.
• Achondroplasia, an AD disorder
that often occurs as a new mutation.
• Note small stature with short limbs,
large head, low nasal bridge,
prominent forehead, and lumbar
lordosis in this typical presentation.

In medical practice, homozygotes for
dominant phenotypes are not often seen
because matings that could produce
homozygous offspring are rare.
 Which mating can produce a D/D
homozygote?
 Practically, only the mating of two
heterozygotes need be considered because
D/D homozygotes are very rare and
generally too severely affected to reproduce
(fitness =0).
Incompletely Dominant Inheritance

Achondroplasia: incompletely dominant skeletal
disorder of short-limbed dwarfism and large head.
 Most achondroplastics have normal intelligence
and lead normal lives within their physical
capabilities.
 A homozygous child of two heterozygotes is often
recognizable on clinical grounds alone; much more
severely affected and commonly do not survive the
immediate postnatal period.

A pedigree of a mating
between two individuals
heterozygous for the
mutation that causes
achondroplasia. The deceased
child, individual III-3, was a
homozygote and died soon
after birth.

Another example is AD familial
hypercholesterolemia, leading to
premature coronary heart disease.
 The rare homozygotes have a more severe
disease, with an earlier age at onset and
much shorter life expectancy.
Cutaneous xanthomas
in a familial
hypercholesterolemia
homozygote.
New Mutation in Autosomal
Dominant Inheritance

In typical AD inheritance, every affected person in
a pedigree has an affected parent, who also has an
affected parent, and so on as far back as the
disorder can be traced or until the occurrence of an
original mutation.
 This is also true, for X-linked dominant pedigrees.
In fact, most dominant conditions of any medical
importance come about not only through
transmission of the mutant allele but also through
inheritance of a spontaneous, new mutation in a
gamete.
Relationship Between New Mutation and
Fitness in Autosomal Dominant Disorders

Once a new mutation has arisen, its survival in the
population depends on the fitness of persons
carrying it.
 There is an inverse relation between the fitness of
a given AD disorder and the new mutation.
 At one extreme are disorders that have a fitness of
zero, and the disorder is referred to as a genetic
lethal. Must be due to new mutations.

The affected individual will appear as an
isolated case in the pedigree.
 If the fitness is normal, the disorder is rarely
the result of fresh mutation; and the
pedigree is likely to show multiple affected
individuals.
Sex-Limited Phenotype in Autosomal
Dominant Disease

AD phenotypes may also demonstrate a sex ratio
that differs from 1:1.
 Extreme divergence of the sex ratio is seen in sexlimited phenotypes, in which the defect is
autosomally transmitted but expressed in only one
sex.
 An example is male-limited precocious puberty
(familial testotoxicosis), an AD disorder in which
affected boys develop secondary sexual
characteristics and undergo an adolescent growth
spurt at about 4 years of age.

In some families, the defect is in the gene that
encodes the receptor for luteinizing hormone
(LCGR); these mutations constitutively activate
the receptor's signaling action even in the absence
of its hormone.
 The defect is not manifested in heterozygous
females.
 Although the disease can be transmitted by
unaffected females, it can also be transmitted
directly from father to son, showing that it is
autosomal, not X-linked.

Males with precocious puberty due to activating
LCGR mutations have normal fertility, and
numerous multigeneration pedigrees are known.
 For disorders in which affected males do not
reproduce, however, it is not always easy to
distinguish sex-limited autosomal inheritance from
X-linkage because the critical evidence, absence
of male-to-male transmission, cannot be provided.
 In that case, other lines of evidence, especially
gene mapping to learn whether the responsible
gene maps to the X chromosome or to an
autosome, can determine the pattern of inheritance
and the consequent recurrence risk.
Pedigree pattern (part of a much larger pedigree) of male-limited precocious
puberty in the family of the child shown in Figure 7-14. This autosomal
dominant disorder can be transmitted by affected males or by unaffected
carrier females. Male-to-male transmission shows that the inheritance is
autosomal, not X-linked. Because the trait is transmitted through unaffected
carrier females, it cannot be Y-linked.
Characteristics of Autosomal
Dominant Inheritance
 The phenotype usually appears in every
generation, each affected person having an
affected parent.
– Exceptions or apparent exceptions (1) cases originating
from fresh mutations and (2) cases in which the
disorder is not expressed (nonpenetrant) or is expressed
only subtly in a person who has inherited the
responsible mutant allele.
 Any child of an affected parent has a 50% risk of
inheriting the trait.
– This is true for most families, in which the other parent
is phenotypically normal. Wide deviation from the
expected 1:1 ratio may occur by chance in a single
family.
 Phenotypically normal family members do not
transmit the phenotype to their children.
– exceptions.
 Males and females are equally likely to transmit
the phenotype, to children of either sex. In
particular, male-to-male transmission can occur,
and males can have unaffected daughters.
 A significant proportion of isolated cases are due
to new mutation. The less the fitness, the greater is
the proportion due to new mutation.
X-LINKED INHERITANCE

Phenotypes determined by genes on the X
have a characteristic sex distribution and a
pattern of inheritance that is usually easy to
identify.
 Approximately 1100 genes are thought to
be located on the X chromosome, of which
approximately 40% are presently known to
be associated with disease phenotypes

There are only two possible genotypes in males
and three in females with respect to a mutant allele
at an X-linked locus.
 A male with a mutant allele at an X-linked locus is
hemizygous for that allele, whereas females may
be homozygous for either the wild-type or mutant
allele or may be heterozygous.
 For example, if XH is the wild-type allele for the
gene for coagulation factor VIII and a mutant
allele, Xh, causes hemophilia A, the genotypes
expected in males and females would be as
follows:
Genotypes
Males
Hemizygous XH
Hemizygous Xh
Females Homozygous XH/XH
Heterozygous XH/Xh
Homozygous Xh/Xh
Phenotypes
Unaffected
Affected
Unaffected
Unaffected
(usually)
Affected
X Inactivation, Dosage Compensation,
and the Expression of X-Linked Genes



