Comparison Between Gene and Chromosome Behaviour

By the start of the twentieth century, cytologists had also established that the total chromosome number is constant across every cell of a given species (a diploid eukaryotic cell always carries two haploid sets, one from each parent), and that every somatic cell of an individual carries an identical genetic complement. This constancy, together with the way chromosomes move during meiosis, does more than just confirm Mendel's laws after the fact -- it opened up entirely new ways to study heredity that Mendel himself had no access to. The parallel is exact at every step: alleles of a Mendelian factor occur in pairs, just as chromosomes occur in homologous pairs; a heterozygote's two different alleles separate cleanly from each other during gamete formation, exactly as the two homologous chromosomes physically separate at meiosis; and while different Mendelian factors on different chromosomes assort independently, factors that happen to sit on the very same chromosome do not assort independently at all, because the chromosome carries them together as a physical unit. Mechanistically, the two alleles of a gene occupy the same locus on the two members of a homologous pair; each chromosome first replicates in the S phase of meiotic interphase, so that each allele now exists as two identical copies, one on each sister chromatid; the homologous chromosomes then separate from each other at anaphase I, physically pulling the two different alleles apart; and finally, at anaphase II, the sister chromatids themselves separate, so that each resulting gamete ends up with exactly one copy of one allele. Species vary enormously in how many chromosomes they c …