Source one: independent assortment of chromosomes
At the first meiotic division, each tetrad lines up and is pulled apart. But the orientation of each pair is random: the maternal homolog of pair 1 may face one pole while the maternal homolog of pair 2 faces the other. Each pair decides independently, so the maternal and paternal chromosomes get mixed and matched across the whole set.
This is the cellular basis of Mendel's law of independent assortment. With 23 pairs in humans, the number of possible chromosome combinations in a single gamete is 2²³ — about 8.4 million — before crossing over even adds its layer.
Source two: crossing over within a chromosome
Crossing over (from the previous guide) trades segments between a maternal and a paternal chromatid. The effect is to build chromatids that are part-maternal, part-paternal — combinations of alleles that neither parent's chromosome carried. The overall reshuffling of alleles, whatever its source, is called genetic recombination.
When we look at two genes, a gamete is called a parental type if it carries the same allele combination the parent inherited, and a recombinant if crossing over (or assortment) has produced a new combination. Counting recombinants vs parental types is the measurement that the rest of this track is built on.
Putting both together
Parent's chromosomes: A B (one homolog, both 'capital' alleles)
a b (other homolog, both 'lowercase')
No crossover between A and B:
gametes are parental → A B and a b
Crossover between A and B on two chromatids:
those chromatids become → A b and a B (recombinant)
A single tetrad with one crossover therefore yields
four gametes: A B (parental), a b (parental),
A b (recombinant), a B (recombinant)Together, independent assortment and crossing over make the pool of possible gametes astronomically large. This variation is the raw material that natural selection later acts upon — but for now, our goal is simply to measure it.