Transcript Patterns of Inheritance

CH. 6: Patterns of Inheritance
Major Concepts:
 Genes are discrete sequences of DNA on





chromosomes; chromosomes consist of DNA and
associated proteins.
Genes are the units of inherited information.
Genes code for several RNA types; mRNA is the
template for proteins.
Inheritance of genes occurs in regular patterns
that can be predicted by the rules of probability.
Genetic variation, from mutation and
recombination, is essential for evolution.
The products of genetic engineering give rise to
ethical consideration of benefits and risks to
human well-being and environmental integrity.
Genes Determine Biological
Potential
Genes
 Nucleotides  Genes (DNA)  Chromosomes
Heredity and Environment
 Genetics: branch of biology that deals
with inherited variation.
Heredity and Environment
Nature vs. Nurture:
 Both heredity and environment influence an
individual’s development.



e.g. Siamese cats inherit genes for enzymes that
produce dark pigment for fur.
The enzymes function best at
temps below normal body temp.,
thus dark markings are at
extremities: ears, face, paws, tail.
One can change coloration by
keeping certain portions of cat’s
body cool.
Heredity and Environment
 The environment has a strong impact on
gene expression.
Heredity and Environment
Studies of twins:
 Fraternal twins: develop from separate eggs
each fertilized by separate sperm cells.
 Identical twins: develop from one zygote
forming two complete embryos.
Heredity and Environment
 If a trait shows up more often in identical
twins than fraternal: characteristic is probably
genetic.
 If a trait differs in identical twins: probably
environmentally influenced.
Heredity and Environment
 Blended inheritance: notion that mixing of
parents’ genes resulted in an “averaging” of
parental characteristics

no longer seriously
considered, as it would
not allow for passing on
of traits separately to
future generations,
which is observed.
Genes
 Information is stored in
genes in the sequence of
nucleotide bases that
make up DNA: a molecular
code.
 The code directs the cell processes involved
in development and function of cells and,
thus, the entire organism.
 Genes provide instructions for the structure,
function and development of a cell/organism.
Genes
 Many genes code for the
synthesis of specific
proteins, e.g. an enzyme, muscle protein,
pigment, regulatory proteins, etc.; other
genes code for various forms of RNA
 Through processes of meiosis and
fertilization, chromosomes are passed on
from generation to generation.
Genes Determine Biological Potential
 Heredity - passing of traits from parent to
offspring
 Genes – basic units of genetic info
 Genetics - study of heredity
 Involves predictions
 referred to as probability - predicts the
chances that a certain event will occur
 Geneticist – one who studies heredity and
the actions of genes.
Genes and Chromosomes
 Prokaryotic chromosomes: single circular
DNA molecule with little protein; generally no
introns.
 ~ 90% of DNA is translated.
Prokaryotic Chromosomes
 Often have small circles of additional DNA:
plasmids.



Plasmids may move from one bacterial cell to
another, thereby introducing genetic variation.
Geneticists use
plasmids to introduce
modified genetic
material into bacterial
cells: genetic
engineering, e.g.
insulin production.
Plasmids carry genes
for antibiotic resistance.
Genes and Chromosomes
 Eukaryotic chromosomes: consist of
long molecules of DNA wrapped around
proteins.
 Only part of the DNA
codes for proteins.
 Some noncoding sections
of DNA consist of
sequences repeating
thousands of times.
Genes and Chromosomes
 Only ~ 1.5% of human DNA codes for
proteins. (We don’t know importance of rest.)



Some introns are involved with gene expression.
Repetitive sequences may serve to stabilize
DNA’s bond with associated proteins.
Mutations can convert inactive DNA sequences
into active genes, or inactivate functional genes 
may be a source of new alleles in natural
selection.
Genes and Chromosomes
 Homologous chromosomes carry same
genes, though not necessarily the same
alleles for those genes.
Genes and Chromosomes
 Chromosomes may be distinguished by their
banding pattern (pattern of dye that occurs when
chromosome is stained (Fig. 8.9, p. 191). Ea.
chromosome has a distinctive banding pattern.
Genes and Chromosomes
 Karyotype: (Fig. 13.11, p. 349) a display of human
chromosomes arranged as homologous pairs.
Karyotypes
 Used in genetic studies of disease to search
for hereditary causes.
 Members of each pair have a specific
banding pattern when dyed, as the stains
bind to specific regions of the chromosomes.
Karyotypes
 White blood cells frequently used for
such studies:


