1. Sex-Linked Inheritance

2. 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

3. There is a 50/50 chance of child being a boy/girl
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.
X X X Y X X X X X Y X Y
There is a 50/50 chance of child being a boy/girl

4. 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

5. 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

6. 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

7. Sex-Linked Punnett Squares
Let’s take a look at…
Sex-Linked
Punnett Squares

8. 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:
Daughter with hemophilia? 0%
Carrier child?
25%
Child with hemophilia?
Two healthy children?
¾ x ¾ = 9/16 (56%)
Healthy girl
Healthy boy
Healthy girl (carrier)
Disease boy

9. 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?
Child with hemophilia?
50%
Carrier child?
Healthy girl (carrier)
Healthy boy
Disease girl
Disease boy

10. If the phenotype is more common in males, the gene is likely sex-linked.

11. 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?
XD Xd
XD Y
Xd Y
Xd Y
XD Y
XD Xd
XD Y ?
XD Xd
XD XD XD Xd XD Y
25%
XD Y Xd Y
25%

12. REVIEW How are sex-linked disorders different from autosome disorders?
Why are sex-linked disorders more common in males?
Write the genotype of a heterozygous dominant female.
Write the genotype of a carrier female.
Examine Kelly and Jim’s Punnett square. What are the chances they would have three children, all of whom are healthy?