In tomato plants, the allele for red fruit (R) is dominant to the allele for yellow fruit (r). Two plants, both heterozygous for fruit colour, are crossed. What is the expected phenotypic ratio of red to yellow fruits in the offspring?

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We start with the given genetic scenario: red fruit (R) is dominant over yellow (r), and both parents are heterozygous (Rr x Rr).

Option 1: 3:1. This matches the classic Mendelian monohybrid cross results for a single dominant-recessive trait, yielding three red offspring for every one yellow.

Option 2: 9:3:3:1. This is the typical phenotypic ratio for a dihybrid cross involving two different traits (the product of two heterozygous loci). Since this problem involves a single gene controlling fruit color, this four-part ratio does not apply here.

Option 3: 1:1. A 1:1 ratio would occur if one parent were heterozygous and the other homozygous recessive (Rr x rr), or in other specific allele configurations, but not for the cross Rr x Rr.

Option 4: 1:3. This would imply the recessive phenotype is more frequent than the dominant one, which contradicts the dominance of R over r in this cross. With a dominant allele present in most offspring, this skewed ratio does not arise from Rr x Rr.

In summary, among the given choices, only the 3:1 configuration aligns with the expected phenotypic outcome of a monohybrid cross between two heterozygous parents for a single dominant allele.

AGRI10051_2025_SM2 Revision Questions Weeks 1-4 UNMARKED

In tomato plants, the allele for red fruit (R) is dominant to the allele for yellow fruit (r). Two plants, both heterozygous for fruit colour, are crossed. What is the expected phenotypic ratio of red to yellow fruits in the offspring?

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