What is a Punnett Square and how do I fill it out

Answer 1

A Punnett square is a way to determine the potential results of a genetic cross when you know the genotype (collection of alleles an individual carries for a trait) of the parents. It's called that because it involves drawing a subdivided square, and was developed by a scientist named Reginald Punnett.

Lets do a simple example problem to demonstrate how to fill one out:

We will determine the results of seed pod color for a cross between two pea plants. One plant has green pods but is heterozygous while the other has yellow pods and is homozygous. Green pods are dominant to yellow pods. We need to find the genotype of all possible offspring, and the probabilities for each.

Step 1: We need to know the genotype of each parent to set up the square. The question gives us this information by telling us phenotype and zygosity for each parent. Lets define symbols quickly, "G" will be a green allele, and "g" will be a yellow allele. A a green heterozygous plant (the first parent mentioned) would have the genotype Gg, since heterozygous means two different alleles. A yellow homozygous plant (the second parent mentioned) would have the genotype gg, since homozygous means two of the same and "g" is our yellow allele symbol. So our cross is Gg x gg!

Step 2: The next step will involve our square, so at this point we would draw a square with one vertical and one horizontal dividing line, resulting in 4 squares in a 2x2 arrangement.

Above the square and down the left side, we're going to write the genotype of each parent, one allele (letter) per column or row. It doesn't really matter which parent goes where, just one across the top and one down the left side.

Step 3: Now, we can fill the inside squares by copying the alleles down each column from the top and across each row from the left side. So each box in the first column gets a "G" because that's the allele on top of that column. The second column boxes all get "g" and then we move on to the left side. Each box in row 1 gets a "g" and the same for row 2.

Step 4: Our Punnett square is now complete but we should review how this answers our example problem. Each box in the Punnett square represents an equally likely outcome for any individual offspring from this cross. Since we have 4 boxes, each one has a 1/4 chance (probability) to occur. You might notice that there are 2 Gg boxes. We can combine those by adding the individual probabilities 1/4 + 1/4 = 1/2, this is the total probability of a Gg offspring. The same is true of gg, 1/4 for each box, 1/4 + 1/4 = 1/2, which is the total probability of a gg offspring. So, the answer to our problem is the offspring will be 1/2 Gg and 1/2 gg.

You can see a completed version of this Punnett square here: https://en.wikipedia.org/wiki/Punnett_square#/media/File:Punnett_Square_(Green_Dominant).svghttps://en.wikipedia.org/wiki/Punnett_square