Chapter 29, Problem 3EQ

Two diploid species of closely related frogs, which we will call species A and species B, were analyzed with regard to the genes that encode an enzyme called hexokinase. Species A has two distinct copies of this gene:  A1 and A2. In other words, this diploid species is A1A1 A2A2. Species B has three copies of the hexokinase gene, which we will call B1, B2, and B3. A diploid individual of species B would be B1B1 B2B2 B3B3. These hexokinase genes from the two species were subjected to DNA sequencing, and the percentage of sequence identity was compared among these genes. The results are shown here.

Percentage of DNA Sequence Identity

	A1	A2	B1	B2	B3
A1	100	62	54	94	53
A2	62	100	91	49	92
B1	54	91	100	67	90
B2	94	49	67	100	64
B3	53	92	90	64	100

If we assume that hexokinase genes were never lost in the evolution of these frog species, how many distinct hexokinase genes do you think there were in the most recent ancestor that preceded the divergence of these two species? Explain your answer. Also explain why species B has three distinct copies of this gene, whereas species A has only two.