The clinical relevance of X inactivation is profound. It
leads to females having two cell populations, one in which
one of the X chromosomes is active, the other in which the
other X chromosome is active.
For example, in Duchenne muscular dystrophy, female
carriers exhibit typical mosaic expression, allowing
carriers to be identified by dystrophin immunostaining.
Depending on the pattern of random X inactivation of the
two X chromosomes, two female heterozygotes for an Xlinked disease may have very different clinical
presentations because they differ in the proportion of cells
that have the mutant allele on the active X in a relevant
tissue (as seen in manifesting heterozygotes).
Immunostaining for dystrophin in
muscle specimens. A, A normal
female (magnification ×480).
B, A male with Duchenne muscular
dystrophy (×480).
C, A carrier female (×240).
Staining creates the bright lines seen
here encircling individual muscle
fibers. Muscle from DMD patients
lacks dystrophin staining. Muscle
from DMD carriers exhibits both
positive and negative patches of
dystrophin immunostaining, reflecting
X inactivation
Recessive and Dominant Inheritance of
X-Linked Disorders

X-linked "dominant" and "recessive" patterns of
inheritance are distinguished on the basis of the
phenotype in heterozygous females. Some Xlinked phenotypes are consistently expressed in
carriers (dominant), whereas others usually are not
(recessive).
 The difficulty in classifying an X-linked disorder
as dominant or recessive arises because females
who are heterozygous for the same mutant allele
in the same family may or may not demonstrate
the disease, depending on the pattern of random X
inactivation and the proportion of the cells in
pertinent tissues that have the mutant allele on the
active versus inactive chromosome.
X-Linked Recessive Inheritance

The inheritance of X-linked recessive phenotypes
follows a well-defined and easily recognized
pattern.
 An X-linked recessive mutation is typically
expressed phenotypically in all males who receive
it but only in those females who are homozygous
for the mutation.
 X-linked recessive disorders are generally
restricted to males and rarely seen among females
(except for manifesting heterozygotes).

Hemophilia A is a classic X-linked recessive
disorder in which the blood fails to clot normally
because of a deficiency of factor VIII.
 The hereditary nature of hemophilia and even its
pattern of transmission have been recognized since
ancient times, and the condition became known as
the "royal hemophilia" because of its occurrence
among descendants of Britain's Queen Victoria,
who was a carrier.
 If a hemophiliac mates with a normal female ?
 Now assume that a daughter of the affected male
mates with an unaffected male ?
Pedigree pattern demonstrating an X-linked recessive disorder such as
hemophilia A, transmitted from an affected male through females to an
affected grandson and great-grandson.
Homozygous Affected Females

Relevant for X-linked color-blindness, a
relatively common X-linked disorder (an
affected male x a carrier female).
 Most X-linked diseases are so rare, unusual
for a female to be homozygous unless
parents are consanguineous
 Affected male x carrier female 
Homozygous affected female
Consanguinity in an
X-linked recessive
pedigree for redgreen color
blindness, resulting in
a homozygous
affected female
Manifesting Heterozygotes and Unbalanced
Inactivation for X-linked Disease

Rare, a female carrier of a recessive X-linked
allele has phenotypic expression of disease =
manifesting heterozygote.
 Have been described for many X-linked recessive
disorders, e.g., color-blindness, hemophilia A &
B, DMD, Wiskott-Aldrich syndrome (an X-linked
immunodeficiency), etc.
 Whether a female heterozygote will be a
manifesting heterozygote depends on a number of
factors:

First, the fraction of cells in which the
normal/mutant allele happens to remain active
(unbalanced or skewed X-inactivation).
 Second, depending on the disorder in question,
females can have very different degrees of disease
penetrance and expression, even if their degree of
skewed inactivation is the same, because of
underlying physiological functioning of genes e.g.,
– In Hunter syndrome (iduronate sulfatase
deficiency), cells with normal allele on active X
can export enzyme to extracellular space, picked up
by cells in which mutant allele on active X and
defect is corrected in those cells
– So, penetrance for Hunter syndrome in
heterozygous females is extremely low even when
X-inactivation deviates from expected random
50%:50% pattern

On the other hand, nearly half of all female
heterozygotes for fragile-X syndrome show
developmental abnormalities.
 In addition to manifesting heterozygotes, the
opposite pattern of skewed inactivation can also
occur.
Characteristics of X-Linked Recessive Inheritance






The incidence of the trait is much higher in males.
Heterozygous females are usually unaffected,
exception?
The gene responsible is transmitted from an affected
man through all his daughters. Any of his daughters'
sons has a 50% chance of inheriting it.
The mutant allele is ordinarily never transmitted
directly from father to son.
The mutant allele may be transmitted through a series
of carrier females; if so, the affected males in a
kindred are related through females.
A significant proportion of isolated cases are due to
new mutation.
X-linked Dominant Inheritance






Regularly expressed in heterozygotes
No male-to-male transmission
For a fully penetrant XD pedigree, all daughters
and none of sons of affected males are affected.
Pattern of inheritance through female is no
different from AD.
The expression is usually milder in females, who
are almost always heterozygotes. Thus, most XD
disorders are incompletely dominant.
Only a few genetic disorders are classified as XD.

E.g., X-linked hypophosphatemic rickets
(a.k.a. vitamin D-resistant rickets)
 Defective gene product is one of the
endopeptidases that activate or degrade a
variety of peptide hormones
 Both sexes are affected but, serum
phosphate level is less depressed and rickets
less severe in heterozgous females.
Pedigree pattern demonstrating X-linked dominant inheritance
X-linked Dominant Disorders with Male Lethality

Some rare genetic defects expressed exclusively or
almost exclusively in females appear to be XD lethal
in males before birth
 Typical pedigrees: transmission by affected female 
affected daughters, normal daughters, normal sons in
equal proportions (1:1:1)
 Rett syndrome meets criteria for an XD that is usually
lethal in hemizygous males. The syndrome is
characterized by normal prenatal and neonatal growth
and development, followed by rapid onset of
neurological symptoms and loss of milestones
between 6 and 18 months of age.
Rett syndrome cont.

