Learn Biology: How to Draw a Punnett Square

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A Punnett square is used to predict the chances of an offspring to have its parents' traits. These squares are most commonly divided into four parts, with each part equalling a 25% chance of the offspring receiving that set of genes. More complicated squares may have more than four parts, though the same basic method applies.
The letters surrounding and within each square represent alleles. They are one part of a gene pair occupying a specific part of a chromosome. All dominate alleles have capital letters, while the recessive ones are lowercase. Dominate alleles will always overpower recessive ones in the expression of the gene.

If the alleles for a parent do not match, they are known as heterozygous. In the image above the Gg is heterozygous. This can happen if there is a dominate and a recessive gene in the parent. If the alleles are the same for that expressed gene, it is known as homozygous. This is seen if both alleles are dominate or if both alleles are recessive; e.g., GG or gg. In order for a recessive gene to be expressed, the alleles must be homozygous.

Step 1:
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Draw the Punnett square. This is done by drawing a square, followed by a straight line up and down and another from side to side. This will quarter, or create 4 equally sized boxes within the square.

Step 2:
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Place the father's alleles on the top of the Punnett square with one letter above each box. Place the mother's alleles on the left hand side of the square, with one letter in front of each box. Be sure to use capital letters for the dominate genes and lower case letters for the recessive alleles. For this example, let's say this square represents the color of a flower. The father has one dominant blue and one recessive orange allele. The mother has two recessive orange alleles.

Step 3:
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Drop the father's alleles down into the squares and bring the mother's across. This will provide you with all possible combinations of alleles for the offspring. Each square represents a 25% chance of the offspring having that combination. If there are squares with the same cominations in them, the squares can be added together to determine the percentage.

Conclusion:
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From the completed square above, we can see that 50% of offspring will be blue since any dominant allele paired with a recessive one will win. There are, however, two homozygous combinations in which both genes are recessive, so 50% of the offspring will be orange. This means that half of the offspring will be blue, while the other half will be orange. Easy, right?

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Hi! My name is Mary Poffenroth. I'm an adjunct professor of biology,

and today we're going to have a little fun with Mendelian Punnett squares.

Now, before we started and jump on in to our Punnett square problems,

let's set down some ground rules. First, I'm going to talk about alleles.

Alleles are going to be like different flavours of genes.

So, you go into the ice cream shop, there's 31 flavours of ice cream. Even though they're all different flavours, they're all still ice cream, right?

Alleles are going to be those different flavours of genes that you have available to you.

The next thing I want to talk about is dominant and recessive.

In any relationship, some of those alleles are going to be dominant, and some of those alleles are going to be recessive.

Those dominant alleles, if they're present, they're going to have more of a say in what the outcome of that cross is going to be,

and the recessives, well, they're going to have little to no say unless there are two recessives that are available.

Also, in each of our crosses, we're going to have one allele from our mother, and one allele from our father.

Now, let's jump into our crosses.

Today, we're going to do a four-square Punnett square.

So, you want to start out by just drawing a square. In that square, you're going to draw a line right down the middle from the top to the bottom, and a line from left to right.

Now, you have four individual squares. Each of those squares is going to be a probability,

and each of those squares is one fourth, right? Or 25%.

Next, we're going to put our parents' alleles on the Punnett square.

Now, it doesn't really matter if you put male or female on the top, but generally people put male on top, female to the left.

For our problem today, we're going to use eye colour.

The eye colour alleles are going to be for our father, are going be brown phenotypically-expressed alleles.

That means that that father has brown eyes, but his genotype, or the genes at play, his alleles are going to be big B, little b.

That means that he's heterozygous, or he has two different alleles, for that trait of eye colour.

It's a little important to note at this time that all of these letters are completely arbitrary.

You can make up any letters you want. However, in any relationship, a capital letter, or a big letter, like, say, big B, is going to be for the dominant,

and a little letter, or a lowercase letter, in this case, little b, is going to be for the recessive.

OK, so we have the father, big B, little B. He's heterozygous, meaning he has two different alleles. What about the mom?

The mom has blue eyes, so phenotypically, her expression that you see on the outside is blue eyes.

Her genotype, or the genes at play, are going to be little b - little b, or homozygous recessive.

All right, it's time to do our cross.

Now, we're going to take one from the top, and one from the left, and we're going to bring them down into those squares.

So we have on our top, big B-little b, then bring another big B-little b down. In our right square at the top, we have little b-little b, and our bottom right square we have little b-little b.

OK, what does that mean for the probability of getting an offspring with what kind of eye colour?

Now remember, each of those squares is going to be 25%.

So, we have a 25 plus 25, or 50% chance of getting an offspring with brown eyes,

and we have a 25 plus 25, or 50% chance of getting an offspring with blue eyes.

And for a fun little extra problem when you're sitting around the dinner table, take the eye colours or the phenotypes of your parents.

Even though you won't know the exact genotypes, or the genetic makeup, behind what you're seeing on the outside,

do some crosses, and see if you can find out the probability that you could have actually had a different eye colour.

Thanks for watching, and if you want to learn more about this subject, click on the link below, or if you want to learn more biology, feel free to click on any links around me.

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