In this video, we're going to take a look at genetic diagrams, which show us all the different combinations of alleles that we can get from two parents. To show you what we mean, let's imagine that there was a single gene that determined how muscular a mouse would be. Let's suppose that the dominant allele, which we can show as a capital A, codes for a normal mouse with average muscle, while the recessive allele which would be a lowercase a, or code for muscular mass.
If you haven't seen this idea of representing alleles with letters before, basically we show the two different alleles that an individual has as upper and lowercase versions of the same letter, with the uppercase letter meaning it's dominant, and the lowercase one being recessive. A typical question for this topic could be something like... Draw a genetic diagram for the cross between a homozygous normal mouse and a homozygous muscular mouse.
Now whenever you have to draw a genetic diagram there are five main things you need to look at and you look at each of them in turn. So first you find the parent's phenotype and genotype which I'll often give you in the question. Then you use that to find out all of the gametes genotypes. and then finally use those to find the offspring's genotypes and then phenotypes. So in our example, we're told that one of the parents is homozygous for normal muscle, which means the genotype will be capital A capital A and the phenotype will be normal.
Meanwhile the other parent is homozygous for the muscular allele, so its genotype will be two lowercase a's, And even though this allele is recessive, it will still be expressed because it's homozygous. So the phenotype is muscular. To work out the gametes, all we do is take the two letters in each of the parent genotypes and split them into two separate circles.
This basically represents the parent cells splitting in two during meiosis. And so we get two gametes from each parent, each one with half of the genetic material. Now, to find the offspring's genotypes, we have to do all the possible combinations of the two parents'gametes.
So for one of these, we would take the capital A gamete from the normal mouse, and combine it with a lowercase a from the muscular mouse to give us a capital A lowercase a genotype. And then we would do this exact same thing three more times, so that we end up with the genotypes of all four of our offspring. This particular case is a bit boring though, because all of the offspring are heterozygous, which remember means that they have one allele of each type. And because normal muscles are dominant to large muscles, all the heterozygous offspring will have the normal phenotype.
We could also show this process using another type of diagram called a Punnett square, which is just a large square split into four smaller squares to give us a 2x2 grid. Then what we do is place one of the parent's gametes at the top, and the other parent's gametes on the left, and then fill in each of the four squares depending on the combination of gametes. So the top left square gets the capital A from the left parent, and the lowercase a from the top parent, and we do the same thing for all of the others. The benefit of Punnett squares is that they show us almost all the same information as genetic diagrams do, but they're much simpler to draw.
The only downside is that they don't show us the phenotypes of the parents or offspring. In this case though, because we can see that all of our offspring are heterozygous, we know that they'll all have the normal phenotype. For a more interesting example, let's use the Punnett square to work out what would happen if two of these heterozygous normal offspring were to mate together. First, as all the offspring had the genotype of AA, we place A and A gametes on the top and the left. Then, as we fill in the square, we get one homozygous dominant offspring with a AA genotype, two heterozygous offspring, and one homozygous recessive offspring with two lowercase a alleles.
So, in terms of the phenotypes, we would have three that are normal because they all have a dominant allele that's being expressed, and then because of our homozygous recessive amounts we'd have one muscular offspring. You might sometimes be asked to write the outcome of genetic crosses as a ratio or probability and in this case we did that we have a three to one ratio of normal to muscular mice or we could say that there is a one in four or 25 percent probability of having muscular offspring. Now just to make sure everything's clear, let's quickly whisk through this example again but we'll do it with a genetic diagram, so we're still crossing two heterozygous mice.
So just like before, we start with these five lines and we know that both parents have the normal phenotype but the big A little a genotype and so both of them will give one big A gamete and one little a gamete. Which means that when we mix them together, we'll get one homozygous dominant, two that are heterozygous, and one that's homozygous recessive. So just like with the Punnett square, the phenotypes would be three normal and one muscly. Although we've been discussing how a single gene determines a particular trait, it's important to remember that Loads of different genes interact, and the outside environment can also play a big part. For example, you might have loads of different genes that code for being tall, but if you didn't get enough food or sleep as a child, then you'll probably still end up being short.
And that's it for this video. So if you enjoyed it, then please do give us a like and subscribe, and we'll see you next time.