Inheritance - Punnett Grid

The question tests whether students understand the relationship between dominant and recessive alleles in a heterozygous (Bb) individual. The key principle is that the dominant allele fully masks the recessive allele.

In dominant-recessive inheritance, the dominant allele (B) is expressed whenever at least one copy is present. A person with genotype Bb has one dominant allele, which is sufficient to produce the dominant phenotype (brown eyes) entirely.

The student is confusing dominant-recessive inheritance with incomplete dominance, where a blended phenotype does occur. In standard dominant-recessive inheritance, no blending happens — the dominant allele fully suppresses the expression of the recessive allele.

The correct explanation is that in dominant-recessive inheritance, B fully masks b, so a Bb individual shows brown eyes only — not a partial or blended expression of blue.

We need to find the correct explanation for why the student's claim — that Bb should produce partly blue eyes — is wrong.

The option 'The statement is correct — Bb individuals always show a blend of the two traits' is wrong. Blending only occurs in incomplete dominance, which is a named exception. In standard dominant-recessive inheritance, Bb individuals show only the dominant phenotype.

'The student is wrong because b is the dominant allele and B is recessive' contradicts standard genetics notation — capital letters represent dominant alleles and lowercase letters represent recessive alleles. This option reverses the convention incorrectly.

'Bb individuals cannot exist because two different alleles cannot occupy the same gene location' is factually incorrect. Heterozygous (Bb) genotypes are completely normal and very common in genetics.

The remaining correct option correctly states that B fully masks b in dominant-recessive inheritance, producing entirely brown eyes in a Bb individual.

Parent 1 is YY (homozygous dominant) and Parent 2 is yy (homozygous recessive). This is a classic cross between two homozygous individuals with opposite genotypes.

Parent 1 (YY) can only produce gametes with Y. Parent 2 (yy) can only produce gametes with y. Every cell in the Punnett grid will therefore be Yy.

All four boxes in the Punnett grid give the genotype Yy (heterozygous). This means 100% of offspring are heterozygous.

Since Y (yellow) is dominant over y (green), all Yy offspring will show yellow seeds. The phenotypic ratio is 4 yellow : 0 green → 100% yellow seeds.

We need to determine the phenotype of offspring when a YY parent is crossed with a yy parent.

'All green seeds' would require all offspring to be yy, but the YY parent always contributes a Y allele, so no offspring can be yy. This option is incorrect.

A 50:50 split occurs in a test cross (Yy × yy), not in a YY × yy cross. Since YY produces only Y gametes, no offspring can receive two y alleles.

The 3:1 ratio (75%:25%) is the classic result of a heterozygous × heterozygous cross (Yy × Yy), not a homozygous dominant × homozygous recessive cross.

All offspring are Yy, and since Y is dominant, they all show yellow seeds — 100% yellow is correct.