Great Migration: Life for Migrants in the City. By the end of 1919, some 1 million blacks had left the South, usually traveling by train, boat or bus; a smaller number had automobiles or even horse-drawn carts.
Genetic variation is increased by meiosis During fertilisation, 1 gamete from each parent combines to form a zygote. Because of recombination and independent assortment in meiosis, each gamete contains a different set of DNA.
Independent assortment is one kind of genetic recombination, one where the chromosomes supplied by one's parents are literally independently assorted into resulting gametes. Note that reassortment is a similar process that occurs within the genomes of certain viruses. One way to view the process of independent assortment, and its impact on the genetic variation of offspring, is that, for the haploid human gametes, there (metaphorically) are 23 buckets and each bucket is to hold one homologue (actually 22 buckets of autosomes and one bucket containing a lone sex chromosome).