Children become spastic and ataxic, develop autistic
features and irritable behavior with outbursts of crying,
and demonstrate characteristic purposeless wringing or
flapping movements of hands and arms.
 Head growth slows and microcephaly develops.
Seizures are common (~50%)
 Surprisingly, mental deterioration stops after a few
years and the patients can then survive for many
decades with a stable but severe neurological disability.
 Most cases caused by spontaneous mutations in an Xlinked MECP2 gene encoding methyl CpG binding
protein 2. ? Thought to reflect abnormalities in
regulation of genes in developing brain.
Typical appearance and hand posture of girls
with Rett syndrome
Rett syndrome cont.

Males who survive with the syndrome usually have two
X chromosomes (as in 47,XXY or in a 46,X,der(X)
male with the male determining SRY gene translocated
to an X) or are mosaic for a mutation that is absent in
most of their cells
 There are a few apparently unaffected women who have
given birth to more than one child with Rett syndrome.
? X-inactivation pattern in a heterozygous female. ?
Germline mosaic ?
Pedigree pattern demonstrating an X-linked dominant disorder, lethal in
males during the prenatal period.
Characteristics of X-Linked Dominant
Inheritance

Affected males with normal mates have no
affected sons and no normal daughters.
 Both male and female offspring of a heterozygous
female have a 50% risk of inheriting the
phenotype. The pedigree pattern is similar to that
seen with autosomal dominant inheritance.
 Affected females are about twice as common as
affected males, but affected females typically have
milder (although variable) expression of the
phenotype.
New Mutation in X-Linked Disorders

In males, genes for X-linked disorders are exposed to selection that
is complete for some disorders, partial for others, and absent for
still others, depending on the fitness of the genotype.
 Patients with hemophilia have only about 70% as many offspring
as unaffected males do; that is, the fitness of affected males is about
0.70.
 Selection against mutant alleles is more dramatic for X-linked
disorders such as DMD. DMD is currently a genetic lethal because
affected males usually fail to reproduce. It may, of course, be
transmitted by carrier females, who themselves rarely show any
clinical manifestation of the disease.
 New mutations constitute a significant fraction of isolated cases of
many X-linked diseases. When patients are affected with a severe
X-linked recessive disease, such as DMD, they cannot reproduce
(i.e., selection is complete), and therefore the mutant alleles they
carry are lost from the population. Because the incidence of DMD
is not changing, mutant alleles lost through failure of the affected
males to reproduce are continually replaced by new mutations.
PSEUDOAUTOSOMAL INHERITANCE

Pseudoautosomal inheritance describes the
inheritance pattern seen with genes in the
pseudoautosomal region.
 Alleles for genes in the pseudoautosomal
region can show male-to-male transmission,
and therefore mimic autosomal inheritance,
because they can cross over from the X to the
Y during male gametogenesis and be passed on
from a father to his male offspring.

Dyschondrosteosis, a dominantly inherited
skeletal dysplasia with disproportionate short
stature and deformity of the forearm, is an
example of a pseudoautosomal condition inherited
in a dominant manner.
 A greater prevalence of the disease was seen in
females as compared with males, suggesting an Xlinked dominant disorder, but the presence of
male-to-male transmission clearly ruled out strict
X-linked inheritance.
 Mutations in the SHOX gene encoding a
homeodomain-containing transcription factor have
been found responsible for this condition.
 SHOX is located in the pseudoautosomal region on
Xp and Yp and escapes X inactivation.
Figure 7-22 Pedigree showing inheritance of dyschondrosteosis due to
mutations in a pseudoautosomal gene on the X and Y chromosomes. The
arrow shows a male who inherited the trait on his Y chromosome from his
father. His father, however, inherited the trait on his X chromosome from
his mother
MOSAICISM

Mosaicism is the presence in an individual or
a tissue of at least two cell lines that differ
genetically but are derived from a single
zygote.
 Mosaicism due to X inactivation is a wellknown phenomenon.
 More generally, mutations arising in a single
cell in either prenatal or postnatal life can
give rise to mosaicism.

Mosaicism for numerical or structural
abnormalities of chromosomes is a clinically
important phenomenon, and somatic mutation is
recognized as a major contributor to many types of
cancer.
 Mosaicism for mutations in single genes, in either
somatic or germline cells, explains a number of
unusual clinical observations, such as segmental
neurofibromatosis, in which skin manifestations
are not uniform and occur in a patchy distribution,
and the recurrence of osteogenesis imperfecta, a
highly penetrant autosomal dominant disease, in
two or more children born to unaffected parents.

The population of cells that carry a mutation in a
mosaic individual could theoretically be present in
some tissues of the body but not in the gametes
(pure somatic mosaicism), be restricted to the
gamete lineage only and nowhere else (pure
germline mosaicism), or be present in both somatic
lineages and the germline, depending on when the
mutation occurred in embryological development.
 Whether mosaicism for a mutation involves only
somatic tissues, the germline, or both depends on
whether during embryogenesis the mutation
occurred before or after the separation of germline
cells from somatic cells.

If before, both somatic and germline cell lines
would be mosaic and the mutation could be
transmitted to the offspring as well as being
expressed somatically in mosaic form.
 Thus, e.g., if a mutation were to occur in a
germline precursor cell, a proportion of the
gametes would carry the mutation.
 There are about 30 mitotic divisions in the cells of
the germline before meiosis in the female and
several hundred in the male, allowing ample
opportunity for mutations to occur during the
mitotic stages of gamete development.
Schematic presentation of mitotic cell divisions. A mutation occurring
during cell proliferation, in somatic cells or during gametogenesis, leads to
a proportion of cells carrying the mutation-that is, to either somatic or
germline mosaicism.

Determining whether mosaicism for a mutation is
present only in the germline or only in somatic
tissues may be difficult because failure to find a
mutation in a subset of cells from a readily
accessible somatic tissue (such as peripheral white
blood cells, skin, or buccal cells) does not ensure
that the mutation is not present elsewhere in the
body, including the germline.
 Characterizing the extent of somatic mosaicism is
made more difficult when the mutant allele in a
mosaic fetus occurs exclusively in the
extraembryonic tissues (i.e., the placenta) and is
not present in the fetus itself.
Somatic Mosaicism

A mutation affecting morphogenesis and
occurring during embryonic development might
be manifested as a segmental or patchy
abnormality, depending on the stage at which the
mutation occurred and the lineage of the somatic
cell in which it originated.
 For example, NF1 is sometimes segmental,
affecting only one part of the body. Segmental
NF1 is caused by mosaicism for a mutation that
occurred after conception. In such cases, the
patient has normal parents, but if he or she has an
affected child, the child's phenotype is typical for
NF1, that is, not segmental.
Germline Mosaicism

There are well-documented examples where
parents who are phenotypically normal and test
negative for being carriers have more than one
child affected with a highly penetrant autosomal
dominant or X-linked disorder.
 Such unusual pedigrees can be explained by
germline mosaicism. Germline mosaicism is well
documented in as many as 6% of severe, lethal
forms of the AD osteogenesis imperfecta, in
which mutations in type I collagen genes lead to
abnormal collagen, brittle bones, and frequent
fractures.

