In the rats identified in Problem 10, a third independently assorting gene involved in the determination of coat color is the C gene. At this locus, the genotype C– permits expression of pigment from genes B and D. The cc genotype, however, prevents expression of coat color and results in albino rats. For each of the following crosses, determine the expected phenotype ratio of progeny.
BBDdcc×BbddCc

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Identify the genotypes of the parents: The first parent has the genotype BBDdcc, and the second parent has the genotype BbddCc. Note that the B and D genes determine coat color, while the C gene determines whether pigment is expressed at all.

Determine the possible gametes for each parent: For the first parent (BBDdcc), the possible gametes are BDc and BDc (since it is homozygous for B and D, and homozygous recessive for c). For the second parent (BbddCc), the possible gametes are BdC, Bdc, bdC, and bdc (since it is heterozygous for B, D, and C).

Set up a Punnett square to determine the genotypes of the offspring: Combine the gametes from each parent to form all possible combinations of offspring genotypes. For example, one combination would be BDc × BdC, and so on.

Analyze the phenotypes of the offspring: The phenotype depends on the interaction of the B, D, and C genes. If the offspring has at least one dominant C allele (C–), pigment will be expressed. The B and D genes will then determine the coat color. If the offspring is cc, it will be albino regardless of the B and D alleles.

Calculate the expected phenotype ratio: Count the number of offspring with each phenotype (e.g., albino, black coat, brown coat) based on the genotypes from the Punnett square. Use the rules of inheritance to determine the proportion of each phenotype in the progeny.

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Independent Assortment

Independent assortment is a fundamental principle of genetics stating that alleles for different genes segregate independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another, allowing for a variety of combinations in offspring. Understanding this concept is crucial for predicting the phenotypic ratios in genetic crosses.

Genotype and Phenotype

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable physical or biochemical characteristics resulting from the genotype and environmental influences. In the context of the question, the genotypes BBDdcc and BbddCc will produce different phenotypes based on the expression of the C gene, which is essential for determining coat color in rats.

Epistasis

Epistasis occurs when the expression of one gene is affected by one or more other genes, leading to interactions that can mask or modify phenotypic outcomes. In this case, the C gene's influence on coat color illustrates epistasis, as the cc genotype inhibits pigment expression regardless of the presence of other color genes, which is vital for calculating the expected phenotype ratios in the progeny.