Transcript document

Heredity:
PedigreesWorking Out
Inheritance Patterns
By Lisa Marie Meffert, PhD
Rice University
Genology - Lee Family of Virginia and Maryland
c1886 Apr. 26.
Prints and Photographs Division,
Library of Congress (LC-USZ62-90145)
BioEd Online
How is gender determined? (text p 318)
Recall that in humans the diploid # of
chromosomes is 46 (23 pairs)
 There are 22 pairs of homologous chromosomes
called autosomes
 The 23rd pair of chromosomes are different in
males and females

How is gender determined? (text p 318)
These two x’mes are called the sex
chromosomes.
 Indicated by the letters X and Y
 Females are homozygous XX
 Males are heterozygous XY

Gender determination (cont’d)
Which chromosomes will determine the gender?
 The male determines gender
 Why?
 What is the expected ratio of males to females?
 Complete a punnett square (XX x XY)

XX
X
x
XY
Y
X
XX
XY
X
XX
XY
Sex Linked Traits
Traits controlled by genes located on the sex
chromosomes are called sex linked traits (most
often the X chromosome)
 Y can’t cover up the effects
 Males either have it or not

Sex Linked Traits
Females can have it, not have it or be carriers
 Carriers can pass the gene, but do not exhibit the
characteristics of the gene
 More about this when we talk about pedigrees

Sex Linked Traits
Nondisjunction (p 271)
The events of meiosis usually proceed
accurately
 Sometimes homologous chromosomes fail to
separate properly
 Anaphase I – chromosome pairs separate (1
to each daughter cell)

Nondisjunction (p 271)
Nondisjunction – both chromosomes of a
homologous pair move to the same pole.
 One gamete has an extra chromosome



(n+1)
The other is short one chromosome

(n-1)
Meiotic Nondisjunction at Meiosis I Animation
Tokyo Medical University Genetics Animations
http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/mutations%20notes_files/image006.jpg
Levels of Genetic Disorders

What are Genetic Disorders?

List of disorders with info
Trisomy

Zygote with one normal gamete and one gamete
with extra x’me



47 x’mes – Down Syndrome
AKA – Trisomy 21
Organism with an extra chromosome often
survives
Monosomy
Organisms are one or more chromosomes short
– usually don’t survive
 Cause of most chromosomal miscarriages
 E.g. Turner syndrome


Tetraploid
Changes in Chromosome Size
Fragile –X
 Results from a faulty crossover event
that results in a longer X chromatid.
 A child receiving this chromosome can
be male or female but mostly boys
because it is a recessive trait to a
normal X.



Their faces are longer,
have trouble with gait,
many have learning differences or
disabilities and autism-like mannerisms.
Cri du Chat
1/20 000 live births, mostly girls
 Deletion of chromosome 5

http://learn.genetics.utah.edu/content/disorders/whataregd/cdc/
William’s Syndrome
1/7500 births
 Deletion of genes on chromosome 7
 Elfin, perfect pitch, trouble spacial relationships,
cognitive processing difficulties, aortic defects

http://learn.genetics.utah.edu/content/disorders/whataregd/williams/index.html
Syndromes
Trisomy 21 – Down syndrome
 Trisomy 13 – Patau’s syndrome
 XO – Turner’s syndrome
 XXX – Trisomy X (metafemales)
 XXY – Klinefelter’s syndrome
 XYY – Jacob’s syndrome
 OY – lethal

Turner syndrome – XO monosomy.
Dwarfism
 Webbed neck
 Valgus of elbow.
 Amenorrhea
Klinefelter’s Syndrome - Trisomy XXY
testicular atrophy
 increase in gonadotropins in urine.

Jacob’s syndrome
Jacob's syndrome is a rare chromosomal
disorder that affects males.
 It is caused by the presence of an extra Y
chromosome.
 Males normally have one X and one Y
chromosome.

Jacob’s syndrome
However, individuals with Jacob's syndrome
have one X and two Y chromosome.
 Males with Jacob's syndrome, also called XYY
males

Patau’s syndrome
Fig 12.2 - Pedigree Chart
Family history that shows how a trait is inherited
over several generations.
 Carriers: those heterozygous for a trait.
 Can determine if





autosomal (occurs equally both sexes)
sex-linked (usually seen in males)
heterozygous (dominant phenotype)
homozygous (dominantdominant phenotype,
recessive recessive phenotype)
Pedigree Symbols (see worksheet 103)
Dominant Pedigree
affected individuals
have at least one
affected parent
 the phenotype
generally appears
every generation
 two unaffected
parents only have
unaffected offspring

Recessive Pedigree
unaffected
parents can have
affected offspring
 affected progeny
are both male and
female

Factors to Consider in Pedigrees

Is the trait located on a sex chromosome or an
autosome?