Pedigrees that could be explained by germline
mosaicism have also been reported for several
other well-known disorders, such as hemophilia
A, hemophilia B, and DMD, but have only very
rarely been seen in other dominant diseases, such
as achondroplasia.
 Accurate measurement of the frequency of
germline mosaicism is difficult, but estimates
suggest that the highest incidence is in DMD, in
which up to 15% of the mothers of isolated cases
show no evidence of the mutation in their somatic
tissues and yet carry the mutation in their
germline.
Pedigree demonstrating recurrence of the autosomal dominant disorder
osteogenesis imperfecta. Both affected children have the same point
mutation in a collagen gene. Their father (arrow) is unaffected and has no
such mutation in DNA from examined somatic tissues. He must have
been a mosaic for the mutation in his germline.

Geneticists and genetic counselors are aware of
the potential inaccuracy of predicting that a
specific autosomal dominant or X-linked
phenotype that appears by every test to be a new
mutation must have a negligible recurrence risk in
future offspring.
 Obviously, in diseases known to show germline
mosaicism, phenotypically normal parents of a
child whose disease is believed to be due to a new
mutation should be informed that the recurrence
risk is not negligible!

Furthermore, apparently non-carrier parents of a
child with any autosomal dominant or X-linked
disorder in which mosaicism is possible but
unproven may have a recurrence risk that may be
as high as 3% to 4%; these couples should be
offered whatever prenatal diagnostic tests are
appropriate.
 The exact recurrence risk is difficult to assess,
however, because it depends on what proportion
of gametes contains the mutation
IMPRINTING IN PEDIGREES
Unusual Inheritance Patterns due to Genomic imprinting
 In some genetic disorders such as PWS and AS, the
expression of the disease phenotype depends on
whether the mutant allele has been inherited from the
father or from the mother, a phenomenon known as
genomic imprinting.
 Imprinting can cause unusual inheritance patterns in
pedigrees, as clearly demonstrated by a rare condition
known as Albright hereditary osteodystrophy
(AHO). AHO is characterized by obesity, short stature,
subcutaneous calcifications, and brachydactyly,
particularly of the fourth and fifth metacarpal bones.
A, Characteristic appearance of a patient with Albright hereditary
osteodystrophy. B, Hand radiograph showing shortened metacarpals and
distal phalanges, especially and characteristically the fourth metacarpal

AHO is inherited as a fully penetrant
autosomal dominant trait. What is unusual,
however, is that in families of individuals
affected by AHO, some but not all of the
affected patients have an additional clinical
disorder known as
pseudohypoparathyroidism (PHP).

In PHP, an abnormality of calcium metabolism
typically seen with a deficiency of parathyroid
hormone occurs but with elevated levels of
parathyroid hormone (hence the use of the prefix
pseudo) that is secondary to renal tubular
resistance to the effects of parathyroid hormone.
 PHP in an individual with the AHO phenotype is
known as pseudohypoparathyroidism type 1a
(PHP1a).
 AHO with or without PHP is caused by a defect in
the GNAS gene. GNAS is involved in transmitting
the parathyroid hormone signal from the surface
of renal cells to inside the cell.

A careful examination of PHP1a pedigrees shows
that some individuals have AHO only, without the
calcium and renal problems, whereas others have
the physical characteristics as a component of
PHP1a.
 When AHO occurs without the renal tubular
dysfunction in families in which other relatives
have PHP1a, it is often referred to as
pseudopseudohypoparathyroidism (PPHP).
 Interestingly, when PPHP and PHP1a occur within
the same family, affected brothers and sisters in
any one sibship either all have PPHP or all have
PHP1a; what does not happen is that one sib will
have one condition while another has the other.

This unusual pattern of inheritance can be explained
by the fact that the defective gene (GNAS) in PHP1a
and PPHP is imprinted only in certain tissues,
including renal tubular cells, so that only the GNAS
allele inherited from the mother is expressed in these
cells while the father's allele is normally silent.
 PHP1a therefore occurs only when an individual
inherits an inactivating mutation in GNAS from his or
her mother; since the paternal copy is not expressed
anyway, these tissues have no normal, functioning
copy of GNAS, and resistance to the effects of
parathyroid hormone ensues.
 There is no imprinting, however, in most of the tissues
of the body. In the tissues without GNAS imprinting,
heterozygotes for one mutant GNAS allele all develop
AHO, which is passed on as a simple autosomal
dominant trait.
Pedigrees of pseudohypoparathyroidism. A, Family with pseudohypoparathyroidism
1a (PHP1a, solid-blue symbols) and pseudopseudohypoparathyroidism (PPHP, halfblue symbols), showing that all PHP1a patients inherit the mutant GNAS gene from
their mothers, whereas all PPHP patients have a paternally derived mutant allele.

The effect of imprinting is also seen in another
form of AD pseudo-hypoparathyroidism,
known as PHP type 1b.
 PHP1b has the calcium abnormalities seen in
PHP1a but without the physical signs of AHO.
 PHP1b is caused by a mutation in upstream
regulatory elements (the "imprinting center")
that control the imprinting of the GNAS gene;
the normal function of these regulatory
elements is to specify that the maternally
inherited GNAS allele, and only that allele, will
be expressed in renal tubules.

