AP® Biology · Unit 5 · Topic 5.2
Independent Assortment and Random Fertilization
During meiosis, each pair of homologous chromosomes lines up and separates independently of every other pair, and random fertilization then combines gametes unpredictably, generating enormous genetic variety among siblings.
Siblings differ because the chromosomes they inherit get shuffled twice. First, during metaphase I of meiosis, each homologous pair lines up at the cell's equator with no regard for how any other pair is lined up. Whether the maternal or paternal copy of chromosome 1 faces one pole is completely independent of which copy of chromosome 2 faces that same pole. With every pair flipping its own coin, the number of possible chromosome combinations in a gamete multiplies fast: for n pairs, there are 2n possible arrangements.
Humans have 23 pairs of homologous chromosomes, so assortment alone gives 223, over 8 million, genetically distinct gametes per parent, before crossing over adds even more variety. Random fertilization then stacks on top: any one of one parent's roughly 8 million possible egg types can pair with any one of the other parent's roughly 8 million possible sperm types, giving about 223 x 223, or 246, possible genetic combinations for a single child. That is why full siblings, barring identical twins, never carry the same genome.
This is also why independent assortment has a limit. It depends on separate homologous pairs moving independently of each other. Two genes sitting on the same chromosome are physically linked: they travel to the same pole together during meiosis I unless crossing over in prophase I swaps one of them onto the other homolog. So linked genes break the 2n logic until recombination separates them, which sets up the next topic, linkage and recombination.
Key terms
- Independent assortment
- The random, independent orientation of each homologous chromosome pair at metaphase I of meiosis.
- Homologous pair
- Two chromosomes, one from each parent, that carry the same genes in the same order.
- Random fertilization
- The unpredictable combination of any one gamete from each parent to form a zygote.
- Linked genes
- Genes located on the same chromosome that tend to be inherited together unless separated by crossing over.
Watch it explained
- How it worksHow does each homologous pair lining up independently multiply the gamete possibilities?
- Worked exampleFind how many genetically different gametes a human can make from assortment alone.
- How it worksWalk me through random fertilization stacking on top of the variation meiosis already made.
- Common mistakeWhy doesn't independent assortment apply to two genes on the same chromosome?
Check yourself
End of topic.