And Mendel actually did not invent the Punnett square, although he was thinking in these terms. It was actually invented by Reginald Punnett in And this is useful to think about the probabilities of various combinations based on what each parent could contribute. So let's say we're talking about the tall plant, and let's say it has two tall versions. So it can contribute a capital T or a capital T. And let's say that this short plant over here has two of the short versions for now.
So it could contribute either a lowercase t, or a lowercase t. And so what are all the possible combinations for its offspring? Well, in one scenario, you could get this capital T from the male parent, and this lowercase t, from the female parent. In another scenario, you could get this capital T from the male parent, and a lowercase t from the female parent.
In this scenario, and I know these look very similar, a capital T from the male parent and this lowercase t from the female parent, and then last but not least, I know this looks repetitive, you could get this capital T from the male parent and this lowercase t from the female parent.
And the reason why in all of these cases, you see a tall plant, is because the tall version, and he coined this term, is dominant. And once again, this was all his hypothesis to explain his results. So this is dominant and the short is recessive. So even if you have one of each, you're actually going to show the dominant trait. We now call that your phenotype, what you show is going to be tall. Now, what's interesting about this hypothesis is it seems to explain what happens in the next generation.
Just so a little bit of notation. The first generation is usually called the P generation for parental, and then the first generation of offspring is known as the F1, F for filial. And that comes filials, which means sun in Greek, and then the generation after that would be F2, that's just a little bit of notation there, but let's think about what would happen at the F2 generation, if you self fertilized, some of these characters right over here.
Well, in that situation, let me draw another Punnett square, on the male parents side, you could contribute either your capital T version, which we now call your dominant allele, or you could contribute the lowercase t version, and on the female parents side and once again for a plant, you can have the same plant that has both the male and the female parent. You could contribute the dominant version, the capital T, or the recessive version, the lowercase t.
Now we see something interesting happen in the offspring. There is a one in four chance you get both capital Ts. The offspring genotypes are YY, Yy, and yy. Of course, yy peas are green.
And while YY and Yy peas have different genotypes, they have the same yellow phenotype. As you can see, there are three yellow peas to one green pea.
This is Mendel's 3 to 1 ratio. My squares are especially useful when tracking the inheritance of more than one trait at a time. Try the problem in this section and you'll see what I mean. The Manx breed of cats is known for being tailless, though some are born with tails. A dominant gene shortens the spine and is the cause for no tail. In a cross between two tailless Manx, you get a litter of kittens where for every 2 tailless kittens, there is one with a tail.
What happened to Mendel's 3 to 1 ratio? Funded by The Josiah Macy, Jr. All rights reserved. Concept 5 Genetic inheritance follows rules. Breeding green with green, the offspring was two greens as well.
Learn more about the misconceptions about genetics and the Punnett squares. When crossbreeding a green and yellow pea plant, the offspring has one copy of the green gene and one of the yellow gene, and the offspring will no longer be genetically pure. But Mendel discovered these plants had only yellow seeds.
In modern terms, a plant with any yellow genetic seed material will yield yellow seeds. Thus, the green seed genetic material is called recessive. Only plants with exclusively green genetic material end up with green seeds.
In the case of peas, having a phenotype that is green, it is sure that the genotype is two green genes. If the phenotype is yellow, it is unclear if the genotype has two yellow genes, or one yellow and one green. If there are two parent plants that are purely green, it means that both their genes are green. To fill in the genetic outcomes of the offspring in a two-by-two grid, one parent is put on the top and one on the left. The plants are crossbred, and the offspring gets a green gene from the top parent and a green gene from the bottom one.
The same thing happens for pure yellow gene parents. In the case of crossing a purely green plant with a purely yellow, the Punnett squares represent the offspring that are all a mixture, with one green gene and one yellow gene. Mendel found that yellow genes are dominant, having at least one yellow gene, the plant looks yellow with both having the same phenotype. This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on Wondrium. Punnett squares also help understand why, when breeding plants that were crossbred themselves, their offspring were three times more likely to be yellow than green.
By setting up the Punnett square, there is a plant on the top with one yellow and one green gene and the same on the left. Either a green or a yellow gene is taken from one plant and combined with the other plant. One option is an offspring with two yellow genes and another is an offspring with two green genes. Punnett squares are very useful to understand the connection between genetics and what people actually see.
A plant with one yellow and one green gene resulting in a yellow pea is not so obvious but making a yellowish-green colored pea is.
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