Punnett Square Basics | Mendelian Genetic Crosses

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This is a simple overview of monohybrid genetic crosses using punnett squares and Mendelian genetics.

Punnett squares measure the probability or likelihood of a genetic outcome based on a specific genetic cross. It doesn’t tell you what will be, only what could be.

For the simplest of punnett squares you’ll cross two genotypes. These parent genotypes will be for the same gene, but may or may not contain different alleles. Let’s look at an example of a genetic cross for eye color, green eyes being dominant and blue eyes being recessive. Let’s use Gg for the maternal genotype and gg for the paternal. So this is our cross, the mother has green eyes and the father has blue eyes and we want to see the probability that their offspring will have either green or blue eyes.

For this cross we will draw a square split into four quarters, like a simple four panel window. This is the punnett square. One of the parental genotypes will go on top, and the other will go on the side. It doesn’t matter which genotype goes where and the order of the letters doesn’t matter, however, we conventionally write the dominant allele first.

So your setup should look like this.

Then you simply carry the letters through to their respective quadrants and match them up, like so. Each quadrant represents a 25% chance that that genotype will result. So in this case we have only two possible genotypes, Gg and gg. And each genotype has 50% chance of showing up in the offspring. Similarly, you will have two possible phenotypes, Green eyes and Blue eyes, with a 50% chance of each.

Let’s look at another example. Let’s cross two genes that are heterzygous for eye color. Gg X Gg. These colors are just for illustration purposes. We again draw our punnett square and put one gene on top and the other down the side, carry the letters through to their respective quadrants, and analyze the results. This cross results in a 1:2:1 genotype ratio, with 25% Homozygous Dominant GG, 50% Heterozygous Gg, and 25% Homozygous Recessive gg. The phenotypic ratio would be a 3:1 ratio with 75% green eyes and 25% Blue eyes.

Hopefully that helps you understand simple punnett squares. Genetic crosses can be much more complicated and I will make some videos in the future explaining how to tackle those. If you have any questions from this video, throw them in the comments, and don’t forget to like and subscribe. Thanks for watching. I’ll catch you next time.