1
In 1902, Walter Sutton
and Theodor Boveri
developed the
Chromosome Theory of
Inheritance.
This theory basically
states that Mendelian
genes have a specific
locus on the chromosome
and that chromosomes
undergo segregation and
independent assortment
during meiosis.
Theodor Boveri
Walter Sutton
This was a link between
mitosis/ meiosis and
Mendel’s findings.
2
Thomas Hunt Morgan was the first person to
associate a specific gene with a specific
chromosome. He worked with the common
fruit fly (Drosophila melanogaster) and, after
many generations, found a male fly with white
eyes. From crosses with this organism, he
discovered sex-linkage.
3
Morgan choose these organisms to do
experiments on for several reasons:
- They are hardy
- Sex easily distinguishable
- Only 4 pairs of chromosomes
- Produce large numbers of offspring
- Can control matings
- Traits that are easily observable
- Can reproduce every 2 weeks
Most observed phenotype is called
wild type (ex. Red eyes). Alternate
phenotypes are called mutant
phenotypes (ex. White eyes)
4
There are a few different systems of sex
determination. They are:
- XY
- XO
- ZW
- Haplo-diploid
5
The XY sex determination
system occurs in mammals
(including humans!); The sex is
determined by the sperm;
Females are XX and males are
XY
6
The XO sex determination system occurs in
grasshoppers, crickets, and most insects;
There is only 1 type of sex chromosome;
XX is female, and XO (only ONE X
chromosome) is male; Sperm cells carry
either an X or no sex chromosome at all.
7
The ZW system of sex determination
occurs in birds, some fish, and some
insects; The egg determines the sex;
ZZ is male and ZW is female…all
sperm carry a Z and eggs have
either a Z or a W
8
The haplo-diploid system of sex determination occurs
in bees and ants. They do not have any sex
chromosomes. Females develop from fertilized eggs
(they are diploid), and males develop from
unfertilized eggs (haploid)
9
The one gene that is on the Y chromosome and is very important is the SRY gene (Sexdetermining Region of the Y). This makes males phenotypically male and instructs the
gonads to develop into testes instead of ovaries. The SRY gene codes for a protein that
regulates many other genes, triggering a cascade of biochemical, physiological, and
anatomical features.
There is very LITTLE crossing over between the X and Y chromosomes because they are
not homologous.
10
Sex linked genes generally mean genes found on the X
chromosome. The human X chromosome contains about 1,100
genes. Because males and females inherit a different number of
X chromosomes, the pattern of inheritance of X-linked genes
differs from that of genes located on autosomes. Fathers can
pass their genes onto all of their daughters and none of their
sons (the sons get their Y chromosomes). Mothers can pass
their traits onto both male and female children. In order for a
son to get a recessive disorder, their mom must either be
afflicted or a carrier of the gene.
If an X-linked
trait is due to
a recessive
allele, a
female will
express the
phenotype
only if she is
homozygous
for that allele.
Heterozygous
females are
carriers for
the recessive
trait. Males
would have
the disorder if
they only
have one
copy because
they only
have 1 X
chromosome.
11
This is a disorder with seeing
certain colors. There are
various tests that can
determine if someone is
colorblind. There are different
types of colorblindness. Red/
green colorblindness is one of
the most common types.
12
Muscular Dystrophy is a sex-linked recessive
disorder characterized by progressive skeletal
muscle weakness, defects in muscle proteins
(specifically dystrophin), and the death of muscle
cells and tissue. Duchenne MD usually affects
only males (females would need to have a father
that also had the disease, but they usually do not
live to reproduce) and is the most common of all
the muscular dystrophies.
Cross section of muscle shows extensive
replacement of muscle fibers by adipose cells
13
Hemophilia is a rare
inherited disorder
where the blood does
not clot correctly.
Afflicted people have
little to none of a
protein that is necessary
for blood clotting. They
can bleed internally
(especially in the knees,
elbows and ankles, and
this can be fatal.
