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Sex-Linked Genes
Sample Problem!
• Show, using a Punnett Square, that there is a
50% of having a boy or a girl when humans
mate (aka get it on).
Determination of Sex
• Girls: XX
(homologous sex
chroms)
• Boys: XY (nonhomol. sex
chroms)
• Only ends of Y
have regions
homologous to X
X
Y
Sex Linked Genes
• Found on sex chroms, mainly X (contains more DNA).
• Easy to spot b/c many males will have trait  XY..if trait
is on X, no competition on Y (less DNA)
Hemophilia-A Sex Linked Gene
Colorblindness- A Sex-Linked Gene
Colorblindness- A Sex-Linked Gene
Colorblindness- A Sex-Linked Gene
Colorblindness- A Sex-Linked Gene
Fruit Fly Genetics!
• Good for genetic studies (Drosophila
melanogaster:
 breed at a high rate
 A generation can be bred every 2 weeks
 Only have 4 pairs of chromosomes
• Wild type = normal phenotype
• Traits alternative to wild type: mutants
Some Traits are Sex Limited and
Expression Depends on Sex of Individual
Let’s do another Sample Problem!
H
Sex Linked Fruit Fly Sample Probs,
Video Clip, and Lab