• Mendel studied autosomal gene traits• 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

KEY CONCEPT The chromosomes on which genes are located can

affect the expression of traits.

Males and females can differ in sex-linked traits.

– Y chromosome genes in mammals are responsible for male characteristics.

– X chromosome genes in mammals affect many traits.

• Male mammals have an XY genotype.

– 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 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

Let’s take a look at…

Lynda is a healthy carrier of colorblindess and her husband, Jim, has no family

history of colorblindness. Lynda’s genotype:

XXd

Jim’s genotype:XY

What is the probability ofgetting:

• A colorblind daughter?0%

• Carrier child?25%

• A colorblind child?25%

• Two healthy children?¾ x ¾ = 9/16 (56%)

Jen is a healthy carrier of hemophilia and Adam has no history in his family.

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%)

Kelly is a healthy carrier of hemophilia, but Joe suffers from the disease.

What is the probability of getting a:

• Healthy son?25%

• Healthy daughter?25%

• Child with hemophilia?50%

• Carrier child?25%

We use PEDIGREES to trace: • Autosomal traits• Sex-linked traits• Autosomal disorders or

diseases• Sex-linked disorders or

diseases

• 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 carrier daughters?

b)What is the probability they will have sons that are colorblind?

?

a) ½ daughter x ½ carrier = 25%b) ½ son x ½ c.b. = 25%

Fill in the X-linked Pedigree