Forelimbs form at the junction of the neck and thoracic vertebrae,the position of which is controlled by HoxC-6 genes.Several kinds of vertebrates from different classes are limbless,including limbless lizards,and amphibians known as caecilians.All descended from limbed forms.On the basis of recent research in genetics,what would you conclude about the functioning of their HoxC-6 genes?
A)The site of expression of their HoxC-6 genes may have moved anteriorly until the level of expression has eliminated the neck region.
B)The site of expression of their HoxC-6 genes would be the same as that of limbed vertebrates but is simply turned off early in development.
C)Their HoxC-6 gene has mutated and serves a different function than in limbed vertebrates.
D)The effect of the HoxC-6 gene is masked by another gene that is dominant to it.

Question

Quizplus · Accepted Answer

The fact that limbless vertebrates descended from limbed forms suggests that their HoxC-6 genes were once involved in limb formation. The current absence of limbs, therefore, suggests that the site of expression of their HoxC-6 genes has moved anteriorly until the level of expression has eliminated the neck region. This would support choice A as the best answer.

Forelimbs Form at the Junction of the Neck and Thoracic