Sex-Linked Traits

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

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

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.

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.

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!

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

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