They can be made to divide easily.
Grow well in culture.
 Chemicals interrupt cell cycle at




metaphase. Why?
Cells are placed on microscope slide
and treated with water to spread
chromosomes apart.
Stains cause the banding pattern to appear.
Unique banding of each chromosome pair enables
researchers to detect missing or extra chromosome
parts and extra chromosomes themselves.
Have also helped with mapping of genes on
chromosomes.
Genes and Chromosomes
Karyotype:
 Allows one to count and identify chromosomes, and
spot any unusual, missing or extra chromosomes
fairly quickly.
Genes and Chromosomes
 Human karyotype made up of 22 pairs of
autosomes, chromosomes that are the same in ♂ &
♀, and one pair of sex chromosomes, chromosomes
that are different in ♂ & ♀.
 ♀: XX; ♂: XY
Probability
 Probability: an area of mathematics that
predicts the chances that a certain event will
occur.
 Using the rules of probability, one can predict
the most probable outcome of randomly
ordered events; the actual outcome, however,
may not match the prediction, i.e. the
prediction is simply that: a prediction, not a
guarantee.
 Investigation 13A: Probability, pp. 748 – 49
(Lab write-up due ___)
Gregor Mendel
 1822 – 1884
 Austrian monk
 Studied science & math at the
University of Vienna
 Formulated the laws of heredity in the early
1860's
 Did a statistical study of traits in garden peas
over an eight year period
 Mendel’s work led to the concept of the gene
- Mendelian Genetics
Why garden peas (Pisum sativum)?
 Can be grown
in a small area
 Produce lots
of offspring
 Produce pure
plants when
allowed to
self-pollinate several generations (truebreeding)
 Can be artificially cross-pollinated
 Garden pea flowers contain both male &
female reproductive parts
 Self-pollination (pollinates itself)
 Cross-pollination (collect pollen from
flowers of one pea plant & transfer to
another)
Mendel and the Idea of Alleles
 4 ways experiments were unique:
 Looked at only one trait at a time
 Used large numbers
 Combined results of many identical
experiments
 Analyzed results using rules of probability
 Thus, Mendel was able to see patterns
of inheritance
Mendel and the Idea of Alleles
 Mendel studied 22 simple traits of pea plants
(e.g. seed color & shape, pod color & shape,
etc.).
 Mendel traced the inheritance of individual
traits & kept careful records of numbers of
offspring.
 He used his math principles of probability to
interpret results.
 Mendel studied pea traits, each of which had
a dominant & a recessive form.
Inheritance of Alleles
 Trait: any characteristic that can be passed from





parent to offspring
Allele: one of two or more possible forms of a gene
(e.g. dominant & recessive)
Dominant: an allele that masks the presence of
another allele of the same gene in a heterozygous
organism, represented by capital letter, e.g. B
Recessive: a trait (allele) whose expression is
masked (hidden) in a heterozygous organism,
represented by lower-case letter, e.g. b
Genotype: genetic makeup of an organism; gene
combination for a trait (e.g. RR, Rr, rr)
Phenotype: observable appearance or trait
determined by the genotype; the physical feature
resulting from a genotype (e.g. tall, short)
Inheritance of Alleles
 Homozygous: The condition (genotype) in which




both alleles are the same form, e.g. RR, rr; can
produce only one type of gamete; also called “pure.”
Heterozygous: The condition in which two alternate
forms (alleles) of a gene are contained within the
organism, e.g. Rr; also called “hybrid.”
Monohybrid cross: a cross (mating)
involving a single trait.
Dihybrid cross: a cross (mating)
involving two traits.
Punnett Square: graphic tool (grid)
used to solve genetics problems.
Inheritance of Alleles
Inheritance of Alleles
 A Punnett Square:
Inheritance of Alleles
 A Punnett Square:
 The dominant (shows up most often) gene
or allele is represented with a capital
letter, & the recessive gene with a lower
case of that same letter (e.g. B, b)
 Mendel’s traits included:
a. Seed shape --- Round (R) or Wrinkled (r)
b. Seed Color ---- Yellow (Y) or Green (y)
c. Pod Shape --- Smooth (S) or wrinkled (s)
d. Pod Color --- Green (G) or Yellow (g)
e. Seed Coat Color --- Gray (G) or White (g)
f. Flower position --- Axial (A) or Terminal (a)
g. Plant Height --- Tall (T) or Short (t)
h. Flower color --- Purple (P) or white (p)
Mendel’s Experiments (cont.)
 1st: Mendel tested each strain of plant he used to
ensure that it was true-breeding (homozygous), i.e.
genetically true, producing offspring identical to
themselves generation after generation.
 2nd: He worked with strains that were true-breeding
in all but one characteristic, e.g. tall vs. short plant
form; green vs. yellow seeds, round vs. wrinkled
seeds, etc.
 This allowed him to follow the pattern of inheritance
of one trait at a time from generation to generation,
e.g. round vs. wrinkled seeds:
Mendel’s Experiments (cont.)
 Parental generation (P1): Crossed round seed-producing
plants with wrinkled seed-producing plants.
 First filial generation (F1): All offspring of the above cross
produced round seeds.
 Second filial generation (F2): ¾ produced round seeds; ¼
produced wrinkled seeds.
 Thus, wrinkled seeds seemed to disappear in one generation
(F1), then reappear in the next (F2).
 Mendel called the round seed condition, “dominant,” and the
wrinkled seed condition, “recessive.”
 He surmised that the recessive form of a trait could only be
manifest when the individual inherited that trait from both
parents, i.e. received two “doses” of the trait.
 This is known as Mendel’s principle of dominance.
Mendel’s Experiments (cont.)
Mendel’s Experiments (cont.)
Mendel’s Experiments (cont.)
 Alleles are not dominant or recessive; only their