When a mutation of the imprinting control region
is inherited from the mother, both the paternal
allele, which is normally silent in kidney tubules,
and the maternal allele, which is silenced in these
tissues because of the deletion, fail to be
expressed, and PHP1b ensues.
 Individuals who inherit the mutation from their
fathers, however, are asymptomatic heterozygotes
because their maternal copy of GNAS, with its
imprinting control region intact, is expressed
normally in these tissues. Outside of the kidney
and a few other tissues, both maternal and paternal
GNAS alleles are expressed independently of any
imprinting, and AHO therefore does not occur.
B, Pedigree of family with PHP1b (solid-blue symbols) due to a deletion
in the imprinting control region. All affected patients inherit the
deletion allele from their mothers; heterozygotes with a paternal allele
are unaffected. Heterozygotes for a deletion mutation in the imprinting
regulatory region of the GNAS gene are indicated by the blue dots.
UNSTABLE REPEAT EXPANSIONS

In all of the types of inheritance presented earlier
in this chapter, the responsible mutation, once it
occurs, is stable from generation to generation.
 In contrast, an entirely new class of genetic
disease has been recognized, diseases due to
unstable repeat expansions. By definition, these
conditions are characterized by an expansion
within the affected gene of a segment of DNA
consisting of repeating units of three or more
nucleotides in tandem (i.e., adjacent to each other).

For example, the repeat unit often consists
of three nucleotides, such as CAG or CCG,
and the repeat will be CAGCAGCAG …
CAG or CCGCCGCCG … CCG.
 In general, the genes associated with these
diseases all have wild-type alleles that are
polymorphic; that is, there is a variable but
relatively low number of repeat units in the
normal population.

As the gene is passed from generation to
generation, however, the number of repeats can
increase (undergoes expansion), far beyond the
normal polymorphic range, leading to
abnormalities in gene expression and function.
 The molecular mechanisms by which such
expansions occur are not clearly understood but
are likely to be due to a type of DNA replication
error known as slipped mispairing.
 The discovery of this unusual group of conditions
has dispelled the orthodox notions of germline
stability and provided a biological basis for such
eccentric genetic phenomena as anticipation and
parental transmission bias.
Table 7-3. Four Representative Examples of
Unstable Repeat Expansion Diseases
Repeat
Number
Disease
Inheritance
pattern
Repeat Gene
Affected
Location in
Gene
Normals
intermediate
Affected
Huntington
disease
Autosomal
dominant
CAG
HD
coding region
<36
36-39
usually
affected
>40
Fragile X
X-linked
CGG
FMR1
5' untranslated
<60
60-200
usually
unaffected*
Myotonic
dystrophy
Autosomal
dominant
CTG
DMPK
3' untranslated
<30
50-80 may
be mildly
affected
Friedreich
ataxia
Autosomal
recessive
AAG
FRDA
intron
<34
36-100
*May have tremor-ataxia syndrome or premature ovarian failure.
>200
80-2000
>100

More than a dozen diseases are known to result
from unstable repeat expansions. All of these
conditions are primarily neurological.
 A dominant inheritance pattern occurs in some, Xlinked in others, and recessive inheritance in still
others. The degree of expansion of the repeat unit
that causes disease is sometimes subtle (as in the
rare disorder oculopharyngeal muscular
dystrophy) and sometimes explosive (as in
congenital myotonic dystrophy or severe fragile
X syndrome).

Other differences between the various
unstable repeat expansion diseases include:
- the length and base sequence of the repeated
-
unit;
the number of repeated units in normal,
presymptomatic and fully affected individuals;
the location of the repeated unit within genes;
the pathogenesis of the disease;
the degree to which the repeated units are
unstable during meiosis or mitosis; and
parental bias in when expansion occurs.
Polyglutamine Disorders
Huntington Disease

Huntington disease (HD) is a well-known disorder
that illustrates many of the common genetic features
of the polyglutamine disorders caused by expansion
of an unstable repeat.
 HD was first described by the physician George
Huntington in 1872 in an American kindred of
English descent. The neuropathology is dominated by
degeneration of the striatum and the cortex.
 Patients first present clinically in midlife and manifest
a characteristic phenotype of motor abnormalities
(chorea, dystonia), personality changes, a gradual loss
of cognition, and ultimately death.

For a long time, HD was thought to be a typical, AD.
Although homozygotes may have a more rapid course
of their disease.
 There are, however, obvious peculiarities in its
inheritance that could not be explained by simple AD
inheritance.
 First, the age at onset of HD is variable; only about
half the individuals who carry a mutant HD allele
show symptoms by the age of 40 years.
 Second, the disease appears to develop at an earlier
and earlier age when it is transmitted through the
pedigree, a phenomenon referred to as anticipation,
but only when it is transmitted by an affected father
and not by an affected mother.

These are now readily explained by the discovery that
the mutation is composed of an abnormally long
expansion of a stretch of the nucleotides CAG, the
codon specifying glutamine, in the coding region of a
gene for a protein of unknown function called
huntingtin.
 Normal individuals carry between 9 and 35 CAG
repeats in their HD gene, with the average being 18 or
19.
 Individuals affected with HD have 40 or more repeats,
with the average being around 46. A borderline repeat
number of 36 to 39, although usually associated with
HD, can be found in a few individuals who show no
signs of the disease even at a fairly advanced age.
 Once an expansion increases to greater than 39,
however, disease always occurs, and the larger the
expansion, the earlier the onset of the disease.
Figure 7-27 Graph correlating approximate age at onset of Huntington
disease with the number of CAG repeats found in the HD gene. The solid
line is the average age at onset, and the shaded area shows the range of
age at onset for any given number of repeats
Figure 7-28 Pedigree of family with Huntington
disease. Shown beneath the pedigree is a
Southern blot analysis for CAG repeat
expansions in the huntingtin gene. In addition to
a normal allele containing 25 CAG repeats,
individual I-1 and his children II-1, II-2, II-4, and
II-5 are all heterozygous for expanded alleles,
each containing a different number of CAG
repeats. I-1, who developed HD at the age of 64
years and is now deceased, had an abnormal
repeat length of 37. He has three affected
children, two of whom have repeat lengths of 55
and 70 and developed disease in their 40s, and
a son with juvenile HD and 103 CAG repeats in
his huntingtin gene. Individual II-1 is unaffected
at the age of 50 years but will develop the
disease later in life. Individuals I-2 and II-3 have
two alleles of normal length (25). Repeat lengths
were confirmed by PCR analysis.

