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.
The collection of all the genes and the various alternative alleles within a population is called a gene pool. Genetic Variation within a population is measured according to the number of different alleles of all genes and the frequency with which they appear.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.