effects on a trait are.
At the molecular level, the genotype, both alleles are
present.
At the level of the organism, the phenotype, the
effects of one allele (the dominant form) may mask
those of the other (recessive form) allele.
Mendel repeated this type of experiment for other
traits (outlined in Fig. 6.10, p. 179).
He calculated the ratio of dominant to recessive
forms for each trait and it was always essentially the
same: the dominant form appeared in approx. ¾ of
the F2 plants, while the recessive form appeared in ¼
of the F2 plants. Thus, the ratio was always 3:1.
(See p. 179)
Inheritance of Alleles
 Mendel did not know about genes. He referred to
dominant and recessive “factors” to describe the
results of his experiments.
 He did not know where these “factors” were located
in cells.
 Hypothesized that only one copy of a factor went into
each sperm or ovum, i.e. if a parent were truebreeding for round seeds, for example, all its
gametes would have the “round-seed factor.”
Similarly for “wrinkled-seed factor.”
 Offspring of a round-seed by wrinkled seed cross
would have one factor of each from each parent: the
principle of segregation.
Inheritance of Alleles
 Genotype: genetic makeup of an organism;
made up of the alleles for any gene, e.g. RR,
Rr, rr.
 Phenotype: the observable appearance or
trait that is determined by the genotype, e.g.
Three distinct genotypes:



BB  phenotype: purple flowers
(dominant)
Bb  phenotype: purple flowers
(dominant)
bb  phenotype: white flowers
(recessive)
Inheritance of Alleles
Inheritance of Alleles
 Mendel also worked with plants that varied in two traits at a






time, e.g. round vs. wrinkled seeds and yellow vs. green seed
color.
Dihybrid cross: a cross that results in offspring that are
heterozygous for two (di) traits.
See Fig. 13.14, p. 353; a Punnett square
F2 Phenotypes occur in 9:3:3:1 ratio (9/16, 3/16, 3/16, 1/16).
Each trait individually displays the 3:1 ratio of a monohybrid
cross.
The genes for the various traits separate independently from
one another . . .
Principle of independent assortment: alleles for one trait
segregate independently of alleles for the other trait during
gamete formation.
Dihybrid Cross
Dihybrid Cross
Dihybrid Cross
Patterns of Inheritance
 Testcross: a cross between an organism
with an unknown genotype and an organism
with the recessive phenotype



The recessive phenotype individual must be
homozygous recessive, thus one knows the
genotype.
The resulting Punnett Square results will allow you
to determine the genotype of the “unknown”
parent.
Quick Lab: Using a Testcross (p. 185)
Patterns of Inheritance
 Mendel worked with garden peas, but his
results apply to inheritance in other
organisms as well,
 e.g cystic fibrosis, which geneticists now
know is a recessive trait.



