In this lesson, we will define the process of independent assortment, examine the principle, and go through some examples. At the end, you can test your knowledge with a short quiz.
I have my mother’s eyes and my father’s nose. My brother has my mother’s nose and my father’s eyes. My brother and I are full siblings with the same two parents, and yet we inherited the traits for noses and eyes (and all other traits) very differently. Nobody would necessarily guess that we are siblings, unless they saw us together with our parents.
Why does this happen? Why aren’t all siblings identical? Physical traits are manifestations of our genes. While studying inheritance, the famous geneticist Gregor Mendel discovered that genes are inherited independently of each other. This concept defines the law of independent assortment.
We cannot look at how genes are inherited without first examining how they are sorted. Gametes are made by the process of meiosis 1, wherein chromosomes containing parent genes are randomly sorted and separated into the gamete cells.
Before they are separated, though, the chromosomes undergo a process of crossing over, where the chromosomes basically swap genes with each other, leading to new chromosome configurations. This helps to ensure that independent assortment takes place. In the diagram, we see that the red and blue chromosomes undergo crossing over, and the result is a mix-match of color on each chromosome.
These colors represent the genes that they have swapped. The resulting gametes each have a unique combination of the genes inherited by their parents, and thus, the genes will manifest themselves independently of each other.
Punnett Square Example
Among his test subjects, Mendel used peas in his genetic experiments that illustrated independent assortment. He could examine how one gene is passed on quite easily, usually with two different alleles, or variations of the gene.
This Punnett Square shows a monohybrid cross, the crossing of two parents who differ by one gene of interest. This cross is examining the gene for color: yellow and green. This gene expresses yellow as a dominant allele and green as a recessive allele. The offspring (four, one in each box) receives one allele from each parent and will manifest their color based on which alleles they receive. If they receive two green alleles, they will be green.
But if they receive one yellow and one green allele, they will be yellow, as yellow is the dominant allele.
With one gene of interest, this is pretty straightforward. But as you know, organisms have many, many genes that code for their diverse and complex characteristics. When we look at two genes together in a dihybrid cross, we see that the two genes will be inherited independently of each other, leading to a much larger possible combination of alleles and thus necessitating a bigger Punnett Square: