In prophase I the homologs , not sister chromatids, are separated. I, crossing-over leading to recombination occurs. The sister chromatids are eventually separated in a second division. Many mitoses of stem cells lead to primary spermatocytes that enter meiosis and generate four spermatids each. In mature human females one egg is produced each month.
Genes are simply particular stretches of the DNA that constitutes chromosomes .
In a population of animals there are many variants of the sequence of each gene, produced by mutation, some neutral, some disadvantageous, and a few advantageous. These variations of each gene are called alleles . In diploids each individual can only have two alleles of each gene, however there are commonly many alleles of each gene circulating in populations. Segregation refers to the reliable separation of homologs and sister chromatids during meiosis leading each gamete to have one copy of each chromosome. Independent assortment refers to the random selection of the paternal and maternal homologs of various chromosomes that are placed in each gamete.