The most classic question I've seen this being taught with is actually more complex in real life (i.e. > 1 gene controls it), but I'll pretend it's controlled by one gene here so you understand how to use Punnett squares. (I usually would prefer to use the
original experiment done to prove this, but it's on pea plants and pea plants just aren't as relatable.)
A heterozygous brown eyed woman has a blue eyed husband and has discovered she is pregnant. What is the chance her child has blue eyes, knowing that the brown eye allele is dominant against the blue eye allele? Draw a Punnett square to show your answer.Step 1: Write down what each allele means.Start off by giving our alleles some names - let's go with 'B' in this case. Write down "Let B = brown eyes and b = blue eyes".
It doesn't necessarily have to be 'B' per se and you can use whatever letter you like, but it is the most logical letter in this instance and will save you some hassle later if you choose a letter in which has very distinctly different capital and lower case letters. (Otherwise, ensure your capital and lower case letters are distinctly different sizes when written.)
Punnett squares usually take on capital and lower case letters, so
be careful with this. (The reason they do this is because genetics isn't always simple and is often dictated by many genes for the one trait. Dominant alleles or the 'wild type' [gene seen most often in nature] allele is
usually denoted as capital letters, while lowercase letters are
usually used for recessive alleles. This is not always the case, but for the majority of cases you'll encounter, this generalisation will suffice.)
Step 2: Draw a grid.In this case, we're only dealing with one gene. Draw out a 3 x 3 grid. All will be explained in a second. (Note: 2 genes = 4 x 4 grid. You will unlikely work with any more than that, but if you're curious,
here's a nice website.)
Step 3: Write down the genotypes of the parentsWe already know that one gene gives two alleles per parent; each kid has one biological 'mother' and 'father', both of which have two alleles each. This gives us two letters per gene per parent, so all of our organisms will have two letters for their genotype in the end.
(a) We know from our question that brown is dominant. In genotype, you can get 4 combinations with 1 gene:
- BB (homozygous dominant - brown eyes phenotype)
- Bb (heterozygous - still brown eyes phenotype, because it has one copy of the brown eye phenotype, which is shown against the blue)
- bB (which we usually denote the capitals first anyway; heterozygous - brown eyes phenotype, refer to Bb above)
- bb (homozygous recessive - blue eyes phenotype)
(b) Thus, Mum can't be 'bb' because she's known to have brown eyes and 'bb' has no brown eye genes (Capital B = brown eyes, remember?). We also know from the question that she's heterozygous - meaning she must be 'Bb'. Since Mum only gives one copy to each of her progeny, write a letter 'B' in the top centre box of the 3 x 3 grid and a 'b' in the top right. If you wish, you can write "mother" above the grid for reference.
(c) We also know that Dad has blue eyes. Blue eyes can only be given by the genotype 'bb'. Any big 'B's here and the brown eye phenotype is given to the progeny, so they have to have two small 'b's instead. Write a small 'b' in the first column, second row (columns go vertically/ downwards, rows go horizontally/ left and right) and another 'b' in the first column, third row. If you wish, you can write down "father" on the left of the grid for reference.
Your incomplete Punnett square should look like this:
incomplete Punnett square
B b
b
b
Step 4: Write down the progeny formed from these combinationsExcept the top left box of the grid, run down each column and row, write in what you get when they meet to get a new genotype with two alleles. (Except for the top left box, there should be four boxes that receive a total of two letters.) For example, when you run down the second column and second row, you'll find that you get B and b, so you'll pencil in "Bb" for the box in the second column, second row. The third column, second row will give "bb". Keep doing this until all of the boxes are filled out in the grid, except that top left box, which is left empty.
Finished Punnett square
The complete Punnett square should look like this:
mother
[blank] B b
father b Bb bb
b Bb bb
The bottom right 4 boxes are the types of progeny you could get from parents with the given genotypes.
Step 5: Answer the questionYay! You've done the Punnett square, now you just need to
answer the actual question. What chance is there that the progeny (aka the child) will be blue-eyed?
Look at your four new combinations in your completed Punnett square. Two of those four are now filled with 'bb' - the blue eyes phenotype. Using a little math, 2/4 simplifies to 1/2 so 50% chance is the answer you need to write down.