The correct answer is None of these choices are correct.
Explanation:
Crossing over doesn't occur only between heterozygous genes, it also doesn't happen only in some chromosomes and it never reduces de variation, otherwise, the crossing over is one of the main causes of variation among species.
Crossing over can be defined as the swapping of the genetic material which occurs in the germ line.
When the sperm and ovum combine together during the process of meiosis then the paired chromosomes from each of the parent align in such a way that the similar DNA sequence form the paired chromosomes cross over on each other.
The crossing over results in the genetic variation.