Autosomal – not on a sex chromosome
Sex Linkage – located on one of the sex
chromosomes



Y-linked - only males carry the trait.
X-linked (recessive) - sons inherit the disease from normal
parents
How is the trait expressed?


Dominant - the trait is expressed in every generation.
Recessive - expression of the trait may skip
generations.
Pedigree Diagrams: I

Basic Symbols
Pedigree Diagrams: II

Basic Symbols for offspring and the
expression of a trait.


The offspring are depicted below the parents.
Filling the symbol with black indicates the
expression of the studied trait.
Marfan’s Syndrome: An Example

Expressed in both sexes.


Thus, autosomal.
Expressed in every generation.

Thus, dominant.
Marfan’s: Genotype the Normal Individuals

Assign codes for the alleles.



Code “m” for the recessive normal allele.
Code “M” for the dominant allele for Marfan’s
syndrome.
Normal individuals must be “mm.”
Marfan’s: Genotype the Affected Individuals
 Affected
one “M.”
individuals must have at least
Marfan’s: Parent-Offspring Relationships



Possibilities for #1 and #2: Heterozygote (Mm) or
homozygous for “M?”
If “MM,” all offspring from a normal mate should be
affected.
Therefore, both must be heterozygotes.
Marfan’s: Parental Genotypes Known
“M” must have come from the
mother.
 The father can contribute only “m.”
 Thus, the remaining genotypes are
“Mm.”

Albinism: An Example

Expressed in both sexes at approximately equal
frequency.


Thus, autosomal.
Not expressed in every generation.

Thus, recessive.
Albinism: Genotype the Affected
Individuals

Assign codes for the alleles.




Code “A” for the dominant normal allele.
Code “a” for the recessive allele for albinism.
Affected individuals must be homozygous for “a.”
First generation parents must be “Aa” because they have
normal phenotypes, but affected offspring.
Albinism: Genotype the Normal
Individuals

Normal individuals must have at least one “A.”
Albinism: Parent-Offspring
Relationships



#1 must transmit “a” to each offspring.
The “A” in the offspring must come from the father.
Normal father could be either heterozygous or homozygous for
an “A.”
**
Albinism: Parental Genotypes are
Known
Both parents are heterozygous.
 Normal offspring could have received an “A”
from either parent, or from both.

Albinism: One Parental Genotype is
Known
Only the genotype of the offspring expressing
albinism are known.
 Normal offspring must have received an “a”
from their affected father.

Hairy Ears: An Example
Only males are affected.
 All sons of an affected father have hairy ears.
 Thus, hairy ears is Y-linked.

Hairy Ears: Female Sex Determination

All females are XX.
Hairy Ears: Male Sex Determination

All males are XY.
Hairy Ears: Gene on the Y
Chromosome

Code “H” indicates the allele on the Y
chromosome for hairy ears.
Hairy Ears: Wild-Type Allele for Normal
Ears

Code “+” indicates the allele on the Y
chromosome for normal ears.
Hemophilia: An Example

In this pedigree, only males are affected,
and sons do not share the phenotypes of
their fathers.


Thus, hemophilia is linked to a sex chromosome–the X.
Expression of hemophilia skips
generations.

Thus, it is recessive.
Extensive bruising of
the left forearm and
hand in a patient with
hemophilia.
Hemophilia:
Expression of the Female Sex
Chromosomes

All females are XX.
Hemophilia:
Expression of Male Sex Chromosomes

All males are XY.
Hemophilia: Genotype the Affected
Individuals

Assign codes for the alleles.



Code “H” for the recessive hemophilia allele.
Code “+” for the wild-type normal allele.
Affected individuals must have an “H” on an X
chromosome.
Hemophilia: Father-Daughter
Relationship

All daughters of an affected father receive an
X chromosome with the “H” allele.
Hemophilia: Genotyping the Normal
Individuals

Normal individuals must have at least one X
chromosome with the wild-type allele, “+.”
Hemophilia: Homozygous or
Heterozygous?




Only males affected
Not Y-linked
Skips a generation: recessive
X-linked
Fig 12.2 - Discussion
 Draw
a punnet square for each
generation
 Assignment
 12.1
worksheets p 89, 97, & 104