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.
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
So for the top left box, we get a dominant allele from each parent, giving us an offspring with two dominant alleles (PP) and a purple flower. If we move to the right along that top row, we get the same dominant allele from the first parent, but now we get a recessive allele from the second parent, giving us a heterozygous offspring (Pp). But even though this offspring has two different alleles, the dominant one is the one that is expressed as a purple flower phenotype, while the recessive white flower color is just 'carried' along in that offspring's genotype. So even though this offspring is different than the first offspring genetically, we would never know by looking at them because physically they would look exactly the same!
Moving on, let's go to the bottom left square. Here, we get the recessive allele from Parent 1 and the dominant allele from Parent 2. This gives us yet another heterozygous offspring that looks just like our homozygous dominant one from the very first square. This individual has one of each allele, Pp, and a purple flower color.
Our last square gives us something very different, though. This final offspring gets a recessive allele from both parents (so pp), meaning that the recessive allele finally gets its chance to shine! No longer repressed by the dominant allele, this homozygous recessive individual with white flowers looks different from the others both inside and out.
In this example, we ended up with a 1:2:1 ratio of possible genotypes (1 x PP, 2 x Pp, and 1 x pp). And it's important to remember that even though three of the potential offspring showed purple flowers physically in their phenotypes, two of those still carry the white flower allele in their genotype, meaning that they can pass that possibility on to their own children in the future.
You can go through this process for any genes that have one dominant and one recessive allele and for any genetic combination of parents. Perhaps one parent is homozygous dominant while the other is homozygous recessive. Or maybe one parent is homozygous (either dominant or recessive) and the other is heterozygous. Just fill in your square appropriately and you'll find all the possible combinations of alleles for the offspring of those two parents.
When you get a chance, thank your parents for getting together and making you. You simply couldn't exist without them! Your genetic makeup, or genotype, comes directly from them, just as you will pass your genetic information directly to your offspring.
You have a lot of genetic information, though, so only a selection will make it to your children. You won't know until it happens what genes do make it, but you can certainly test the possibilities to determine what the chances are of passing certain alleles, or different versions of genes, on to the next generation.
The easiest way to do this is with a Punnett square, which is designed to show the possible combinations of alleles an offspring might inherit from two parents. When working with dominant and recessive alleles, you can also predict which phenotypes, or physical characteristics, would occur as well. And no matter if the two parents are homozygous, meaning they have two of the same alleles, or heterozygous, having two different alleles, simply line up the alleles of each parent along the sides of the Punnett square and fill in the boxes inside to get your answers.
After you've completed this lesson, you should have the ability to:
- Differentiate between genotype and phenotype
- Define allele, homozygous and heterozygous
- Explain the purpose of and how to use a Punnett square