Question

Students taking a genetics exam were expected to answer the following question by converting data to a “meaningful ratio” and then solving the problem. The instructor assumed that the final ratio would reflect two gene pairs, and most correct answers did. Here is the exam question:
<IMAGE>
Convert the data to a meaningful ratio that allows you to explain the inheritance of color. Determine the number of genes involved and the genotypes that yield each phenotype.”
b. A number of students failed to reduce the ratio for two gene pairs as described above and solved the problem using three gene pairs. When examined carefully, their solution was deemed a valid response by the instructor. Solve the problem using three gene pairs.

Accepted Answer

Step 1: Begin by examining the original data provided (such as counts of different phenotypes) and express these counts as a ratio. This ratio should reflect the relative frequencies of each phenotype observed in the offspring. Step 2: For the three-gene pair approach, consider that each gene pair segregates independently according to Mendel's laws. Since each gene pair has two alleles, the total number of genotype combinations is $2^3 = 8$. List all possible genotype combinations and their expected phenotypic outcomes based on dominant and recessive allele interactions. Step 3: Assign phenotypes to each genotype combination by determining how the alleles at each of the three loci contribute to the color trait. This may involve identifying which alleles are dominant or recessive and how they interact (e.g., additive effects, epistasis). Step 4: Calculate the expected phenotypic ratios by multiplying the probabilities of each genotype combination. For example, if each gene pair segregates 1:1 for dominant to recessive alleles, the expected frequency of each genotype is the product of individual gene probabilities. Step 5: Compare the calculated phenotypic ratios from the three-gene model to the observed data. Use this comparison to confirm the number of genes involved and to assign genotypes to each phenotype, explaining how the three gene pairs together determine the color trait.

Key Concepts

Mendelian Inheritance and Gene Pairs

Phenotypic Ratios and Data Reduction

Polygenic Inheritance and Multiple Gene Interactions