Which characteristics evolved independently in mammals and birds?

endothermy

Explanation:

This term describes the ability of some animals to use an internally generated heat to maintain body temperature. Their body temperature tends to stay steady regardless of environment. When it's cold outside such animals increase their metabolic heat production to keep their body temperature fixed. For this reason, the internal body temperature of an endotherm is more or less independent of the temperature of the environment.

answer; a male inherits only one allele of the x-linked gene controlling hair color;

the answer would be..

b. sucking