The Law of Segregation
Now we've already learned that the Law of Segregation states that alleles at a given locus segregate into separate gametes. Basically, all that means is that each parent can only contribute one allele of any given gene to a gamete. Now, how does the Law of Segregation apply to our flying hamster studies? Let's consider Adrian's hamster experiment more closely.
Naming the Generations in Genetic Crosses
Remember that he inferred that his two populations of brown hamsters and white hamsters were homozygous for the brown, or 'BB' allele, and white, or 'bb' allele, respectively. To determine which allele was dominant, he performed a cross.
When a series of genetic crosses are performed, we refer to the first generation as the parental or P generation. The organisms in the P generation are typically what are called 'true breeding.' A true breeding strain of hamsters always produce progeny that are the same as the parents. So for instance, in Adrian's case, when he mated brown hamsters with brown hamsters, he always got brown hamsters. Because Adrian's brown and white strains were true breeding, we can infer that the genotype in this generation was 'BB' (for brown) and 'bb' (for white).
The progeny of the P generation are referred to as the first filial, or more commonly, F1 generation. Similarly, the progeny of the F1 generation are referred to as the F2 generation. If we had further progeny that we were going to keep track of, that would be the F3 generation, and so on and so forth.
The only genotype that can be produced in the F1 generation of this mating scheme is 'Bb' because I can get a 'B' from this hamster parent and a 'b' from this hamster parent. The fact that we can only get a 'Bb' genotype and that all the progeny in this generation are brown, supports Adrian's hypothesis that brown is a dominant allele.
Notice that the P generation cross was pretty simple and it's pretty easy to keep track of the genotypes when there's only one genotype that can be produced. Unfortunately, that's not always going to be the case and it really becomes necessary to have a system for keeping track of all the genotypes that are possible.
For instance, Adrian decides to cross two of the F1 hamsters. What is he going to get in the F2 generation?
The Punnett Square
Geneticists use an aid known as a Punnett square to help them determine what genotypes and phenotypes to expect. A Punnett square is basically just a chart that can be used to predict genotypic and phenotypic ratios of offspring in a genetic cross.
Adrian's cross between two heterozygotes is often referred to as a monohybrid cross. A hybrid is just basically something derived from a mixed origin. For instance, a hybrid car can operate based on either gasoline or electric-based power.
In this case, what we're talking about is a cross between hamsters that are heterozygous at a single gene (the coat color gene). Let's see how a Punnett square will help us determine the outcome of the cross.
So why don't you get a piece of paper and let's do this together? Go ahead and pause the video if you need to. Are you all ready?
First off, let's go ahead and write the genotype for the mother in the middle of the screen. Let's draw the genotype for the father over here on the left part of the screen. We have one parent that's 'Bb' and, in fact, our second parent is also 'Bb.' These are the possible alleles that these parents can contribute to their progeny.
Let's draw a square that encompasses those alleles. Let's divide the square into four equal sections so that each of the letters is on one side of the box.
This is really simple now. All we're going to do is we're going to take the letter from the top of the column and the letter from the left of the column and put them into each of these boxes.
I'm going to get 'BB' here, a 'Bb' here, another 'Bb' and finally I'm going to get a 'bb'. So all we've done here is we've used the Punnett square to help us represent the gametes segregating from the mother and the father into the new offspring.
In this case, this progeny got a 'B' from mom and a 'B' from dad and ends up with a 'BB' genotype. Similarly, this hamster got a 'b' from mom and a 'b' from dad and is 'bb'.
It happens that these two hamsters got a 'B' from dad in this case and a little 'b' from mom in this case, but this hamster also has the same genotype; however, it got it's 'B' from mom and it got a 'b' from dad.
Remember that we said that this is a monohybrid cross. It's a monohybrid cross because it's a cross between two parents that are both heterozygous at a single locus that we're studying, in this case, the coat color gene.
One common misconception is that the Punnett square is telling you that you're going to get four progeny all the time from whatever the cross is, in this case, these two heterozygous parents. And that the four progeny are going to be these four genotypes that appear in the Punnett square. That's not exactly the case. As you know, the number of progeny produced in any mating vary from mating to mating in general.
What the Punnett square is telling us is not the number of progeny that we're going to get, but the ratio in which the genotypes are going to occur. What it's telling us is that for any given progeny that's produced by this cross, there's a 1/4 chance that it'll have a 'BB' genotype; there's also a 1/4 chance to have a 'bb' genotype; and finally, there's a 50% chance that any progeny produced from this mating is going be 'Bb' in genotype.
So the Punnett square is telling us that we're going to get a 1:2:1 ratio of genotypes, but what we're really interested in is phenotypes because that's what we can actually observe. Let's look and see what phenotypes this cross is going to produce.
We know that 'BB' is going to produce brown hamsters, and we know that 'bb' is going to produce white hamsters. What about 'Bb'? What is it going to produce? If you remember from our studies of alleles, we know that the 'B' allele is dominant over the 'b' allele. So 'Bb' will produce brown hamsters. This means that although we predict a 1:2:1 ratio of genotypes, we predict a 3:1 ratio of phenotypes. There's three times as many brown hamsters as white hamsters because the brown allele masks the white allele in the two heterozygotes.
Note the fact that the white phenotype has mysteriously reappeared after having mated two brown hamsters together. This is another reinforcement of this idea of dominant versus recessive. Basically, a recessive phenotype can be masked in the F1 heterozygotes, but can then be uncovered in the F2 generation.
Based on his experimental results, Adrian can confidently conclude that hamster coat color is a single gene trait in which brown is dominant over white.
To summarize, the P generation refers to the organisms in the initial step in a series of genetic crosses. The F1 generation refers to the progeny of the P generation. The F2 generation refers to the progeny of the F1 generation.
A Punnett square is a chart that can be used to predict the genotypic ratio of offspring produced by a genetic cross. Based on the genotypic ratio, a phenotypic ratio can also be predicted.
A monohybrid cross is a mating between two organisms heterozygous at a given locus. A monohybrid cross always produces a 1:2:1 ratio of genotypes of homozygous dominant, to heterozygous, to homozygous recessive traits. A monohybrid cross produces a phenotypic ratio of 3:1 between the dominant phenotype and the recessive phenotype.
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