What is a Punnett Square? | Best Biology Review

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Punnett Square


A punnett square is a diagram of every possible combination of a maternal allele with its paternal counterpart for each gene investigated in a crossbreeding experiment. That means if you were going to try to crossbreed two plants, say one that had red blooms and one that had white blooms, you could do a punnett square to predict how many different plants would come out red and how many would come out white and how many might come out pink.


This just would put out all the options. From that, you could figure out the ratios. If you planted 100 of these plants, or crossbred 100 of these plants, you could predict how many of them would come out which color, if colour was the trait you were looking at. Let’s look at some examples. A monohybrid cross considers only one trait in the cross. “Mono” means “one”. We’re only going to be considering one trait in the cross.


We’re going to look at a tall maternal pea plant and a short paternal pea plant. We’re looking at peas. The big T is going to represent the dominant allele and the lower case t is going to represent the recessive allele. Since the big T is dominant, any time the big T is present, it’s going to take precedence. The big T means “tall”. Since we have two big T’s, we know this is going to be a tall plant.


If we see a big T and a little t, it’s still going to be a tall plant, because a big T is present. The only way we get a short plant is with two little t’s, because it is recessive. You must have both alleles be short alleles to get a short pea plant. Let’s look at how this looks in the punnett square. What we’re looking at is a tall parent that was dominant and a short parent that had both alleles recessive.


Let’s combine them. We’re going to put the big T with the little t in this box. Big T with little t in this box. Big T with little t in this box. Big T with little t in this box. Since we have four that are a combination of a big T and little t, this would give you a phenotypic ratio of 4. We only have tall plants in this combination, because every time a dominant allele will be present.


The phenotypic ratio means what the plant’s going to look like. This will be tall, tall, tall, tall. It wouldn’t matter if we had a pair of alleles that were both the dominant or if we had a pair like this where there’s a dominant and recessive. It would still be tall. That’s all we’re looking at is what it looks like, not what the alleles actually- what the letters of our alleles are in the punnet square.


Since they’re all going to look tall, our phenotypic ratio is 4. They will all four be tall. Let’s look at a different one. I’m going to draw a line to keep us separated. This one’s going to be a dihybrid cross. A dihybrid cross considers two traits in the cross. “Di” means “two”, “mono” means “one”. We’ve got our dihybrid cross that’s going to compare two different traits. We’re going to look at a tall, wrinkled parent and a tall, round parent.


Again, the T’s are going to be the same. If we have two big T’s it will be tall, a big T and a little t (tall), two little t’s will be short. Your R’s are going to stand for whether it’s round or wrinkled. The big R is the dominant allele, and it codes for round peas. Round peas are the norm. Wrinkled peas are going to be that recessive gene. A big R and a little r is still going to give you round peas, because it includes a dominant allele.


The two little r’s would give you the wrinkled peas, because that is the recessive allele. If all you have are the two recessives, you’re going to get those wrinkled peas. This is a parent that has a dominant and a recessive allele for tall and two dominant alleles for round peas, and another parent that has a dominant and recessive gene for tall and two recessive genes for wrinkled peas.


We’re going to look at what we get in our punnett square with this. W have T/T, R/r. This is going to give us tall and round, because we have two dominants and a dominant and recessive. Moving across, we have two big T’s, a big R and a little r. Again, we’re going to get tall and round. Next, we’ve got T/t, R/r. Again, it still includes two dominants, so we’re going to have tall and round. T/t, R/r.


We still have a dominant T and a dominant R, so it’s going to be tall and round. Moving on. T/T, R/r. Tall, round. If you notice, these two were the exact same. What you’re going to put all the way across is going to be the same as what you had above it. Since we had a T/R, T/R, the results going across the board are going to be the same. All of these will also be tall and round.


If you wanted to go across and check them all, you definitely could and just look at it. Big T with this little t? We’ve got it. Big R with this little r? We’ve got it. Now, we’ve got a different one: t/R. Now we’re going to have a big T with a little t. T/t. You always want to put the dominant allele first whenever you’re writing it. Then, you’ve got R/r. This is still going to code for tall and round, because you have a dominant allele for the tall and a dominant allele for round.


Next, you’re going to have T/t, R/r. Again, tall around. Unless we have two recessives, we’re going to continue to have tall and round. Now, we’ve got t/t, R/r. Now we have two recessive genes, so we’re going to have a short plant, but it’s still going to be round, because we have that big R. The next one: t/t, R/r. Again, we’re going to have a short, round plant. This is going to be the same way that we did the top row.


Since this was a t/R and this is a t/R, the results are going to be the same across the board. T/t, R/r. Now we get to our T/t, R/r for these last two, because we’ve got t/t, R/r for both of those. They’re going to code the same as the ones above them: Tall and round, tall and round, short and still round, and then short and round. Now, we want to do our phenotypic ratio. We want to say how many of them look the same.


We know we have a lot that are tall and round, so let’s count and see how many we’ve got: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. We’ve got 12. The majority of our plants are going to be tall with round peas. Then, we have 1, 2, 3, 4 that are short and round. Our ratio would be 12:4. Our phenotypic ratio that just tells us what the plants look like, because if we were looking at the alleles themselves, some of the tall plants have two dominants for T and some of them have a dominant and a recessive for T.


Either way, they came out tall. Since they look tall, they get counted in the phenotypic ratio. Punnett squares can be kind of fun and they are like what you would do whenever you’re graphing in math. You just have to make sure that you put every possibility out there. You put every allele along each side, and then make sure you fill it in correctly. Pay attention to how the alleles are going to code.


Do you have to dominants coding for a dominant? A dominant and recessive coding for a dominant? Do you have those two recessive coding for a recessive gene? You want to look at every possible option so that you can make an accurate prediction with your punnett square.



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Last updated: 10/23/2018

 

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