Independent Assortment

∞ generated and posted on 2016.12.06 ∞

Random separation of homologous pairs during anaphase I of meiosis.

Independent means not influenced by something else and Assortment here is a verb meaning to distribute. Independent Assortment means the distribution of things where those things don't influence each other's distribution, in this case the chromosomes inherited from different parents (mom and dad) into different daughter cells during meiosis.

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). The parent cell to these gametes, however has 23 pairs of homologues (again, actually 22 pairs of autosomes, or homologous pairs, and one pair of sex chromosomes). For each of those pairs, one homologue came from "Mom" while the other came from "Dad".

Only Mom's or Dad's chromosome can be placed in each bucket, not both in the same bucket (since that leads to trisomies). As a consequence, there are 23 chances where either Mom's or Dad's homologues end up in a bucket, which has the effect of randomly sorting their chromosomes such that each gamete ends up with a complete set of chromosomes – 22 different homologues plus one sex chromosome – that represents some random assortment of Mom's and Dad's chromosomes.

Describing chromosomes as "Mom's" and "Dad's" actually is a bit misleading and this is due to the other kind of genetic recombination that takes place during meiosis, that is, crossing over, or what I often describe instead (and more generally) as molecular recombination. Crossing over occurs prior to independent assortment, during prophase I rather than anaphase I. As a consequence, the individual chromosomes that are present within a cell at metaphase I, rather than being distinctly "Mom's" and "Dad's" chromosomes, instead are molecular combinations of Mom's and Dad's chromosomes, with the molecular recombination occurring between homologous pairs of chromosomes.

The result of all of this molecular recombination is a vast increase over an already vast amount of genetic variation – here, different combinations of alleles – that is generated during meiosis. No two gametes are likely to be genetically identical and, as a consequence, no two products of fertilization, e.g., you and me, are likely to be genetically identical. Identical twins, though potentially an exception to this "rule", instead are, by definition, the product of only a single fertilization event.