In a complete dominance monohybrid cross between a pure breeding yellow-flowered plant Y and a pure breeding…

Analyze the question

This is a monohybrid cross between two pure-breeding (homozygous) parents. The question asks about the F1 generation. It says the cross shows complete dominance, and the yellow allele is written as capital Y, so yellow is dominant and white (y) is recessive.

Concept

• Pure-breeding means homozygous: YY (yellow) and yy (white).
• Each parent gives only one type of gamete.
• With complete dominance, the dominant allele fully masks the recessive allele in a heterozygote.

Solution

Parents: YY × yy

Gametes: Y and y

Punnett square:

| | y | y | |---|---|---| | Y | Yy | Yy | | Y | Yy | Yy |

All the F1 offspring are Yy (heterozygous). Because Y is completely dominant over y, every one of them shows the yellow phenotype.

• Genotype ratio: 100% Yy
• Phenotype ratio: 100% yellow

Checking the options

• A (50% yellow): this would come from a test cross (Yy × yy), not from this cross.
• B (75% yellow): this is the F2 result (Yy × Yy gives 3 yellow : 1 white).
• C (all yellow): correct.
• D (all white): wrong, because the dominant allele is present in every F1 plant.

Answer: C. All yellow flowers

Exam tip: In a cross between two pure-breeding parents with contrasting traits, the F1 is always uniform and shows the dominant trait. The 3:1 ratio appears only in the F2, when the F1 plants are crossed with each other.