Chapter 20, Problem 23P

Three-spined stickleback fish live in lakes formed when the last ice age ended $10,000$ to $15,000$ years ago. In lakes where the sticklebacks are prey for larger fish, they develop $35$ bony plates along their body as armor. In contrast, sticklebacks in lakes where there are no predators develop only a few or no bony plates.

a. In crosses between fish of the two different morphologies, the lack of bony armor segregates as a recessive trait that maps to the ectodermal dysplasia (Eda) gene. Comparisons between the Eda $-$ coding regions of the armoured and nonarmored fish revealed no differences. How can you explain this result?

b. Loss $-$ of $-$ function mutations in the coding region of the homologous gene in humans result in loss of hair, teeth, and sweat glands, as in the toothless men of Sind (India). What does this suggest about hair, teeth, and sweat glands in humans?