Answer:
they swap alleles
Explanation:
when homologous chromosomes cross over during meiosis, they exchange segments of DNA/swap alleles. this means that the chromosomes now have a completely unique combination of alleles, which causes variation. It is also random which of these unique chromosomes end up in which daughter cell (the gametes) and since fertilisation is a random and independent event, which unique male gamete that fertilises which unique female gamete is also random. This means the resultant zygote has a unique combination of alleles that is unlike either of their parents.
