1. Sex-Linked Traits

2. The Sex Chromosomes: Humans have 46 chromosomes in every cell.
2 of these chromosomes make us male or female.
Female: XX Male: XY

3. The Y chromosome
Has information for male traits.
Few other genes.

4. The X chromosome
Contains many genes, most of which are not on the Y chromosome.
This means men only get 1 copy of the gene.
If they get a recessive allele, they show it.
Women need 2 recessive alleles to show disease
Less likely!
So, men get “sex-linked” diseases more often.

5. Sex-Linked Disorders
Colourblindness: Can’t see the difference between certain colours (ex. red and green).
Hemophilia: “Bleeders” – don’t produce scabs, so continue to bleed from cuts.
Muscular dystrophy: Muscles lose their ability to repair themselves.

6. Punnett Squares for Sex-Linked Traits
1. Write a let statement!
Let XN = no disorder Xn = disorder Y = male
2. Determine the parent’s genotypes (letters).
3. Draw Punnett Square.
4. Fill in Punnett Square.
5. Answer question.
Write as an Exponent!
No exponent! EVER!

7. Sample Problem Colourblindness is a recessive sex-linked trait.
Ahmed, who has “normal” vision, marries Sue, who is a carrier (heterozygous) for normal vision.
What is the chance of one of their kids being colourblind?
Solution:
Let XN=not colourblind Xn=colourblind Y=male
Ahmed: male not colourblind: XNY
Sue: female, heterozygous: XNXn

8. XNXn x XNY XN Xn
All girls have normal vision, but one is a “carrier”.
Half of the boys will be colourblind.
25% of children will be colourblind. XN XN XN Xn XN Y XN Y Xn Y