Which of the following could explain why the phenotype of a healthy female child may show differences from her healthy biological brother (apart from the gender)? 1. independent assortment after the formation of the female zygote 2. crossing over between a chromosome from the sperm and a chromosome from the egg 3. random alignment of homologous chromosomes at the equator of the cell during metaphase I (imat 2018 q25)
Independent assortment of homologous chromosomes in metaphase I of meiosis I leads to variation (and thus differences between individuals). However, this only occurs during gamete formation, so only when sex cells divide by meiosis to form sperm and egg cells. Once formed, the sperm and egg cells fuse and the zygote only divides by mitosis to form a mature individual (so no crossing over).
I couldn’t get that