Random Assortment of Chromosomes: Definition & Explanation

Random Assortment of Chromosomes: Definition & Explanation
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  • 0:00 What are Chromosomes?
  • 1:00 When, Where, and Why?
  • 2:30 How are Chromosomes Sorted?
  • 3:15 Lesson Summary
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Lesson Transcript
Instructor: Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

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.


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).

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