Question

In certain plants, the genes for purple flowers and tall height (AB), is incompletely dominant over the genes for white flowers and dwarf height (ab).
What is the expected phenotypic ratio of the offspring, in a cross involving AaBb X AaBb parents?
A. 1:1:1:1
B. 9:3:3:1
C. 3:6:3:1:2:1
D. 1:2:1:2:4:2:1:2:1

Answer 1

Hint: Interaction of genes may be either allelic or non-allelic. Allelic interaction occurs between alleles of the same gene that are located in identical loci on different homologous chromosomes. On other hand, non-allelic interactions take place between the alleles of different genes that are located at different loci on the same or different chromosomes.

Complete Answer:
- Incomplete dominance is the condition in which the dominant allele is only partially expressed in the heterozygote so that the phenotype of the heterozygote is apparently intermediate between those of dominant and recessive homozygotes.
- Complete dominance and complete recessiveness are extreme cases in which the dominant allele is fully and completely expressed in the heterozygote. So, the heterozygote will be phenotypically identical to the dominant homozygote. This is the typical Mendelian inheritance.
- According to the question, the cross has been made between AaBb X AaBb Where AB is incompletely dominant over ab. Due to incomplete dominance the character intermediate between the two will be expressed.

	AB	Ab	aB	ab
AB	AABB(Purple tall)	AABb(Purple medium)	AaBB(Blue tall)	AaBb(blue, medium)
Ab	AABb(Purple medium)	AAbb(Purple short)	AaBb(blue, medium)	Aabb(blue, short)
aB	AaBB(blue, tall)	AaBb(blue, medium)	aaBB(white, tall)	aaBb(white, medium)
ab	AaBb(blue, medium)	Aabb(blue, short)	aaBb(white, medium)	aabb(white,short)

It will produce offspring of colours purple, white and blue and height tall, short and medium. So, the phenotypic ratio purple tall: purple medium: purple short: blue tall: blue medium: blue short: white tall: white medium : white short is 1:2:1:2:4:2:1:2:1

So the correct answer is D.

Note: In between two extremes, there may be present transitional degrees of dominance called incomplete dominance.
In this case, some sort of interaction is believed to occur between the dominant and recessive alleles. As a result of it, the dominant allele is only partially expressed and intensity dominance gets reduced in the heterozygote.