You are performing crosses between two mice that are both heterozygous (D1D2) fo

Question

You are performing crosses between two mice that are both heterozygous (D1D2) for a mutation in the centromere region. Assume that the D1 allele is found in 90% of eggs produced by heterozygous females, but that sperm produced by heterozygous males exhibit the normal 1:1 ratio of D1 to D2 genotypes.

a. Which of Mendel’s rules is violated by the pattern of transmission exhibited by heterozygous females? Briefly explain why.

b. What genotypic segregation ratio will this monohybrid cross produce? Show your calculations but no written explanation is necessary. c. Explain why it is biologically reasonable to imagine that a mutation in the centromere would alter the genotype ratios in oogenesis but not in spermatogenesis.

Explanation / Answer

a. Law of segregation is violated, which states that alleles separate independently at the allelic pair. But in heterozygous females, the alleles have separated in a different manner.

b. D12DD2 female crossed with D1D2 male.

Female gametes are D1 and D2 in ratio 9:1

Male gametes are D1 and D2 in ratio 1:1

Genotypes produced will be D1/D1:D1/D2:D2/D2 = 9:10:1

c. Males are hemizygous. So, crossing over does not occur in X and Y chromosome. If a mutation is present on centromere of X chromosome, it will be passed on to the offspring as it is; because there is no homologous chromosome to pair with.

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