Transcript Sex-Linked Traits

 Humans have 46 chromosomes in every cell.
 2 of these chromosomes make us male or female.
 Female: XX
Male: XY
 Has information for male traits.
 Few other genes.
 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.
 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.
 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.
 Colourblindness is a recessive sex-linked trait.
 Ahmed, who has “normal” vision, marries Sue, who is a
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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
•All girls have
normal vision, but
one is a “carrier”.
•Half of the boys
will be colourblind.
•25% of children
will be
colourblind.