How, then, does an individual come to have an
expanded CAG repeat in his or her HD gene?
 Most commonly, he or she inherits it as a
straightforward autosomal dominant trait from an
affected parent who already has an expanded
repeat (>36).
 In contrast to stable mutations, however, the size
of the repeat may expand on transmission,
resulting in earlier onset disease in later
generations; on the other hand, repeat numbers in
the range of 40 to 50 may not result in disease
until later in life, thereby explaining the agedependent penetrance.

In this pedigree, individual I-1,
now deceased, was diagnosed
with HD at the age of 64 years
and had an expansion of 37
CAG repeats.
 Four of his children inherited
the expanded allele, and in all
four of them, the expansion
increased over that found in
individual I-1
 Individual II-5, in particular, has
the largest number of repeats
and became symptomatic during
adolescence. Individual II-1, in
contrast, inherited an expanded
allele but remains asymptomatic
and will likely develop the
disease sometime later in life.

On occasion, unaffected individuals carry alleles
with repeat lengths at the upper limit of the normal
range (29 to 35 CAG repeats) that, however, can
expand during meiosis to 40 or more repeats.
 CAG repeat alleles at the upper limits of normal
that do not cause disease but are capable of
expanding into the disease-causing range are
known as premutations.
 Expansion in HD shows a paternal transmission
bias and occurs most frequently during male
gametogenesis, which is why the severe earlyonset juvenile form of the disease, seen with the
largest expansions (70 to 121 repeats), is always
paternally inherited.

Expanded repeats may continue to be unstable
during mitosis in somatic cells, resulting in some
degree of somatic mosaicism for the number of
repeats in different tissues from the same patient.
 The largest known group of HD patients lives in
the region of Lake Maracaibo, Venezuela; these
patients are descendants of a single individual who
introduced the gene into the population early in
the 19th century.
 About 100 living affected persons and another
900, each at 50% risk, are currently known in the
Lake Maracaibo community.
 High frequency of a disease in a local population
descended from a small number of individuals,
one of whom carried the gene responsible for the
disease, is an example of founder effect.
Spinobulbar Muscular Atrophy and Other
Polyglutamine Disorders

In addition to HD, other neurological diseases are
caused by CAG expansions encoding polyglutamine,
such as X-linked recessive spinobulbar muscular
atrophy and the various autosomal dominant
spinocerebellar ataxias.
 These conditions differ in the gene involved, the
normal range of the repeat, the threshold for clinical
disease caused by expansion, and the regions of the
brain affected.
 They all share with HD the fundamental
characteristic that results from instability of a stretch
of repeated CAG nucleotides leading to expansion of
a glutamine tract in a protein.
Fragile X Syndrome





The fragile X syndrome is the most common heritable
form of moderate MR and is second only to Down
syndrome among all causes of MR in males.
The name refers to a cytogenetic marker on the X
chromosome at Xq27.3, a "fragile site" in which the
chromatin fails to condense properly during mitosis.
The syndrome is inherited as an X-linked disorder with
penetrance in females in the 50% to 60% range.
The fragile X syndrome has a frequency of at least 1 in
4000 male births and is so common that it requires
consideration in the differential diagnosis of MR in
both males and females.
Testing for the fragile X syndrome is among the most
frequent indications for DNA analysis, genetic
counseling, and prenatal diagnosis.
Figure 7-30 The fragile site at
Xq27.3 associated with X-linked
mental retardation

The disorder is caused by another unstable repeat
expansion, a massive expansion of another triplet
repeat, CGG, located in the 5' untranslated region
of the first exon of a gene called FMR1 (fragile X
mental retardation 1).
 The normal number of repeats is up to 60, whereas
as many as several thousand repeats are found in
patients with the "full" fragile X syndrome
mutation.
 More than 200 copies of the repeat lead to
excessive methylation of cytosines in the promoter
of FMR1; this interferes with replication or
chromatin condensation or both, producing the
characteristic chromosomal fragile site, a form of
DNA modification that prevents normal promoter
function or blocks translation.

Triplet repeat numbers between 60 and 200 constitute a
special intermediate premutation stage of the fragile X
syndrome.
 Expansions in this range are unstable when they are
transmitted from mother to child and have an
increasing tendency to undergo full expansion to more
than 200 copies of the repeat during gametogenesis in
the female (but almost never in the male), with the risk
of expansion increasing dramatically with increasing
premutation size.
 Carriers of premutations can develop an adult-onset
neurological disorder of cerebellar dysfunction and
neurological deterioration, known as the fragile Xassociated tremor/ataxia syndrome.
 In addition, approximately one quarter of female
carriers of premutations will experience premature
ovarian failure by the age of 40 years.
Figure 7-29 Characteristic
facial appearance of a
patient with the fragile X
syndrome
Figure 7-31 Frequency of expansion of a premutation triplet repeat in FMR1 to a full mutation in
oogenesis as a function of the length of the premutation allele carried by a heterozygous
female. The risk of fragile X syndrome to her sons is approximately half this frequency, since
there is a 50% chance a son will inherit the expanded allele. The risk of fragile X syndrome to
her daughters is approximately one-fourth this frequency, since there is a 50% chance a
daughter would inherit the full mutation, and penetrance of the full mutation in a female is
approximately 50%
Myotonic Dystrophy

Myotonic dystrophy (dystrophia myotonica, or DM) is
inherited as an autosomal dominant myopathy
characterized by myotonia, muscular dystrophy,
cataracts, hypogonadism, diabetes, frontal balding, and
changes in the electroencephalogram.
 The disease is notorious for lack of penetrance,
pleiotropy, and its variable expression in both clinical
severity and age at onset.
 The DM congenital form, is particularly severe and may
be life-threatening as well as a cause of MR.
 Virtually every child with the congenital form is the
offspring of an affected mother, who herself may have
only a mild expression of the disease and may not even
know that she is affected. Thus, pedigrees of DM, like
those of HD and fragile X syndrome, show clear
evidence of anticipation.

