Exam 27: Phylogenies and the History of Life

Deletions in mice in the Hoxa3 gene,on the A cluster,result in abnormal formation of glands in the neck.Deletions in the homologous Hoxd3 gene,on the D cluster,result in abnormal formation of the neck skeleton.The protein-coding sequences of Hoxa3 and Hoxd3 are about 50% the same.Researchers inserted a functional copy of the coding sequence of Hoxd3 in place of the coding sequence of a nonfunctional Hoxa3.They left all promoters,enhancers,splicing signals,and processing signals of Hoxa3 in place.Normal neck gland development was restored.What can you conclude? (J.M.Greer,J.Puetz,K.R.Thomas,and M.R.Capecchi.2000.Maintenance of functional equivalence during paralogous Hox gene evolution.Nature 403:661-65.)

What is still lacking in the hypothesis of an asteroid impact as the cause of the end-Cretaceous mass extinction?

Which of the following likely has the most Hox genes?