T4 assignment with answers S - Biology 2C03 Genetics...

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Biology 2C03: Genetics Tutorial 4 Assignment 1. Two gene loci, A and B , assort independently, and alleles A and B are dominant over alleles a and b . Indicate the probabilities of producing the following (1 mark): a. An AB gamete from an AaBb individual? a. 1/4 b. An AB gamete from an AABb individual? b. 1/2 c. An AABB zygote from a cross AaBb × AaBb ? c. 1/16 d. An AaBb zygote from a cross AaBb × AABB ? d. 1/4 e. An Aabb zygote from a cross AaBb × AAbb ? e. 1/4 f. An AB phenotype from a cross AaBb × AaBb ? f. 9/16 g. An AB phenotype from a cross aabb × AABB ? g. 1 (100%) h. An aB phenotype from a cross AaBb × AaBB ? h. 1/4 2. A geneticist is examining a culture of fruit flies and discovers a single female with strange spots on her legs. The new mutation is named melanotic . When a female melanotic fly is crossed with a normal male, the following progeny are produced: 123 normal females, 125 melanotic females, and 124 normal males. In subsequent crosses with normal males, melanotic females are frequently obtained, but never any melanotic males. Provide a possible explanation for the inheritance of the melanotic mutation. (Hint: The cross produces twice as many female progeny as male progeny.)(2 marks) These observations can be explained by a single X-linked locus with two segregating alleles. The skewed sex ratio (2 female:1 male) in the F 2 suggests a recessive lethal allele on the X chromosome that kills males that carry the lethal allele in one copy on their one X chromosome. The phenotype of the female parent also suggests that the allele is dominant for the melanotic trait.
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