Question

When a garden pea plant with violet flowers was crossed with another plant with white flowers, 50% of the progeny bore violet flowers. (a) Work out the cross. (b) Mention the type of cross and its significance. (c) How does the inheritance pattern of flower colour in snapdragon differ from the above?

Answer 1

Hint: This is an example of a monohybrid cross where the dominant gene for colour and the recessive gene for colour are interacting. Progeny ratios will depend on the parent genotype.

Complete answer:
If 50% of the progeny bore violet flowers, it means that the other 50% were white, as this kind of dominance is complete. Genotype of a white flower (recessive) would be vv, but a violet flower could be Vv or VV. That is, homozygous or heterozygous. If we cross the heterozygous plant, Vv, with the homozygous recessive white vv, the possible combinations in the offspring are only two: Vv or vv. This means 50% of the offspring will be Violet (heterozygous) and 50% would be white (homozygous).

Parent Alleles	V	v
v	Vv	vv
v	Vv	vv

(2 out of 4 are Vv or violet heterozygous; 2 out of 4 are vv or white homozygous.)
This type of cross between an individual with the dominant phenotype but whose genotype is unknown and one with the homozygous recessive genotype is called a Test Cross. It is commonly used to identify the genotype of the individual exhibiting the dominant characteristic.
Inheritance of flower colour in the snapdragon is an example of incomplete dominance. Crossing homozygous red flowers with homozygous white flowers results in all pink flowers. This is because the dominant red allele is incompletely dominant over the white. A heterozygous self-cross would give the ratio of 1:2:1 for red:pink:white flowers, which would also be the genotype ratio of homozygous red: heterozygous pink: homozygous white.
Note: While in incomplete dominance individuals have a phenotype in-between the two distinct options, in codominance as seen in human blood groups, both alleles are expressed. Type A parent crossed with type B parent can give AB offspring.