What would be the genotypes of the female F1 flies? ____phenotype? ______ What would be the genotypes of the male F1 flies? ___phenotype? _______ Do a Punnett square for the F2 results if one of the F1 females mated with one of the F1 males.
ACTIVITY 5: Sex-linked inheritance
In many animals, the sex of an individual is determined by which sex chromosomes (X or Y) are inherited. In mammals (and the fruit fly Drosophila), females have 2 X chromosomes and males have one X and one Y. The X and Y are an unusual pair of homologous chromosomes. The X is large and contains thousands of genes, while the Y is small and contains few genes. Most genes on the X are not found on the Y, so males possess only one copy of each X-linked gene and females have 2 copies. This means that X-linked recessive traits are expressed more often in males.
Eye color is a sex-linked trait in Drosophila. The allele for red eyes is dominant to the allele for white eyes. Because this gene is on the X chromosome, the alleles are written as XR and Xr.
A. Do a Punnett square for a cross between a homozygous red-eyed female (XR XR ) and a white-eyed male (Xr Y).
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What would be the genotypes of the female F1 flies? ____phenotype? ______
What would be the genotypes of the male F1 flies? ___phenotype? _______
Do a Punnett square for the F2 results if one of the F1 females mated with one of the F1 males.
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*To show results of sex-linked traits:
1. Divide
2. List the phenotypes, with the corresponding genotype (in parentheses) next to each phenotype.
Males:
Females:
B. A sex-linked trait in humans: hemophilia = blood clotting disorder.
Normal clotting (XN) is dominant to hemophilia (Xn).
Do a Punnett square for a normal clotting (homozygous dominant) woman and a man with hemophilia.
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*To show results of sex-linked traits:
1. Divide phenotypes into males and females first, keep results as fractions (or %).
2. List the phenotypes, with the corresponding genotype (in parentheses) next to each phenotype.
Males:
Females
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