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Name___________ Sex-linked Traits Sex-linked traits are traits whose genes are found on the sex chromosomes (X or Y). Most sexlinked traits are found on the X chromosome; in fact, since they’re only found on the X chromosome we can guess that they are found on the fourth “leg” that the X has instead of 3 legs on the Y. Further, sex-linked traits are usually recessive. Sex-linked traits usually are carried by females and show up in males. Here’s why: If a woman gets a sex-linked recessive trait on one X chromosome, she can still get the dominant trait on her other X chromosome to cover it up. If a man gets a sex-linked recessive trait on one X chromosome, he doesn’t have a second X chromosome in order to cover it up. So let’s look at an example. Baldness is a sex-linked recessive trait, so here are the genotypes and phenotypes. Genotype XBXB XBXb XbXb XBY Xb Y Phenotype Normal Female Normal (carrier) Female Bald Female Normal Male Bald Male So you can see, with just one recessive baldness allele a man will be bald, while it takes two recessive baldness alleles for a woman to be bald. For the problems below, draw and complete the Punnett Square and give the percent possibility for each genotype and phenotype. 1. What are the possible results for a mating between a female carrier for baldness and a normal male? Possible Genotypes XBXB = XBXb = XbXb = XBY = XbY = Possible Phenotypes Normal Female = Normal (carrier) Female = Bald Female = Normal Male = Bald Male = 2. What are the possible results for a mating between a bald female and a normal male? Possible Genotypes XBXB = XBXb = XbXb = XBY = XbY = Possible Phenotypes Normal Female = Normal (carrier) Female = Bald Female = Normal Male = Bald Male = 3. What are the possible results for a mating between a normal woman and a bald man? Possible Genotypes XBXB = XBXb = XbXb = XBY = XbY = Possible Phenotypes Normal Female = Normal (carrier) Female = Bald Female = Normal Male = Bald Male = 4. If you’re a male and your dad is bald, will it have any effect on you? Explain. 5. Color-blindness is another sex-linked recessive trait. Show a Punnett square in order to tell me the parents’ possible genotypes and phenotypes for a color-blind girl. Genotypes XCXC XCXc XcXc XCY XcY Phenotypes Normal Female Normal (carrier) Female Color-blind Female Normal Male Color-blind Male Mother’s possible genotype(s) _________ and phenotype(s) ________________ Father’s possible genotype(s) __________ and phenotype(s) ________________ 6. Show the Punnett square and results for the mating between a carrier woman and a colorblind man. Possible Genotypes XCXC = XCXc = XcXc = XCY = XcY = Possible Phenotypes Normal Female = Normal (carrier) Female = Color-blind Female = Normal Male = Color-blind Male = 7. If a boy is color-blind and his mother and maternal grandmother are not, will his maternal grandfather know if he’s wearing a green or red sweater? Show the Punnett square to support your answer. 8. If a color-blind man and his normal vision wife have a color-blind baby girl, what is the wife’s phenotype and genotype? Show the Punnett square to support your answer.