In the 1800s, hemophilia was
widespread among the royal
families of Europe. Queen Victoria
of England passed the allele to
several of her descendants.
Intermarriage with royal family
members of other nations, such as
Spain and Russia, further spread
this X-linked trait.
14
In FEMALES, one X chromosome becomes almost
completely inactivated in development and turns into
a Barr Body. Most of the genes in the Barr Body are
NOT expressed, although some are. Which X
chromosome becomes inactivated is random, thus
females become a mosaic of 2 types of cells. The X
chromosome becomes inactivated by being
methylated.
15
Linked genes are
genes that are
located on the
same chromosome.
They are usually
inherited together.
If they are NOT
inherited together,
it is due to crossing
over. The results
of crosses with
linked genes do
not follow normal
Mendelian
inheritance and are
different from
what would be
expected according
to the law of
independent
assortment.
Evidence:
Testcross
16
A recombinant is an
offspring that has a
different combination
of traits than either of
the parents.
The recombination
frequencies can be
used to make genetic
linkage maps.
17
 Most
of the offspring from
the Drosophila testcross in
Morgan’s experiment for
body color and wing size
had parental phenotypes.
 That suggested that the two
genes (body color and wing
size) were on the SAME
chromosome, since the
occurrence of parental
types with a frequency
greater than 50% indicates
that the genes are linked.
 About 17% of offspring,
however, were
recombinants.
 Morgan proposed that
some mechanism must
occasionally break the
physical connection
between genes on the same
chromosome.
 This process, called
crossing over, accounts for
the recombination of linked
genes.
Crossing over produces
genetic recombination of
linked genes
18
Sturtevant
Sturtevant was a grad student that worked with Morgan
on the flies. He discovered the first method of gene
mapping using the recombination frequencies. The
frequencies of recombination are used to determine the
distance between genes on a chromosomes. Sturtevant
predicted that the farther apart two genes are, the higher
the probability that a crossover will occur between them
and, therefore, the higher the recombination frequency.
19
Genetic Map – ordered list of genes
on a chromosome
Linkage Map – genetic map based
on recombination frequencies
1% recombination
frequency = 1 map
unit
Max value = 50%
If genes are greater
than 50 map units
apart, they behave as
if they are on separate
chromosomes
(crossing over is
almost certain to
20
occur)
Nondisjunction –
when chromosomes
do not separate
correctly during
meiosis
Aneuploidy – the
term for an incorrect
number of
chromosomes;
monosomy, trisomy,
and polyploidy are
all examples of
aneuploidy
21
Trisomy – having an extra copy of one
chromosome; instead of having 2 copies you
have 3; written as 2n + 1
Monosomy – having only one copy of a certain
chromosome, instead of the normal 2 copies;
written as 2n - 1
Trisomy 18
Monosomy 23
Polyploidy – having a whole extra set of
chromosomes; Ex. 3N or 4N; common in plants; lethal
in MOST animals although there are fish and
amphibian polyploid species; polyploids are more
nearly normal in phenotype than aneuploids
22
23
A deletion is when a piece of a chromosome is lost.
This can be very bad because if that missing piece
contains important genes, the phenotype of the
afflicted person will be dramatically changed.
24
A duplication is when a section of a
chromosome (a couple genes) is
copied and included twice. This can
greatly affect the individual.
However, its usually not as bad as
missing the genes all together
(deletion).
25
An inversion occurs when a
piece of a chromosome breaks off
and reattaches in the wrong
orientation. Because some genes
are regulated by other genes near
them, this can affect gene
expression.
26
A translocation is when a piece
of two chromosomes break off
and attach to non-homologous
chromosomes. It yields
“recombinant” chromosomes.
Duplications and
translocations are typically
harmful. Reciprocal
translocations or inversions
can alter phenotype because a
gene’s expression is
influenced by its location
among neighboring genes.
