Which best describes color blindness? (Please answer ASAP if you know the answer!)

B. Males need only one copy of the color-blind allele to show the trait.

Explanation:

Typical red-green color blindness is a recessive X-linked trait. This means that females, which normally have two X chromosomes, would potentially need to carry two affected alleles in order to manifest the trait. A female carrying a single affected allele would be an asymptomatic carrier, albeit capable of passing the altered allele on to her offspring of either sex.

Males, in contrast, have a single X chromosome. This means that, should their (single) allele be affected, they will be colorblind. As for offspring, none of the colorblind man’s sons will inherit the affected gene from their father (since he will have passed on his Y chromosome to them), whereas all of his daughters will inherit the affected allele (since they receive the X chromosome, of which he only has one).

Note that, as seen above, there are (infrequent) scenarios where females may end up with two affected alleles and thus be colorblind (e.g. female offspring of a colorblind man and an asymptomatic female carrier).