M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Gregor Mendel's law of independent assortment states that when genes are inherited, they are inherited independent of each other. Linked genes are exceptions to the law of independent assortment because two genes are located on the same chromosome, but this is generally mitigated when chromosomes cross over.
One of the basic principles of inheritance or genetics is the law of independent assortment otherwise known as Mendel's second law. And this is the idea that the inheritance of one trait does not influence the ihnheritance of another. i.e. they are sort to the gametes independent of each other. Now this is caused by the way that the genes get shuffled and your chromosomes randomly assort themselves during the process known as meiosis which is the creation of gametes.
Now I'm going to be talking about a couple of different alleles or genes in this context. One of them that I'll refer to is the ability to roll your tongue or the inability to roll your tongue. Tongue rolling is a dominant allele, non-rolling is a recessive allele.
There's another gene for earlobe shape. The dominant version or allele is the free or detached earlobe shape while the recessive allele creates a not attached earlobe. So if we're talking about somebody who is homozygous sorry, heterozygous for both traits, they're big e little e and they cross with somebody who's also big e little e and big r little r. Let me put that little r in there, good. What you do is instead of using the smaller punnett square that you're familiar with, you wind up using the larger punnett square that has 16 boxes. Now this person I'm going to put their gametes on the left hand side. The second person I'll put across the top.
So this person can give every gamete one of their two r alleles. So they'll give him the big r and they'll give them the big e. Similarly they could give the exact same big r only in combination with the little e. The big e and little sorry, the big r and the big e do not have to travel together. They can be separate. You can have the little r go with the big e or you can have the little r go with the little e. This person, with the identical genotype can also produce a big r and a big e. Big r little e, a little r big e or a little r little e. With me so far? Great.
So now that I have my gametes on the outside, I start creating my offspring on the inside. So this sperm swims to meet that egg and I get a big r big r, and I get a big e big e. So, that's how I'd fill this out. Now, I can spend a lot of time filling out all 16 of these but let's just skip to the end and see what it would look like.
So here we have a completed punnett square demonstrating the effects of Mendel's second law, the law of independent assortment. And we can see we get this broad array of different kinds of offspring. Now I've color coded them to make them easier to figure out what their phenotypes are. You'll notice all these guys here are red. That represents individuals or offspring who are dominant for both traits. And if we count it up we'll see that the rollers with the detached earlobe shape, people who look like myself would be nine out of these 16 offspring. Those who are rollers they can roll their tongue they've got the attached earlobe shape, they would be three of our 16 offspring. We'd have an identical amount of non-rollers but with detached earlobes and we'd have one out of our 16 offspring would be non-roller attached earlobe shapes. So that's how the law of independent assortment plays out when you're talking about punnett squares and what effects you get with inheritance but a lot of times teachers like to ask you to not just to know how to use it but they want you to know why it works. And that goes back again to the process of meiosis where you have one cell create 4 possible gametes. Now I could go through this and talk about homologous chromosomes but I found that a lot of times it's just simpler to use shoes because much like the homologous chromosomes, they too come in pairs.
So I'm going to use these black men's dress shoes to represent the chromosome pair the homologous chromosome pair that carries the big e allele and the little e allele. I'm going to use the white sneakers to represent the big r and little r carrying chromosomes. So these are homologous shoes very similar, not identical. This is a right, this is a left and these are homologous shoes. Not identical. One's a left, one's a right and they carry different versions of the same gene. This one keeps trying to go for a chromosomal mutation, a deletion of the little e allele.
So, when you undergo meiosis metaphase 1 of meiosis, let me raise these up, we could have these two line up on the same side, these two line up on the other side. And when they separate from their gametes, I've lined up with one gamete that has the combination little e and big r and I'll have another one that has the combination little r big e. But how these shoes line up has no influence on how those shoes line up. So I could have done it like this. Let me swap this over. And now, here we go.
Here, when I wind up creating this gamete I get the big r big e together in one gamete. Here I get the little r and little e in one gamete. Each outcome is equally probable. So there's no influence that the black shoes have on the white shoes. They're independent of each other. That's the law of independent assortment.