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Modern Genetics
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1 2 3 4 5 6
Morgan and Drosophila
Sex determination and chromosomes
Sex linked traits
Sex linked human disorders
Autosomal dominant human disorders
Autosomal recessive human disorders
T.H. Morgan and Drosophila
 Fruit
flies have
only 4
chromosomes
 Easy to store
 14 day life cycle
Sex Determination and Chromosomes
Each human cell contains 46
chromosomes:
22 pairs of autosomes
1 pair of sex chromosomes
Sex chromosomes are designated as:
“X” and “Y”
XX= female
XY= male
The sex of a human is genetically
determined at fertilization when a
sperm cell containing either the X or Y
chromosome unites with an egg cell
containing the X chromosome.
Sex Determination
Female and Male Sex Chromosomes
X
X
X
Y
C
C
c
c
C
H
h
H
s
S
s
Who gives the sons the eye color gene? Mom or Dad?
Sex-Linked Traits
 Morgans
work with Drosophila
demonstrated that genes for
certain traits are located on the X
chromosome and do not appear
on the Y chromosome.
 Genes found on the X
chromosome are said to be sexlinked genes
Recessive traits that are
sex-linked occur more
frequently in males
than in females.
For the recessive trait to
show in a female, she
must be homozygous
recessive.
The gene must be
present on both of her
X chromosomes( X X).
Human Disorders associated
with Sex-Linked Genes
Hemophilia- disease in which the blood does
not clot properly.
2. Colorblindness- inability to see certain
colors, most commonly red and green.
* Both of these disorders are more common in
males than in females because a female will
not show the disorder as long as she has one
normal gene. Females who are
heterozygous for a sex-linked trait are said
to be carriers for that trait.
1.
Color blind punnett square
C = normal color vision
c = color blind
Answer these questions



Which child is colorblind? Explain in
terms of the alleles he or she inherits.
Which child is a carrier? Explain in terms
of the alleles he or she inherits.
If the father had been red-green
colorblind and the mother was still a
carrier, which children would be
colorblind? Which children would be
carriers? Can a son be a carrier? Create
a Punnett square to find out.
hemophilia
H = normal
h = hemophilia
What do you think?
 Can
a father with hemophilia
pass the disease on to his son if
the mother is homozygous for
the normal allele?
 What will the genotype of the
daughters be?
 What percentage of the
daughters will be carriers of the
disease?

Autosomal Dominant Disorders
Normally two working copies of every
gene in each individual.
 In dominant genetic disease, one copy
of the gene is altered by mutation and
causes the disease
 achondroplasia (a form of dwarfism),
neurofibromatosis, and Huntington
disease.

Try some examples
(3) Joan and John both have achondroplasia.
They are trying to decide whether to have
children. What is the probability that any fetus
they produce will be homozygous dominant for
achondroplasia and suffer skeletal
abnormalities causing a spontaneous abortion?
 25%
a
A

A
a
AA
Aa
Aa
aa
(4) Zach's mother has achondroplasia but Zach
does not. What is the probability of Zach passing
achondroplasia to his children?
 0%
 (5) Margaret, who is normally statured is
married to a man who has achondroplasia. What
is the probability that their first child will have
achondroplasia?
 50%

Autosomal Dominant
parent who shows the trait will pass
the mutation on to half of his/her
children with an equal chance for
sons and daughters to be affected.
 Children who do not have the trait
will generally not pass the disease on
to their children.
 observed in each generation, usually
without skipping a generation.

Autosomal Recessive Disorders
both parents must be carriers (i.e., they are
clinically normal but have one mutation of a
particular gene)
 both must pass the mutation to a child in order
for that child to be affected.
 This inheritance pattern is distinctive in that the
parents and other relatives of the person with
the disease appear to be completely normal,
while 25% of their brothers and sisters will
share the same disease

Autosomal Recessive
mutation travels unobserved (silently)
within the family and is expressed by
siblings in a single generation
 sickle cell anemia, cystic fibrosis,
Tay-Sachs disease, and
phenylketonuria.

Gene Linkage
 Every
organism has thousands of genes
 Genes that are on the same chromosomes
make up a linkage group
 Humans have 23 linkage groups
 Linked genes do not separate
independently
 Linked genes will not follow the dihybrid
ratio of 9:3:3:1
 What’s going on here?
Crossing Over (not this kind )
 Exchange
of pieces of
homologous chromosomes
 Occurs during synapsis of the
first meiotic division when
chromotids are in close contact
 Distant genes are separated
more often than closely linked
 Creates new gene linkages
 Thus new variations (think
genetic diversity and evolution!)
bbcc x BBCC becomes bbcC x BBcC
Multiple Gene Inheritance (Polygenic)
When 2 or more independent genes affect
one characteristic
 Example:

– If the length of an ear of corn was controlled
by two genes Aa and Bb with A and B being
dominant for long, the dihybrid cross would
yield a range of phenotypes between pure
dominant and pure recessive.
– The ear with the most dominant letters is the
longest (AABB)
– What would the genotype be of the shortest
ear?