Question

In southwest Bakersfield there is a peculiar species known simply as the "runner". Research in the Szick lab has focused on the genetics of this organism. It has been determined that the size of the runner's brain (measured as the circumference of the head in centimeters) is under the control of two genes. Alleles for either of the genes can either an additive allele or a non-additive allele and the effect of the additive alleles is approximately equal. Assume that a true-breeding individual with a head circumference of 20.2cm is crossed to a true-breeding individual with a head circumference of 37.4cm. What is the best way to represent the genotype of the individual with the smaller head?

		aabb
		Aabb
		AAbb
		AABb
		AaBb

Answer 1

As both are a true-breeding individual the possible genotype is

aa - represents small head circumference (20.2 cm)

bb - represents large head circumference (37.4 cm)

So the best genotype to represents the individual is aabb  

This is because both are homozygous in nature and have additive effect which are approximately equal.

Aabb cannot be true as Aa is hetrozygous

AAbb cannot be true as additive effect which are approximately not equal.

AABb cannot be true as Bb is hetrozygous

AaBb cannot be true as Aa and Bb is hetrozygous