Genetics and Punnett Squares: Getting Traits from Parents

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  • 0:00 Genes Have Variations
  • 1:53 Punnett Squares
  • 6:41 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

You are the genetic product of your parents, but you could have ended up differently if they had passed on different genetic information to you. Punnett squares are a useful tool to help us identify all the possible genotypes of the offspring of two parents.

Genes Have Variations

Whether you like it or not, you are a product of your parents. They each passed some of their genetic information on to you, which is what makes you who you are. But your sisters and brothers aren't exactly like you, even though they came from the same parents. This is because there is some variation in what genetics we get from our parents. If we got all of the genetic information from our folks, we would have way too much - essentially two sets of everything. And then our own kids would have this plus what they got from your partner, and it would quickly spiral out of control.

Instead, we get a selection of genetic information from each parent. And there's different versions of each gene you get, called alleles. You get one allele of a gene from each parent (for a total of two alleles per gene), and the combination of all your alleles is what makes you unique.

It's possible to get the same allele from each parent, or you could get different alleles from each one. Alleles determine all kinds of things: flower color in plants, eye color in humans, whether or not you have a certain disease, and more. When you get the same allele from each parent, then that allele is what is expressed physically in your phenotype.

For example, if you got a brown-eyed allele from each parent, you would have brown eyes.

But if you got a different allele from each parent, say one blue eye allele and one brown eye allele, then whichever allele is dominant is the one that is expressed. Dominant alleles determine an organism's appearance and are represented with capital letters. The other allele you inherited is called the recessive allele, which is genetic information that does not affect an organism's appearance (represented by lower case letters). It's still part of your genetic makeup, your genotype, and you can still pass this genetic information on to your children, but you might not ever know it's there because it just sits silently in the background.

Punnett Squares

Because dominant alleles essentially mask recessive ones, you can't tell just by looking at someone what their genotype is. And the chances of having certain traits is different depending on which dominant and recessive traits your parents have.

There's a fairly simple way to determine the likelihood of the genotype of an individual if you know the genotypes of the parents. We use something called a Punnett square to show the possible combinations of alleles an offspring might inherit from two parents. It's named after Reginald C. Punnett who invented this method.

A Punnett square is called such because it's, well, a square! Along the left side of the square are the two alleles of Parent 1, and across the top are the two alleles of Parent 2. Inside the overall square are four smaller squares, each representing one of four possible allele combinations that could occur in an offspring. It's pretty easy to use, too. Simply fill in the inside boxes with whatever allele corresponds to it from each parent.

If for example, both parents have two of the same alleles, called homozygous ('homo' means 'same'), then all their offspring would also have two of the same alleles. It doesn't matter which is dominant or recessive in this case because both parents have the same genetic makeup. So if both parents have two dominant alleles (AA), then all of their children will have two dominant alleles (AA). Or, if both parents have two recessive alleles (aa), then all of their children will also have those two recessive alleles (aa).

That's a pretty easy example, so let's look at a more complex possibility. Let's use plant flower color as an example. For this example, let's say that the dominant allele is purple flower color, represented by a capital letter 'P,' and the recessive allele is white flower color, represented by a lowercase letter 'p.'

Now, what if, instead of both parents having the same two alleles, they each have one dominant and one recessive allele (Pp)? This would make each parent heterozygous because they have two different alleles for a given gene ('hetero' means 'different'). In this case, we could have different possible outcomes for their offspring because now we have some genetic variety.

How would this look in a Punnett square? Well, both the left and top sides of the square would be marked with a capital P and a lowercase p. Then we just fill in the internal boxes with whatever allele is next to it.

Punnett square for example
example of Punnett square with white and purple

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