Understanding Independent Assortment: Study with Video Lessons, Practice Problems & Examples

Mendel's second law, known as the law of independent assortment, states that alleles of two genes assort independently during gamete formation. This means that the inheritance of one trait does not affect the inheritance of another. In a dihybrid cross, combinations of alleles can occur freely, resulting in various gametes. If genes assort independently, any combination of alleles can be formed, unlike linked genes, which are inherited together. Understanding this principle is crucial for grasping Mendelian inheritance and predicting genetic outcomes in offspring.

Gamete Genetics and Independent Assortment

So this is a short topic, but I made it its own separate thing because it is so important that you understand this, one of Mendel's postulates or laws or whatever you want to say. Because you're going to get a lot of questions that start with something like assuming Mendelian inheritance or assuming genes are assorted independently. And essentially what that's talking about is this law right here. So even though it's a short topic, it's super important. And so, the Independent Assortment is Mendel's second law, so you may see it as that. And pretty much what it does, what it states are that alleles of two genes assort independently. And so, as soon as you understand what that means, it makes complete sense, but sometimes this definition can be a little confusing. And so what we typically do is we perform a dihybrid cross to look at two genes and the offspring that, mating those two genes between two organisms produce. And so, the really important thing here is realizing that independent assortment is focusing on more than one gene. In this case, it's usually defined as two, but it can be more than that. Right? It can be as many genes as you want, but just knowing that each gene assort independently. So what does sorting independently mean? For this, I'm going to do an example. So for this type, we have a genotype AaBb. So a, we'll say, is for yellow and b is for shape. So we have two genes, one is affecting color and one is affecting shape. So gene a, that's color, and gene b, the shape. Now, what does it mean sorting independently? Well, it's talking about gamete formation. Remember gametes are the sex cells, these are the sperm or the egg. So if we were to ask what's the genotype of the gametes, whenever you produce this, this is the gametes are telling you whether or not the genes are sorted independently. So we have these two genes, a and b. Both of them have two different alleles. So if they assort independently, meaning that each gene or each allele is going to its own gamete independent of the other, genes in one of our gametes. Well, the first thing we focus on is the first gene. So that's one, uppercase A, and lowercase a. And because there's two sets, we just repeat this. So each gamete gets one a allele, either the uppercase A or the lowercase a. And because there are four, we just, you know, double it. So two get uppercase A's and two get lowercase a's. Then, because the genes are sorting independently, that means that we can now do the second gene which is gene b. And we do the same thing. Oh, let me actually change the color here. So we can, really see it. AB, little b. And, so now, what are the next two? So before, we just repeated it, but this time we have to do it backward. And the reason we have to do it backward is because we want this gamete, which has two uppercase letters to be different than this gamete, which has the uppercase A and the lowercase b. Now, this is an example of sorting independently. So it's not that these two are always seen together. If it was sorting non-independently, that would mean that the A and the B always were together. The uppercase A and uppercase B were always together, and the lowercase a and lowercase b were always together. Because they would be together, they would be what we're going to refer to as linked in the future. We haven't talked about that yet. But for now, just know that if it's sorting independently, any combination of gametes can be made. Can be uppercase A, lowercase b, uppercase both, lowercase both, any combination of the alleles can be made if they're sorting independently. If they're not sorting independently, then you can only get one combination that occurs. So, hopefully, that makes sense. So this right here is the example of independent assortment. This is what you're going to see or what they're going to try to get you to answer anytime it starts out with, like, assuming Mendelian inheritance or anything like that. This is the way that it's done, is that the A alleles sort of go into their gametes completely independent of the B alleles. Then the B alleles come in, they do whatever they want, And if you have more alleles, C, D, E, they all store independently. They all go into their own gametes completely independent of the other alleles. So that's independent assortment, let's now move on.

Which of the following gametes cannot be formed from the genotype AaBBCc?

• A. aBC
• B. ABC
• C. Abc
• D. ABc

Which of the following gametes cannot be formed from the genotype DDeeFfGG?

• A. DeFG
• B. DDeFG
• C. DefG

Which of the following gametes cannot be formed from the genotype HhJjKK?

• A. HJK
• B. hjK
• C. hjk
• D. HjK

Here’s what students ask on this topic:

What is Mendel's law of independent assortment?

Mendel's law of independent assortment, also known as Mendel's second law, states that alleles of two or more different genes assort independently during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another trait. For example, in a dihybrid cross involving two genes, the alleles for each gene segregate into gametes independently, resulting in various combinations of alleles. This principle is crucial for understanding genetic variation and predicting the outcomes of genetic crosses.

How does a dihybrid cross demonstrate independent assortment?

A dihybrid cross involves two genes, each with two alleles. For example, consider a cross between organisms with genotypes AaBb. During gamete formation, the alleles for each gene assort independently, leading to four possible combinations: AB, Ab, aB, and ab. When these gametes combine during fertilization, they produce offspring with a variety of genotypes. The phenotypic ratio typically observed in the F2 generation of a dihybrid cross is 9:3:3:1, demonstrating that the alleles of the two genes assort independently.

What is the significance of independent assortment in genetics?

Independent assortment is significant because it contributes to genetic diversity. By allowing alleles of different genes to assort independently, it ensures that offspring have a variety of genetic combinations. This principle is fundamental to Mendelian inheritance and helps predict the outcomes of genetic crosses. It also explains why siblings can have different combinations of traits, even though they share the same parents.

Can you explain the difference between independent assortment and linkage?

Independent assortment refers to the random distribution of alleles of different genes into gametes, meaning the inheritance of one gene does not affect the inheritance of another. In contrast, linkage occurs when genes are located close to each other on the same chromosome and tend to be inherited together. Linked genes do not assort independently and are often passed on as a group, reducing genetic variation. Understanding the difference between these concepts is crucial for predicting genetic outcomes.

How does independent assortment affect genetic variation?

Independent assortment increases genetic variation by allowing different combinations of alleles to be passed on to offspring. During gamete formation, alleles of different genes assort independently, resulting in a variety of possible genetic combinations. This process, combined with other mechanisms like crossing over and random fertilization, contributes to the genetic diversity observed within populations. This diversity is essential for evolution and adaptation to changing environments.