I am guessing its the chlorophyll. <span />
<span>tubulin, the microtubule protein</span>
<span>If an island had 1000 cats on it, an ecologist would use the word 'population' to describe all the members of that species</span>
Because the three-horned alien is heterozygous, we know that three must be dominant to four, because the gene for the three horns is "hiding" the gene for four horns. Therefore, the three-horned alien has the genotype Tt (T for three horns, and t for four horns). The four horned alien must be tt, because that is the only way that a recessive trait may be seen. If you solve the punnet square on a cross between Tt and tt, you end up with half three (heterozygous) and half four (homozygous recessive) it is a bit easier to explain with something a little "closer to home" if you want me to explain it again, just say so, I don't mind!
The answer is A. Transport of water and minerals to the shoot system
