A yellow maize is planted and all the fruits obtained are of yellow seeds. When they are croos-bred, yellow…

Analyze the question: A yellow maize plant gives only yellow seeds. When these are cross-bred, the offspring show yellow and white seeds in a 3:1 ratio. We need to say what kind of trait yellow is.

Concept: This is Mendel's monohybrid inheritance. A 3:1 phenotypic ratio in the F2 generation appears when two heterozygous parents (Yy × Yy) are crossed. The trait that shows in the larger group (the "3") is the dominant trait. The trait that reappears in only one quarter of the offspring is the recessive trait.

Solution:

1. The parent plants give only yellow seeds, and these yellow seeds are heterozygous (Yy), since white appears later.
2. Cross: Yy × Yy

| | Y | y | |---|---|---| | Y | YY | Yy | | y | Yy | yy |

3. Genotypes: 1 YY : 2 Yy : 1 yy
4. Phenotypes: 3 yellow (YY, Yy, Yy) : 1 white (yy)

Yellow appears in three-quarters of the offspring and masks the white allele in heterozygotes. The white trait is hidden in the first generation and only reappears when two copies (yy) are present. So yellow is the dominant trait.

Checking the other options:

• A. Non-heritable: wrong, because the trait is passed on to offspring.
• B. Sex-linked: wrong, because maize seed colour is not carried on sex chromosomes, and the ratio does not depend on sex.
• C. Recessive: wrong, because the recessive trait is the white one (the "1" in the ratio).

Correct answer: D, a dominant trait.

Exam tip: In a 3:1 ratio, the bigger number is the dominant trait and the smaller number is the recessive trait. Remember "3 Dominant, 1 recessive."