Meiosis：練習題

In a species of plant, the gene for flower color (R) shows incomplete dominance, where RR is red, Rr is pink, and rr is white. A separate gene for plant height (T) shows complete dominance, where T is tall and t is short. If two plants with the genotype RrTt are crossed, what is the probability of producing an offspring that is both pink and short?

In a dihybrid cross of two pea plants (AaBb x AaBb), where the genes are located on different chromosomes and show complete dominance, what is the probability that an offspring will have a phenotype that matches only one of the two homozygous recessive traits (i.e., either aaB_ or A_bb)?

In a pedigree analysis of a rare human disorder, it is observed that affected fathers never pass the trait to their sons, but all of their daughters become carriers. When a carrier female and an unaffected male have children, 50% of their sons are affected. Which mode of inheritance does this pattern most likely represent?

In Drosophila, the genes for body color (B = gray, b = black) and wing shape (V = normal, v = vestigial) are located on the same chromosome. A cross between a heterozygous gray, normal-winged fly (BbVv) and a black, vestigial-winged fly (bbvv) produced the following offspring:
Gray, Normal: 415
Black, Vestigial: 385
Gray, Vestigial: 105
Black, Normal: 95
Calculate the recombination frequency between these two genes.

In a specific breed of domestic cats, a gene located on the X chromosome controls coat color: the allele for black fur (\(X^B\)) and the allele for orange fur (\(X^O\)) are codominant. Heterozygous females (\(X^B X^O\)) exhibit a tortoiseshell pattern (patches of black and orange). Additionally, an autosomal gene for spotting (S) is dominant over the non-spotting allele (s), where (S) results in white patches on the fur.

(a) A tortoiseshell female with no white spotting is crossed with an orange male that is heterozygous for the spotting gene. Determine the genotypes of the parents.
(b) Calculate the probability that their offspring will be a calico female (a female with black, orange, and white patches). Show your calculation steps.
(c) Explain the phenomenon of X-inactivation (Lyonization) and how it specifically results in the tortoiseshell phenotype in females rather than a uniform blend of colors.
(d) If a male cat is found to have a tortoiseshell phenotype, propose a chromosomal abnormality that could explain this observation.

In a species of flowering plants, the gene for flower color has two alleles: red (R) and white (r), exhibiting incomplete dominance, where the heterozygote (Rr) is pink. A second gene for height has two alleles: tall (T) and dwarf (t), where tall is completely dominant. These two genes are located on the same chromosome and are 20 map units apart.

A plant with pink flowers and a tall phenotype (genotype RrTt) is test-crossed with a plant that has white flowers and is dwarf.

(a) Determine the expected phenotypes and their corresponding ratios among the offspring, assuming the RrTt parent has the alleles RT on one chromosome and rt on the homologous chromosome.
(b) Explain how the distance between the two genes affects the frequency of recombinant phenotypes observed in the offspring.
(c) Calculate the probability of obtaining an offspring that is both pink-flowered and dwarf.