F: allele for no disease
f: allele for cystic fibrosis
(Read story from p. 183 in green Biology text)


What are the genotypes of Bob, Mary and Lisa?
What are the chances that Bob and Mary’s next child will
have cystic fibrosis?
7.2 Multiple Alleles & Alleles Without
Dominance
 Incomplete
Dominance: A
heterozygous condition
in which both alleles at
a gene locus are
partially expressed,
often producing an
intermediate
phenotype. (Fig. 8.7,
p. 190)
P1 ♂: Red;
P2 ♀: White
F1: All Pink
F2: 25% Red; 50% Pink; 25%
White
7.2 Multiple Alleles & Alleles Without
Dominance
 Codominance: The situation in which a
heterozygote shows the phenotypic effects of both
alleles fully & equally, (e.g. blood group antigens).
R = allele for red flowers
W = allele for white flowers
red x white  red & white spotted
RR x WW  100% RW
7.2 Multiple Alleles & Alleles Without
Dominance
 Codominance: The situation in which a
heterozygote shows the phenotypic effects of both
alleles fully & equally, (e.g. mottled coat coloration
in a cow).
7.2 Multiple Alleles & Alleles Without
Dominance
 Multiple alleles: Genes that have more than two
types of alleles; provide another example of
codominance, e.g. three alleles for blood type in
humans (Table 8.2, p. 190):





Genotype
IAIA or IAi
IBIB or IBi
I AI B
ii
 Blood phenotype
 A
 B
 AB (codominance)
 O
 (Three different alleles exist, but an individual has
only two of the alleles.)
Blood Types
7.2 Multiple Alleles & Alleles Without
Dominance
7.2 Multiple Alleles & Alleles Without
Dominance
 Multifactorial Inheritance: Inheritance
determined by the combined effects of
genetic and environmental factors.
7.2 Multiple Alleles & Alleles Without
Dominance
 Discontinuous (Discrete) Traits: traits
like cystic fibrosis, that are either
present or absent.
 Generally
controlled by a single pair of
alleles.
 Continuous Traits: traits that vary
across a broad range, e.g. height,
weight, intelligence, hair color, skin
color, eye color in humans.
7.2 Multiple Alleles & Alleles Without
Dominance
 Continuous variation is the result of
multifactorial inheritance, the
interaction of at least several genes with
a large number of possible
environmental variables, e.g. skin
pigment, melanin, is result of four
genes; exposure to sun also plays a
role.
7.2 Multifactorial Inheritance
 Cleft lip and spina bifida also result from
combination of genes and
environmental conditions in mother’s
womb.
7.2 Multiple Alleles & Alleles Without
Dominance
 Polygenic inheritance (p. 206): Multiple
genes influence phenotype, but there is no
environmental influence.
 Incomplete dominance, codominance,
multiple alleles, and multifactorial inheritance
don’t yield the same ratios as Mendel’s
simple crosses, but are still considered a form
of Mendelian inheritance, i.e. they are the
result of genes residing on chromosomes that
are transmitted by meiosis during sexual
reproduction.
7.2 Multiple Alleles & Alleles Without
Dominance
 Epistasis (from Gk. “to stand upon”):
 The phenotypic expression of a gene at one locus
alters that of a gene at a second locus, e.g. coat
color in labrador retrievers.
 Gene 1: Coat color – Black (B) or Brown (b)
 Gene 2: Allows (E) or disallows (e) coat pigment
to be deposited.
 EE or Ee: coat will be black (BB or Bb) or brown
(bb)
 ee: coat will be yellow
7.2 Multiple Alleles & Alleles Without
Dominance
 Epistasis:
7.2 Multiple Alleles & Alleles Without
Dominance
 Pleiotropy: Genes that have multiple
phenotypic effects
 Pleiotropic
alleles are responsible for the
multiple symptoms associated with certain
hereditary diseases, e.g. cystic fibrosis.
7.2 Multiple Alleles & Alleles Without
Dominance
 Some genetic information is passed on
differently: the DNA of chloroplasts and
mitochondria is passed in a different
manner.
Endosymbiont Theory:
Tracing Human Evolution:
Tracing Human Evolution:
7.2 Multiple Alleles & Alleles Without
Dominance
 Bacteria do not undergo meiosis, but
exhibit a great variety of non-Mendelian
forms of inheritance.
7.3 Linked Genes
 There are many genes on each chromosome.
 Genes that are found close to one another on a
chromosome are said to be linked; they are,
therefore, frequently inherited together (Fig. 7.11, p.
211).
 As an example: the gene for ABO blood type is
found on chromosome 9; so is an oncogene that
may be partially responsible for certain types of
cancer.

