Suppose an F₁ dihybrid (round yellow plant from Problem 16) is crossed to the pure-breeding green, round parental strain. Use a forked-line diagram to predict the phenotypic distribution of the resulting progeny.

Verified step by step guidance

1. Identify the genotypes and phenotypes of the F₁ plant and the pure-breeding green, round parental strain. Typically, the F₁ plant is heterozygous for the traits, while the pure-breeding parent is homozygous recessive for both traits (green color and round shape).
2. Determine the alleles contributed by each parent. The F₁ plant will produce gametes with different allele combinations depending on whether the genes assort independently or are linked, while the pure-breeding parent will contribute only recessive alleles for both traits.
3. Set up the forked-line diagram by first branching according to one gene's alleles from the F₁ parent, showing the probability of each allele being passed on (usually 1/2 for each allele if heterozygous).
4. From each branch, further branch according to the second gene's alleles from the F₁ parent, again showing the probability of each allele being passed on. Multiply the probabilities along the branches to find the combined probability of each genotype.
5. Translate each genotype into its corresponding phenotype (e.g., green or yellow color, round or wrinkled shape) and sum the probabilities of genotypes that produce the same phenotype to predict the phenotypic distribution of the progeny.

Key Concepts

Forked-Line Diagram

A forked-line diagram is a visual tool used to calculate the combined probabilities of independent genetic events. It breaks down complex crosses into sequential branches representing each gene's segregation, allowing easy multiplication of probabilities to predict phenotypic ratios in offspring.

F₁ Generation and Pure-Breeding Strains

The F₁ generation results from crossing two pure-breeding parental strains, producing heterozygous offspring. Pure-breeding strains are homozygous for specific traits, ensuring consistent phenotype expression, which is essential for predicting progeny outcomes in subsequent crosses.

Phenotypic Ratios and Mendelian Inheritance

Phenotypic ratios describe the proportion of different observable traits in offspring, determined by Mendelian inheritance patterns. Understanding dominant and recessive alleles and their segregation during gamete formation helps predict these ratios in genetic crosses.