Download Sex-Linked Inheritance

1
Two copies of each autosomal gene
affect phenotype
• Mendel studied autosomal gene
traits (these genes are on
autosomes), like hair texture.
• The “either-or” traits that were
seen in your tutorial.
• Genes found on autosomes (1-22
for humans) = autosomal traits
• Genes found on sex chromosomes
(pair 23 for humans) = sex-linked
traits
2
Sex Determination
• Sex determined by genes
found on the X and Y
chromosome
– Females: egg produced
with an X chromosome
– X chromosome genes in
mammals affect many traits.
• Males: ½ sperm cells
contain X; and the other ½
contain Y
– Chromosome genes in
mammals are responsible for
male characteristics.
There is a 50/50 chance of
child being a boy/girl
3
Males & Females have Different
Inheritance Patterns
– ALL of a male’s sex-linked
genes are expressed.
– Males have no second
copies of sex-linked
genes.
– Because of this, males
can NEVER be “carriers”
of sex-linked traits or
disorders.
– Ex: Colorblindness,
Hemophilia, Muscular
dystrophy
4
Sex-Linked Disorders
• Defined: Inherited conditions found on X
chromosome
• Usually recessive
• Females: XX chromosomes
XH XH = healthy
XH Xh = healthy carrier
Xh Xh = disease
• Males: XY chromosomes
XHY = healthy
XhY = disease
• Rare in women (back-up X chromosome)
• Ex: Colorblindness, Hemophilia, Muscular
dystrophy
Sample Colorblind Tests
4 Sex-Linked Traits:
1. Normal Color Vision:
A: 29, B: 45, C: --, D: 26
2. Red-Green Color-Blind:
A: 70, B: --, C: 5, D: -3. Red Color-blind:
A: 70, B: --, C: 5, D: 6
4. Green Color-Blind:
A: 70, B: --, C: 5, D: 2
6
Let’s take a look at…
7
Jen is a healthy carrier (XH Xh) of hemophilia and
Adam has no history in his family (XH Y).
What is the probability of
getting:
Healthy girl
Healthy girl
(carrier)
Healthy boy
Disease boy
Daughter with hemophilia?
0%
Carrier child?
25%
Child with hemophilia?
25%
Two healthy children?
¾ x ¾ = 9/16 (56%)
Kelly is a healthy carrier of hemophilia, but Jim suffers
from the disease.
What is the probability of
getting a:
Healthy girl
(carrier)
Disease girl
Healthy boy
Disease boy
Healthy son?
25%
Healthy daughter?
25%
Child with hemophilia?
50%
Carrier child?
25%
• If the phenotype is more common in males, the
gene is likely sex-linked.
Let’s try a sex-linked problem:
A man and a woman marry.
XD Xd
The woman is a carrier of
colorblindness, an X-linked disorder.
XD Y
They have 4 children, 3 boys and one
girl.
Xd Y
Two of the boys are colorblind, and
the girl is a carrier.
b)What is the probability they will
have a child that is colorblind?
25%
XD Xd
XD Y
XD Y
?
XD
Xd
XD
XD XD
XD Xd
Y
XD Y
Xd Y
The carrier daughter marries a
healthy man.
a)What is the probability they will
have a carrier? 25%
Xd Y
11
REVIEW
1) How are sex-linked disorders different from
autosome disorders?
2) Why are sex-linked disorders more common in
males?
3) Write the genotype of a heterozygous dominant
female.
4) Write the genotype of a carrier female.
5) Examine Kelly and Jim’s Punnett square. What are
the chances they would have three children, all of
whom are healthy?