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Sex-Linkage (X-Linked Traits) Drosophila melanogaster Thomas Hunt Morgan – (the father of modern genetics) He found genes located on the X chromosome that regulate fruit fly eye color. Genes on the X chromosome are said to be Xlinked Thomas Hunt Morgan was studying evolution and was looking for mutations in fruit flies (Drosophila). Morgan and his “F1s” (His two daughters) Morgan noticed that while most fruit flies had red eyes, there were a few whiteeyed fruit flies. He also noticed that ones that had white eyes were all males. Morgan’s Experiment: 1) Morgan crossed a true-breeding, red-eyed female with a white-eyed male, he got all redeyed flies in the F1 generation. 2) When he crossed two F1 red-eyed flies, he got three red-eyed flies to every one white-eyed fly. ALSO-the white-eyed flies were male. As stated by Morgan the gene for eye color is located on the X chromosome. R = red color r = …white complelely color dominant over the… Sex-linked genes • Use XR for dominant allele and Xr for recessive. Y stands for Y chromosome • Let’s look at some genotypes and phenotypes: • XRXR=red-eyed female • XRXr=red-eyed female • XrXr=white-eyed female • XRY=red-eyed male • XrY=white-eyed male. Sex-linked genes • Red female with white male: all red • Heterozygous female with red male: 1:1 red:white males Sex-linked genes • Heterozygous female with white male: all females red, all males white. Male fruit flies have white eyes more often than females • Because females get a second chance at a dominant gene, they’re usually red-eyed. • Males only get one chance at a dominant. • Males are more likely to express recessive traits if the gene is on the X chromosome. (hemophilia, color-blindness, muscular dystrophy.) Other interesting Drosophila variations: Other Sex-linked genes • Human examples of sex-linked traits – Red-green colorblindness Other Sex-linked Traits • Hemophilia is another example of a sexlinked recessive trait. • The different possible alleles are XH, Xh, and Y • Absence of the dominant allele is necessary for trait expression – XH XH is a normal woman – XH Y is a normal man – XH Xh is a woman who is a carrier – Xh Y is an affected man – Xh Xh is an affected woman – Males cannot be carriers A Sample Cross for Hemophilia XH Xh XH XH XH X H Xh Y XH Y Xh Y Normal male Carrier female Sex-linked genes • Hemophilia and history: Czar Nicholas II. X-Linked Dominant Traits • The presence of the dominant allele causes the expression of the trait • A female who inherits the dominant allele will have the condition, but a male will have a more severe form because he has no other X chromosome to offset it • An affected male who has children with an unaffected female cannot pass the trait to his sons (males get their X from mom) X-Linked Dominance: CGH • Congenital Generalized Hypertrichosis • Extremely dense hair growth • “Werewolf” syndrome X-Linked Dominance Xc Xc XC XC Xc X C Xc Y Xc Y Xc Y Affected Male Unaffected Female Only the daughters have the trait