Which of the following statements about crossing over is true?
a) it occurs only in males.
b) it occurs only in some chromosomes.
c) it occurs only between genes that are heterozygous.
d) it results in reduced genetic variation among gametes.
e) none of the above

E) None of the above

Explanation:

Crossing over happens during meiosis in diploid individuals. Homologous chromosomes (one set that came from an individual's mother and the other set that came from the individual's father) pair during prophase I, and when they are aligned some parts of the homologous chromatids can be exchanged, resulting in recombinant gametes. This increases genetic variation, because the generated gametes are different from any of the original chromosomes the individual had.