Question

according to the dihybrid diagram above, which of the follow about the offspring?
a there is an even number of yellow peas and green pea offspring
b all the offspring have only recessive traits
c there are only two genotypes that are homozygous rryy and rryy
d all the offspring have only dominant traits

Explanation:

To solve this, we recall dihybrid cross (Mendel's experiment with pea plants, seed color: yellow (dominant, Y) and green (recessive, y)). In a typical dihybrid cross (e.g., RrYy × RrYy for F₂), the phenotypic ratio for seed color (yellow:green) is 3:1 (yellow is dominant). Let's analyze each option:

• Option A: In a dihybrid cross for seed color (Y/y), the number of yellow (Y_) and green (yy) offspring. From the 9:3:3:1 ratio (or focusing on Y/y: 3/4 yellow, 1/4 green). If total offspring count follows Mendelian ratios, the number of yellow (3 parts) and green (1 part) – but "even number" isn't guaranteed by the ratio. Wait, maybe the original diagram (though not fully visible) – but let's check other options.

• Option B: "All offspring have only recessive traits" – impossible, since dominant alleles (Y, R) are present in parents (e.g., RrYy), so offspring can have dominant traits. Eliminate B.

• Option C: "Only two genotypes that are homozygous (RRYY and rryy)" – No, homozygous genotypes for both traits: RRYY, RRyy, rrYY, rryy. So more than two homozygous genotypes. Eliminate C.

• Option D: "All offspring have only dominant traits" – No, recessive traits (like green seeds, yy, or wrinkled, rr) will appear (e.g., rryy has recessive traits). Eliminate D.

Wait, maybe I misread Option A. Let's re-express: In a dihybrid cross for seed color (Y/y), the ratio of yellow (Y_) to green (yy) is 3:1. If the total number of offspring is a multiple of 4 (common in Mendelian ratios, e.g., 16 in F₂), then yellow (12) and green (4) – both even. Or if total is 4, yellow 3 (odd)? No, wait, maybe the diagram shows a specific count. But among the options, A is the only plausible one. Let's confirm:

• B: Wrong (dominant traits exist).

• C: Wrong (homozygous genotypes: RRYY, RRyy, rrYY, rryy – four, not two).

• D: Wrong (recessive traits like yy or rr will be present).

Thus, A is correct.

Answer:

A. There is an even number of yellow peas and green pea offspring