Geneticists almost always find these genes together, i.e.
little or no evidence of independent assortment.
7.3 Linked Genes
 Quiz Question: Why do geneticists
find little or no evidence of
independent assortment among the
genes for blood type and a particular
oncogene involved in certain types of
cancer?
7.3 Linked Genes
 Linked genes do not always remain
together, but the closer on the chromosome
they are to one another, the more likely it is
that they will remain together (less chance
of the chromosome crossing over right
between the genes in question).
 The farther apart two genes are from one
another on a chromosome, the more likely a
break will occur somewhere between them.
7.3 Linked Genes
7.3 Linked Genes
 If two genes are far enough apart on a
chromosome, the principle of independent
assortment may apply to their inheritance,
i.e. the genes exhibit no linkage.
7.3 Linked Genes
 Genetic maps/Linkage maps:
 Geneticists can use the frequency with which
two linked traits become separated to determine
the relative distance between the two genes on
the chromosome.
 From this information, they can construct genetic
maps, diagrams of the locations of genes on
chromosomes (See Fig. 7.10, p. 211).
7.3 Linked Genes
7.3 Linked Genes
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Human karyotype made up of 22 pairs of
autosomes, chromosomes that are the same in ♂ &
♀, and one pair of sex chromosomes, chromosomes
that are different in ♂ & ♀.
 ♀: XX; ♂: XY
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Other species:




Some insects have an X-O system: ♀: XX; ♂: X
(no Y chromosome exists).
Birds, some fish, and some insects have a Z-W
system: ♂: ZZ; ♀: ZW.
Some plants have an XY system.
Most plants and some animals have no sex
chromosomes; sex is determined by a single pair
of alleles.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 During meiosis, chromosomes separate.
 Each ovum contains an X chromosome.
 Males produce two types of sperm though:
some with X chromosome; some with Y.


If ovum is fertilized by X-bearing sperm, zygote
develops into ♀
If fertilized by Y-bearing sperm, ♂.
 Thus, in humans, the father determines
sex of offspring.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Thomas Hunt Morgan: studied fruit flies
(Drosophila melanogaster) at Columbia University.
 Noticed differences among certain flies, e.g. white
eyes, instead of red; short wings, instead of long;
yellow or black bodies, instead of gray.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Considered these differences mutations, variations
of normal genetic material; alternative forms of a
gene.
 Mutations are how new alleles come to be formed.
 Some mutations are beneficial (provide new
material for evolution); some have no effect on
organism at all. In other words, not all mutations
are bad or deleterious.
 Mutations are the
raw material for evolution.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Not all mutations are bad or deleterious.
 Mutations are the
raw material for evolution.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 Morgan et al. studied
mutations by mating
mutant flies with
nonmutants. Most
mutations were
inherited according to
Mendelian patterns,
except . . .
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
. . . White eyes:
 Morgan crossed white-eyed ♂ with normal redeyed ♀’s (Fig. 7.3, p. 201):
 F1 generation: all red eyed (as expected, if white
eye allele was recessive).
 F2 generation: only males displayed white eyes.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
P1 Generation:
XRXR x XrY
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 In another experiment, he crossed white-eyed ♀’s
with red-eyed ♂’s:
 F1 generation: only ♀’s were red-eyed; all ♂’s had
white eyes (because they inherited their single X
chromosome from their mother).

XR  red eyes
Xr  white eyes
Y chromosome does not contribute to eye color

Key to understanding: ♂ = XY; ♀= XX


7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
P1 Generation:
XrXr x XRY
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
 X-linked inheritance: An X-linked trait is a trait
whose gene is only on the X chromosome.
 Over 300 X-linked traits have been identified in
humans, including hemophilia, red-green colorblindedness, and Duchenne muscular dystrophy.
** X-linked traits occur more often in males than
females.
Autosomal traits occur in both sexes more or
less equally.
7.1 X-Linked Traits Show a Modified
Pattern of Inheritance
7.4 X-Linked Traits Show a Modified
Pattern of Inheritance
Color Blindness:
Color Blindness:
7.4 X-Linked Traits Show a Modified
Pattern of Inheritance

Hmwk:
1.
2.
Redraw figure (next slide) on paper.
Complete the genotypes for all individuals.
X-Linked Traits

Hmwk:
XBXB
XBY
XBXb
XbY
XbXb
= male
= female
Blue = color blind
= male
X-Linked Traits
= female

