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.
Variation is high when there are many different alleles of all genes and when there are many different combinations of those alleles. However, genetic variation is constantly changing. Different alleles of a single gene can appear and disappear from time to time within a group.ecombination (Crossing Over). This is the exchange of genetic material between adjacent chromatids on the homologous chromosomes. This random exchange of DNA results in many new combinations of alleles on chromosomes, creating great potential for variation.