Mendel crossed peas having green seeds with peas having yellow seeds. The F₁ generation produced only yellow seeds. In the F₂, the progeny consisted of 6022 plants with yellow seeds and 2001 plants with green seeds. Of the F₂ yellow-seeded plants, 519 were self-fertilized with the following results: 166 bred true for yellow and 353 produced an F₃ ratio of 3/4 yellow: 1/4 green. Explain these results by diagramming the crosses.

Verified step by step guidance

1. Step 1: Identify the traits and their dominance. From the problem, yellow seeds are dominant over green seeds. Represent the dominant allele for yellow seeds as 'Y' and the recessive allele for green seeds as 'y'.
2. Step 2: Analyze the F₁ generation. Mendel crossed green-seeded plants (yy) with yellow-seeded plants (YY). The F₁ generation would all be heterozygous (Yy), as they inherit one dominant allele (Y) from the yellow-seeded parent and one recessive allele (y) from the green-seeded parent. This explains why all F₁ plants have yellow seeds.
3. Step 3: Analyze the F₂ generation. When the F₁ plants (Yy) are self-fertilized, the expected genotypic ratio in the F₂ generation is 1:2:1 (YY:Yy:yy), and the phenotypic ratio is 3:1 (yellow:green). This matches the observed F₂ results of 6022 yellow-seeded plants and 2001 green-seeded plants. Use a Punnett square to confirm this ratio.
4. Step 4: Examine the self-fertilization of F₂ yellow-seeded plants. Of the 519 yellow-seeded plants self-fertilized, 166 bred true for yellow seeds, meaning they were homozygous dominant (YY). The remaining 353 produced an F₃ ratio of 3/4 yellow: 1/4 green, indicating they were heterozygous (Yy). This aligns with the expected genotypic ratio of 1:2 for YY:Yy among yellow-seeded F₂ plants.
5. Step 5: Diagram the crosses. Create Punnett squares for each generation: (1) the P generation cross (YY x yy) to show the F₁ generation, (2) the F₁ self-fertilization (Yy x Yy) to show the F₂ generation, and (3) the self-fertilization of F₂ yellow-seeded plants (YY and Yy) to show the F₃ generation. Label the genotypes and phenotypes clearly in each diagram.

Key Concepts

Mendelian Inheritance

Mendelian inheritance refers to the principles of heredity established by Gregor Mendel through his experiments with pea plants. He discovered that traits are inherited in discrete units, now known as genes, and that these traits can be dominant or recessive. In this case, yellow seeds are dominant over green seeds, which explains why the F₁ generation only produced yellow seeds.

Genotypic and Phenotypic Ratios

Genotypic ratios represent the genetic makeup of offspring, while phenotypic ratios represent the observable traits. In the F₂ generation, the observed ratio of yellow to green seeds (6022:2001) approximates a 3:1 ratio, consistent with Mendel's law of segregation. The self-fertilization of yellow-seeded plants further illustrates this, as the 3/4 yellow to 1/4 green ratio in the F₃ generation indicates the presence of both homozygous and heterozygous yellow-seeded plants.

Punnett Square

A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross. It visually represents the combination of alleles from the parents, allowing for the calculation of expected ratios of traits. In this scenario, constructing a Punnett square for the F₁ and F₂ generations would clarify how the dominant yellow seed trait is expressed and how the recessive green seed trait re-emerges in the F₂ generation.