Sex-Linked Inheritance
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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. 3
There is a 50/50 chance of child being a boy/girl
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
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Sex-Linked Disorders• Defined: Inherited conditions found on X
chromosome• Usually recessive• Females: XX chromosomes
XH XH = healthyXH Xh = healthy carrierXh Xh = disease
• Males: XY chromosomes
XHY = healthyXhY = disease
• Rare in women (back-up X chromosome)• Ex: Colorblindness, Hemophilia, Muscular
dystrophy
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
Sample Colorblind Tests
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Let’s take a look at…
Sex-LinkedPunnett Squares
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Jen is a healthy carrier (XH Xh) of hemophilia and Adam has no history in his family (XH Y).
What is the probability ofgetting:
Daughter with hemophilia?0%
Carrier child?25%
Child with hemophilia?25%
Two healthy children?¾ x ¾ = 9/16 (56%)
Healthy girl Healthy boy
Healthy girl (carrier)
Disease boy
Kelly is a healthy carrier of hemophilia, but Jim suffers from the disease.
What is the probability of getting a:
Healthy son?25%
Healthy daughter?25%
Child with hemophilia?50%
Carrier child?25%
Healthy girl (carrier)
Healthy boy
Disease girl Disease boy
• 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.
The woman is a carrier of colorblindness, an X-linked disorder.
They have 4 children, 3 boys and one girl.
Two of the boys are colorblind, and the girl is a carrier.
The carrier daughter marries a healthy man.
a)What is the probability they will have a carrier?
b)What is the probability they will have a child that is colorblind?
?
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XD Xd
XD Xd
XD Y
XD Y XD YXd YXd Y
25%
25%
XD Xd
XD
Y
XD XD XD Xd
XD Y Xd Y
REVIEW1) 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?