Mendel crossed peas having round seeds and yellow cotyledons (seed leaves) with peas having wrinkled seeds and green cotyledons. All the F₁ plants had round seeds with yellow cotyledons. Diagram this cross through the F₂ generation, using both the Punnett square and forked-line, or branch diagram, methods.

Verified step by step guidance

1. Identify the traits and their dominance relationships: Round seeds (R) are dominant over wrinkled seeds (r), and yellow cotyledons (Y) are dominant over green cotyledons (y). The parental cross is between homozygous round yellow (RRYY) and homozygous wrinkled green (rryy) peas.
2. Determine the genotype of the F₁ generation: Since all F₁ plants have round seeds and yellow cotyledons, their genotype must be heterozygous for both traits (RrYy).
3. Set up the Punnett square for the F₂ generation by crossing two F₁ individuals (RrYy × RrYy). Create a 4x4 grid with the possible gametes from each parent (RY, Ry, rY, ry) along the top and side, then fill in the genotypes for each box by combining the alleles.
4. Analyze the Punnett square to determine the phenotypic ratios of the F₂ generation by grouping genotypes into phenotypes: round yellow, round green, wrinkled yellow, and wrinkled green, based on dominant and recessive allele combinations.
5. Construct the forked-line (branch) diagram by first determining the probability of each seed shape genotype (RR, Rr, rr) from the cross Rr × Rr, then branching each of these with the probabilities of cotyledon color genotypes (YY, Yy, yy) from Yy × Yy, and multiply the probabilities along the branches to find the overall phenotypic ratios.

Key Concepts

Mendelian Inheritance and Dominance

Mendelian inheritance describes how traits are passed from parents to offspring through dominant and recessive alleles. In this cross, round seed shape and yellow cotyledons are dominant traits, while wrinkled seeds and green cotyledons are recessive. Understanding dominance helps predict phenotypes in F₁ and F₂ generations.

Punnett Square Method

The Punnett square is a grid used to predict the genotypic and phenotypic ratios of offspring from a genetic cross. It organizes all possible allele combinations from the parents, allowing visualization of how traits segregate and combine in the F₂ generation.

Forked-Line (Branch) Diagram

The forked-line method breaks down multi-trait crosses by calculating probabilities for each trait independently, then multiplying them to find combined genotype or phenotype ratios. This method simplifies complex dihybrid crosses by showing stepwise segregation of alleles.