27
Although the frequency of aneuploid
zygotes may be quite high in humans,
most of these alterations are so
disastrous to development that affected
embryos are spontaneously aborted long
before birth (miscarriage). There are
several human disorders that are due to
some type of chromosomal alteration 
trisomy, monosomy, deletions,
duplications, inversions, and
translocations:
- Down Syndrome
- Klinefelters Syndrome
- Turners Syndrome
- Cri du chat
- CML (leukemia)
Males with an extra Y
chromosome (XYY)
tend to be somewhat
taller than average.
Trisomy X (XXX), which
occurs once in every
1,000 births live female
births, produces healthy
females.
A baby with Down Syndrome
28
Trisomy 21 affects one in 700
children born in the United States.
Most cases of Down syndrome result
from nondisjunction during gamete
production in one parent. The
frequency of Down syndrome
increases with the age of the mother.
Trisomy 21 may be linked to some
age-dependent abnormality in a
meiosis I checkpoint that normally
delays anaphase until all the
kinetochores are attached to the
spindle.
Although chromosome 21 is the smallest
human chromosome, trisomy 21 severely
alters an individual’s phenotype in
specific ways. Individuals with Down
syndrome have characteristic facial
features, short stature, correctable heart
defects, and developmental delays.
They are susceptible to respiratory
infection, mental retardation, and have
an increased risk of developing leukemia
and Alzheimer’s disease. All males and
half of females with Down syndrome are
sexually underdeveloped and sterile.
- Trisomy-21
(aneuploid
condition)
29
- XXY; phenotypically male
- Male sex organs (but smaller); sterile
- May have feminine characteristics
- Usually have normal intelligence but
may have slightly subnormal
intelligence
30
- Genotype = XO
- Monosomy 23
- Aneuploid condition
- Phenotypically female
- Sex organs don’t mature
- Sterile
- Intelligence is normal
- Only known case of
monosomy in humans
- When given estrogen
replacement therapy, girls with
Turner syndrome develop
secondary sex characteristics
31
- Deletion in a part of Chromosome 5
- These individuals are mentally retarded,
have small heads with unusual facial
features, and have a cry like the mewing
of a distressed cat
- Death in infancy or early childhood
32
CML is an acquired leukemia, resulting in
a change in a person's DNA (NOT
genetically inherited). This change results
in an uncontrolled growth of white cells.
This uncontrolled growth can bring about
abdominal discomfort due to an enlarged
spleen. Other symptoms may include
excessive sweating (night sweats), weight
loss, and an increased sensitivity to warm
temperatures.
This is due to an injury to the DNA
that results in a shortened
Chromosome 22.
33
Genetic Imprinting is the phenomenon of the
phenotype of the gene depends on whether it
arrived via the egg or sperm.
Ex. Deletion in part of Chromosome 15…if it
came from mom, Angelman; Dad = PraderWilli
34
-Due to imprinting
- Deletion in chromosome 15
- Allele from dad
-Mental retardation
- Obesity
- Short stature
- Unusually small hands and feet
35
- Due to imprinting
- Deletion in chromosome 15
- Allele from mom
- Spontaneous laughter
- Jerky movements, and other motor and
mental symptoms
36
-Due to Imprinting
- Abnormal X chromosome
where the tip hangs on by a
thread
- Mental retardation if inherited
from mom
37


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

Extranuclear or cytoplasmic genes are found in
mitochondria and chloroplasts.
These organelles reproduce themselves and transmit
their genes to daughter organelles.
Organelle genes do not display Mendelian inheritance.
Because a zygote inherits all its mitochondria from the
ovum, all mitochondrial genes in most animals and
plants demonstrate maternal inheritance.
Several rare human disorders are produced by mutations
to mitochondrial DNA.
These disorders primarily affect the ATP supply by
producing defects in the electron transport chain or ATP
synthase.
 Other mitochondrial mutations may contribute to
diabetes, heart disease, and other diseases of aging,
such as Alzheimer’s disease.
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