DM is also associated with amplification of a
triplet repeat, in this case a CTG triplet located in
the 3' untranslated region of a protein kinase gene
(DMPK).
 The normal range for repeats in DMPK is 5 to 30;
carriers of repeats in the range of 38 to 54
(premutations) are usually asymptomatic but have
an increased risk of passing on fully expanded
repeats.
 Mildly affected individuals have about 50 to 80
copies; the severity increases and age at onset
decreases the longer the expansion.
Myotonic dystrophy, an autosomal dominant condition with variable
expression in clinical severity and age at onset. The grandmother in this
family (left) had bilateral cataracts but has no facial weakness or muscle
symptoms; her daughter was thought to be unaffected until after the birth
of her severely affected child, but she now has moderate facial
weakness and ptosis, with myotonia, and has had cataract extraction.
The child has congenital myotonic dystrophy

Severely affected individuals can have more than
2000 copies. Either parent can transmit an
amplified copy, but males can pass on up to 1000
copies of repeat, whereas really massive
expansions containing many thousands of repeats
occur only in female gametogenesis. Because
congenital DM is due to huge expansions in the
many thousands, this form of myotonic dystrophy
is therefore almost always inherited from an
affected mother.
Friedreich Ataxia

Friedreich ataxia (FRDA), a spinocerebellar ataxia,
constitutes a fourth category of triplet repeat disease.
 The disease is inherited in an AR pattern, in contrast
to HD, DM, and fragile X syndrome. The disorder is
usually manifested before adolescence and is
generally characterized by incoordination of limb
movements, difficulty with speech, diminished or
absent tendon reflexes, impairment of position and
vibratory senses, cardiomyopathy, scoliosis, and foot
deformities.
 In most cases, Friedreich ataxia is caused by
amplification of still another triplet repeat, AAG,
located this time in an intron of a gene that encodes a
mitochondrial protein called frataxin, which is
involved in iron metabolism.

In normal individuals, the repeat length varies
from 7 to 34 copies, whereas repeat expansions in
the patients are typically between 100 and 1200
copies.
 Expansion within the intron interferes with normal
expression of the frataxin gene; because Friedreich
ataxia is recessive, loss of expression from both
alleles is required to produce the disease.
 In fact, 1% to 2% of FRDA patients are known to
be compound heterozygotes in whom one allele is
the common amplified intronic AAG repeat
mutation and the other a nucleotide mutation
Similarities and Differences Among
Unstable Repeat Expansion Disorders

A comparison of HD (and the other polyglutamine
neurodegeneration diseases) with the fragile X
syndrome, DM, and FRDA reveals some
similarities but also many differences
 Although unstable repeat expansions of a
trinucleotide are involved in all four types of
disease, the expansion in the polyglutamine
diseases is in the coding region and ranges from
40 to 120 copies of the CAG, whereas the repeat
expansions in fragile X syndrome, DM, and
FRDA involve different triplet nucleotides,
contain hundreds to thousands of repeated triplets,
and are located in untranslated portions of the
FMR1, DMPK, and FRDA genes, respectively.

Premutation expansions causing an increased
risk for passing on full mutations are the rule in
all four of these disorders, and anticipation is
commonly seen in pedigrees of the dominant
and X-linked diseases (HD, fragile X
syndrome, and DM).
 However, the number of repeats in premutation
alleles in HD is 29 to 35, similar to what is seen
in DM but far less than in fragile X syndrome.

Premutation carriers can develop significant
disease in fragile X syndrome but are, by
definition, disease-free in HD and DM. The
expansion of premutation alleles occurs in the
female primarily in FRDA, DM, and fragile X
syndrome; the largest expansions causing juvenile
onset HD occur in the male germline.
 Finally, the degree of mitotic instability in fragile
X syndrome, DM, and FRDA is far greater than
that seen in HD and results in much greater
variability in the numbers of repeats found among
cells of the same tissue and between different
somatic tissues in a single individual.
CONDITIONS THAT MAY MIMIC MENDELIAN
INHERITANCE OF SINGLE-GENE DISORDERS

A pedigree pattern sometimes simulates a
single-gene pattern even though the disorder
does not have a single-gene basis.
 It is easy to be misled in this way by
teratogenic effects; by certain types of
inherited chromosome disorders, such as
balanced translocations; or by
environmental exposures shared among
family members.

Inherited single-gene disorders can usually
be distinguished from these other types of
familial disorders by their typical mendelian
segregation ratios within kindreds.
 Confirmation that a familial disease is due
to mutations in a single gene eventually
requires demonstration of defects at the
level of the gene product, or the gene itself.

There is also a class of disorders called segmental
aneusomies, in which there is a deficiency or
excess of two or more genes at neighboring loci
on a chromosome, due to a deletion or a
duplication or triplication of an entire segment of
DNA.
 Here the phenotype, referred to as a contiguous
gene syndrome, results from alterations in the
copy number of more than one gene and yet shows
typical mendelian segregation ratios, with a
usually dominant inheritance pattern, because the
segmental aneusomy is passed on as if it were a
single mutant allele.

Examples include:
– autosomal dominant Parkinson disease due to a
triplication of an approximately 2-Mb region of
chromosome 4q;
– autosomal dominant velocardiofacial syndrome,
where the phenotype is caused by deletions of
millions of base pairs of DNA encoding
multiple genes at 22q11.2; and
– the X-linked syndrome of choroideremia (a
retinal degeneration), deafness, and mental
retardation, caused by a deletion of at least
three loci in band Xq21
MATERNAL INHERITANCE OF DISORDERS CAUSED
BY MUTATIONS IN THE MITOCHONDRIAL GENOME

Some pedigrees of inherited diseases that could
not be explained by typical mendelian inheritance
of nuclear genes are now known to be caused by
mutations of the mitochondrial genome and to
manifest maternal inheritance.
 Disorders caused by mutations in mitochondrial
DNA demonstrate a number of unusual features
that result from the unique characteristics of
mitochondrial biology and function.
The Mitochondrial Genome

The mt genome consists of a circular chr., 16.5 kb.
 Most cells contain at least 1000 mtDNA molecules,
distributed among hundreds of individual mt.
 A remarkable exception is the mature oocyte, which has
more than 100,000 copies of mtDNA, composing about
one third of the total DNA content of these cells.
 Mitochondrial DNA (mtDNA) contains 37 genes. The
genes encode 13 polypeptides that are subunits of
enzymes of oxidative phosphorylation, two types of
rRNA, and 22 tRNAs required for translating the
transcripts of the mitochondria-encoded polypeptides.

