Random assortment of chromosomes is one of the reasons why some siblings look alike and others look so different from each other. Read about the what, why and how of this key, yet arbitrary, mechanism contributing to genetic variation.
What are Chromosomes?
The main evolutionary advantage of sexually reproducing organisms is being able to produce offspring that are different from one another, demonstrating genetic variation rather than being clones. There are three mechanisms responsible for this: random assortment of chromosomes (chromosomes are sorted into daughter cells randomly), crossing over (chromosomes switch chunks of DNA) and random fertilization (chance alone is responsible for which sperm meets which egg). In this lesson we will focus on the random, independent assortment of chromosomes.
Chromosomes are ‘packages’ of tightly packed DNA that are found in the cell nucleus. DNA exists as chromosomes to keep itself organized and contained, mainly so it’s easy to move around when it’s time for the cell to divide.
Humans have 23 pairs of chromosomes. Since we reproduce sexually, one chromosome in each pair comes from the mother and one from the father. The two chromosomes in each pair are called homologous chromosomes because they have the same genes on them.
When, Where and Why?
During mitosis, which is regular cell division, all 46 chromosomes copy themselves, form a line along the center of the cell and split up, so that one copy goes to each of the daughter cells. No sorting is needed here.
Sorting of chromosomes only takes place during meiosis, which is the type of cell division cells that produces sperm and eggs inside sex organs. Chromosomes need to be sorted in this case because each of the daughter cells should receive only half of the set of chromosomes (one from each of the homologous chromosome pairs mentioned above). When sperm and egg come together during fertilization, the new organism produced will have a full set of chromosomes (i.e. 46). For humans, any more than 46 would result in a non-viable zygote (with the exception of an extra chromosome in pairs number 13, 21, 18 or X, in which case the offspring survives but has some developmental abnormalities).
Random assortment of chromosomes produces many variations among daughter cells, giving rise to genetic diversity in offspring. A cell with 2 pairs of chromosomes gives 4 possible chromosome combinations in daughter cells (2^2). Human cells, with 46 pairs of chromosomes, result in 8,388,608 possible daughter cell combinations (2^23). If you add to that the randomness of crossing over and fertilization of eggs by sperm, you get trillions of combinations. That’s why there are so many different people in the world!
How Are Chromosomes Sorted?
Chromosomes are simply assorted by chance. Random assortment of chromosomes refers to the way chromosomes get organized into daughter cells during gamete (sperm and egg) formation. It means that each sperm and each egg will have different combinations of chromosomes, some of which will have come from the person’s mother and others from the father. This happens because homologous chromosomes line up along the cell center with no rhyme or reason other than being across from their partner, shown here in this image:
This explains why some children can look very similar to one of their grandparents (a greater proportion of chromosomes from that person ended up in the winning sperm/egg), and also why siblings can look so different from one another (many different chromosome combinations).
One of the primary mechanisms for creating the genetic diversity in the human race is the random assortment of chromosomes, which is when chromosomes are sorted into daughter cells randomly. This random sorting of chromosomes happens during meiosis, which is the type of cell division that produces sperm and eggs in side sex organs. This random assortment simply occurs by chance, and this ultimately results in lots of variation, which is why the human race isn’t just an army of clones.