Chapter 23, Problem 29ESP

In 1988, Horst Wilkens investigated blind cavefish, comparing them with members of a sibling species with normal vision that are found in a lake [Wilkens, H. (1988). Evol. Biol. 25:271–367]. We will call them cavefish and lakefish. Wilkens found that cavefish eyes are about seven times smaller than lakefish eyes. F₁ hybrids have eyes of intermediate size. These data, as well as the F₁ × F₁ cross and those from backcrosses (F₁ × cavefish and F₁ × lakefish), are depicted below. Examine Wilkens’s results and respond to the following questions:

1. (a) Based strictly on the F₁ and F₂ results of Wilkens’s initial crosses, what possible explanation concerning the inheritance of eye size seems most feasible?
2. (b) Based on the results of the F₁ backcross with cavefish, is your explanation supported? Explain.
3. (c) Based on the results of the F₁ backcross with lakefish, is your explanation supported? Explain.
4. (d) Wilkens examined about 1000 F₂ progeny and estimated that 6–7 genes are involved in determining eye size. Is the sample size adequate to justify this conclusion? Propose an experimental protocol to test the hypothesis.
5. (e) A comparison of the embryonic eye in cavefish and lakefish revealed that both reach approximately 4 mm in diameter. However, lakefish eyes continue to grow, while cavefish eye size is greatly reduced. Speculate on the role of the genes involved in this problem.