More than 100 different rearrangements and 100
different point mutations have been identified in
mtDNA that can cause human disease, often
involving the central nervous and musculoskeletal
systems (e.g., myoclonic epilepsy with ragged-red
fibers).
 The diseases that result from these mutations show
a distinctive pattern of inheritance because of three
unusual features of mitochondria: replicative
segregation, homoplasmy and heteroplasmy,
and maternal inheritance.
Replicative Segregation

The first unique feature of the mt. chromosome is
the absence of the tightly controlled segregation
seen during mitosis and meiosis of the 46 nuclear
chromosomes.
 At cell division, the multiple copies of mtDNA in
each of the mitochondria in a cell replicate and
sort randomly among newly synthesized
mitochondria.
 The mitochondria, in turn, are distributed
randomly between the two daughter cells. This
process is known as replicative segregation.
Homoplasmy-Heteroplasmy

The second feature arises from the fact that most
cells contain many copies of mtDNA molecules.
 When a mutation arises in the mtDNA, it is at first
present in only one of the mtDNA molecules in a
mitochondrion. With replicative segregation,
however, a mitochondrion containing a mutant
mtDNA will acquire multiple copies of the mutant
molecule. With cell division, a cell containing a
mixture of normal and mutant mtDNAs can
distribute very different proportions of mutant and
wild-type mitochondrial DNA to its daughter
cells.

One daughter cell may, by chance, receive
mitochondria that contain only a pure population
of normal mtDNA or a pure population of mutant
mtDNA (a situation known as homoplasmy).
 Alternatively, the daughter cell may receive a
mixture of mitochondria, some with and some
without mutation (heteroplasmy).
 Because the phenotypic expression of a mutation
in mtDNA depends on the relative proportions of
normal and mutant mtDNA in the cells making up
different tissues, reduced penetrance, variable
expression, and pleiotropy are all typical features
of mitochondrial disorders.
Homoplasmy and Heteroplasmy

Figure 7-33 Replicative segregation of a heteroplasmic mitochondrial mutation.
Random partitioning of mutant and wild-type mitochondria through multiple rounds of
mitosis produces a collection of daughter cells with wide variation in the proportion of
mutant and wild-type mitochondria carried by each cell. Cell and tissue dysfunction
results when the fraction of mitochondria that are carrying a mutation exceeds a
threshold level. N, nucleus.
Maternal Inheritance of mtDNA

The final mtDNA is its maternal inheritance. Sperm
mitochondria are generally eliminated from the
embryo, so that mtDNA is inherited from the mother.
Thus, all the children of a female who is homoplasmic
for a mtDNA mutation will inherit the mutation,
whereas none of the offspring of a male carrying the
same mutation will inherit the defective DNA.
 Maternal inheritance in the presence of heteroplasmy in
the mother is associated with additional features of
mtDNA genetics that are of medical significance. First,
the number of mtDNA molecules within developing
oocytes is reduced before being subsequently amplified
to the huge total seen in mature oocytes. This
restriction and subsequent amplification of mtDNA
during oogenesis is termed the mitochondrial genetic
bottleneck.

Consequently, the variability in the percentage of
mutant mtDNA molecules seen in the offspring of a
mother with heteroplasmy for a mtDNA mutation
arises, at least in part, from the sampling of only a
subset of the mtDNAs during oogenesis.
 As might be expected, mothers with a high
proportion of mutant mtDNA molecules are more
likely to produce eggs with a higher proportion of
mutant mtDNA and therefore are more likely to
have clinically affected offspring than are mothers
with a lower proportion.
 One exception to maternal inheritance occurs when
the mother is heteroplasmic for deletion mutation in
her mtDNA; for unknown reasons, deleted mtDNA
molecules are generally not transmitted from
clinically affected mothers to their children.

Figure 7-34 Pedigree of Leber hereditary optic neuropathy, a form of
spontaneous blindness caused by a defect in mitochondrial DNA.
Inheritance is only through the maternal lineage, in agreement with the
known maternal inheritance of mitochondrial DNA. No affected male
transmits the disease.

Although mitochondria are almost always
inherited exclusively through the mother, at least
one instance of paternal inheritance of mtDNA has
occurred in a patient with a mitochondrial
myopathy.
 Consequently, in patients with apparently sporadic
mtDNA mutations, the rare occurrence of paternal
mtDNA inheritance must be considered.
Characteristics of Mitochondrial Inheritance

All children of females homoplasmic for a mutation
will inherit the mutation; the children of males
carrying a similar mutation will not.
 Females heteroplasmic for point mutations and
duplications will pass them on to all of their children.
However, the fraction of mutant mitochondria in the
offspring, and therefore the risk and severity of
disease, can vary considerably, depending on the
fraction of mutant mitochondria in their mother as well
as on random chance operating on small numbers of
mitochondria per cell at the oocyte bottleneck.

Heteroplasmic deletions are generally not
heritable.
 The fraction of mutant mitochondria in different
tissues of an individual heteroplasmic for a
mutation can vary tremendously, thereby causing
a spectrum of disease among the members of a
family in which there is heteroplasmy for a
mitochondrial mutation.
 Pleiotropy and variable expressivity in different
affected family members are frequent.
FAMILY HISTORY AS PERSONALIZED
MEDICINE

An accurate determination of the family pedigree is an
important part of the work-up of every patient.
 Pedigrees may demonstrate a straightforward, typical
mendelian inheritance pattern; one that is more atypical,
as is seen with mitochondrial mutations and germline
mosaicism; or a complex pattern of familial occurrence
that matches no obvious inheritance pattern.
 Not only is a determination of the inheritance pattern
important for making a diagnosis in the proband, but it
also identifies other individuals in the family who may
be at risk and in need of evaluation and counseling.

Despite the sophisticated cytogenetic and
molecular testing available to geneticists, an
accurate family history, including the
family pedigree, remains a fundamental tool
for all physicians and genetic counselors to
use in designing an individualized
management and treatment plan for their
patients.