Punnett Square Basics | Mendelian Genetic Crosses

Thanks for stopping by today. We're talking about the basics of Punnett squares.

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.

And we use big G, little g for the maternal

genotype and little g, little g for the paternal,

so this is our cross.

The mother has green eyes and the father has blue eyes. 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 really doesn't matter,

but we conventionally write the dominant allele first,

if there is a dominant allele.

So, your setup should look like this,

then you simply carry letters through to their

respective quadrants and match them up like

so.

All right, each quadrant will represent a 25% chance that, that genotype will result

in the offspring.

So, in this case, we have only two possible genotypes, big G, little g and little g,

little g.

And each genotype has a 50% chance of showing up in the offspring.

Similarly, you have two possible phenotypes, green eyes and blue eyes

with a 50% chance of each.

Let's look at another example,

we'll cross two genes that are heterozygous for eye color in this example.

So, big G, little g crossed with big G, little g,

these colors are just for illustration purposes.

So, we again will draw our Punnett square and put one gene on top

and the other down the side

and then carry letters through to their respective quadrants

and analyze the results.

So, this cross results in a 1 to 2 to 1 genotype ratio

with 25% homozygous dominant or big G, big G;

50% heterozygous or big G, little g;

and 25% homozygous recessive or little g, little g.

The phenotypic ratio would be a 3 to 1 ratio with 75%

green eyes and 25% blue eyes.

Hopefully, that helps you understand simple Punnett squares a little better.

Genetic crosses can be much more complicated, and I'll make a video

sometime in the future explaining how to

tackle those more complicated genetic crosses.

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.