Two plants with red flowers were back crossed, which of the following results indicate that the plants are…

Analyze the question: Two red-flowered plants are crossed, and the result shows they are heterozygous (Rr). Red (R) is dominant over white (r).

Concept: A heterozygous plant carries one dominant and one recessive allele (Rr). When two heterozygotes are crossed with each other, the offspring follow the classic Mendelian monohybrid ratio of 3 dominant : 1 recessive.

Solution:

Cross: Rr × Rr

| | R | r | |---|---|---| | R | RR | Rr | | r | Rr | rr |

• Genotypes: 1 RR : 2 Rr : 1 rr
• Phenotypes: 3 red : 1 white, which is 75% red and 25% white

Checking the other options:

• B (50% red, 50% white): This is a test cross (Rr × rr), which involves a white parent, not two red plants.
• C (100% white): Impossible, since red parents with at least one R allele would produce some red offspring.
• D (100% red): This would result from RR × RR or RR × Rr, so it does not show that both parents are heterozygous.

A note on the wording: "Back crossed" is loosely used here. A true backcross or test cross uses a recessive (white) parent. Since both parents are red, the question really describes crossing the two red plants together, and only option A (3:1) indicates that both are heterozygous.

Correct answer: A. 75% red and 25% white

Exam tip: A 3:1 ratio in the offspring means both parents are heterozygous. A 1:1 ratio means a test cross (heterozygous × homozygous recessive). If you get all dominant offspring, at least one parent is homozygous dominant.