XBXB
XBXb
Hmwk:
XBY
XBXB
XBXb
XbY
XBXb
XBY
XBXb
XBY
XBXB
XBY
XbY
XbY
XBY
XBY
XBXb
XBXB
XBXb
XBXB
XBXb
XBY
XBY
XBXB
XBXb
Blue = color blind
XBY
XBXb
XBXB
XBXb
XBXB
XBXb
XBY
XbY
XBXb
XBXb
XbY
XbY
XBXb
Investigation: Pedigree Analysis
 P. 218 in text
 Is supertasting an X-linked trait?
 Complete handouts for credit as lab
activity
 See pedigree on next slide
T1T1 = Supertaster
= male
T1T2 = Medium taster
T2T2 = Nontaster
= female
T1T1 = Supertaster
= male
T1T2 = Medium taster
T1T2
T2T2
T2T2 = Nontaster
T1T2
T1T2
T2T2
T1T2
T1T2
T2T2
T1T2
T1T1
T1T2
T2T2
T1T2
T2T2
T1T2
T2T2
T1T2
T2T2
T1T2
T1T1
T1T2
T1T1
T1T2
T1T2
T1T2
T1T1
T1T2
T2T2
Jack
T1T2
T1T1
T2T2
?
= female
T1T2
T1T1
T1T2
T2T2
T1T1
T1T2
T2T2
Jill
T1T2
T2T2
T1T2
T2T2
T1T2
T2T2
?
T1T2
T2T2
T1T2
T2T2
T1T2
T2T2
T1T2
T1T2
T1T2
T2T2
T2T2
T1T2
T1T1
Nondisjunction (p. 192)
 Abnormal numbers or types of chromosomes
can result in certain developmental errors.
 Result of nondisjunction.
Nondisjunction
 Down’s Syndrome (AKA Trisomy 21):
 A condition characterized by distinctive features of the eyes,
mouth, hands, and sometimes internal organs, retarded
mental development (though the degree of delayed
development varies greatly).
 Down’s syndrome individuals have 47 chromosomes,
instead of 46; extra chromosome 21,
 Trisomy: Having three copies of a given
chromosome (Fig. 7.16, p. 217)
 Syndrome: a group of symptoms associated with a
particular disease or condition.
Nondisjunction
 Down’s Syndrome (Trisomy 21)
Nondisjunction
 Turner syndrome (45, X karyotype): a condition
resulting from having only one X chromosome
and no Y chromosome.
 Usually short, underdeveloped and sterile ♀.
 XXX syndrome (47, XXX karyotype):
 ♀ with limited fertility; may have slight
intellectual impairment.
Nondisjunction
 Klinefelter syndrome
(47, XXY karyotype):
 ♂; often tall and sexually
underdeveloped.
Nondisjunction
 These conditions occur when chromosome pairs do
not separate in meiosis, an event called
nondisjunction.
 Results in the formation of abnormal gametes, i.e.
some sperm or ova get extra chromosomes; some
get too few.
 When these gametes fuse with normal gametes
abnormal development usually occurs.
Nondisjunction
Nondisjunction
 Evidence suggests that every cell must contain at
least two of each type of chromosome for the embryo
to develop.
 An exception is the X chromosome, 45. A missing X
chromosome, or extra sex chromosomes (X or Y)
usually permit a fetus to develop, though that
development may be abnormal.
 Except for trisomy 21, an extra autosome usually
results in the death of the fetus. Most spontaneously
aborted fetuses have abnormal chromosome
numbers.
Nondisjunction
13.10 Nondisjunction
 Mary Lyon: British geneticist
 Proposed that early in development of a normal ♀, one X
chromosome in each body cell is inactivated.
 Based proposal on finding that in each cell nuclei of ♀’s, but
not ♂’s, a darkly-staining body appears: a Barr body. (See
Fig. 7.4, p. 203)
Nondisjunction
 ♀’s with three X chromosomes have two Barr
bodies, suggesting that all but one X
chromosome are rendered inactive.


How many would someone with Turner syndrome
have?
What about XXX syndrome?
Nondisjunction
 May be one reason trisomies of X
chromosomes are not as disruptive as
autosomal trisomies.
Nondisjunction
 Some gene function on the condensed X
chromosomes is apparently maintained, resulting in
the abnormalities associated with extra X
chromosomes.
Nondisjunction
Chromosomal Abnormalities
 Some abnormalities result from altered
chromosome structure, e.g.




Deletion: a piece missing
Inversion: a section reversed
Duplication: a piece attaches to a pairing partner
Translocation: a piece attaches to another, unrelated
chromosome.
Chromosomal Abnormalities
 Some abnormalities result from altered chromosome
structure, e.g.
 Deletion: a piece missing
Chromosomal Abnormalities
 Inversion: a section reversed
Chromosomal Abnormalities
 Duplication: a piece attaches to a pairing partner
Chromosomal Abnormalities
 Translocation: a piece attaches to another,
unrelated chromosome.