According to the Laws of Probability, the probability of flipping 2 coins and getting 2 tails is:
P(coin1=tails AND coin2=tails) =
P(coin1=tails) x P(coin2=tails) = 0.5 x 0.5 = 0.25
Likewise, the probability of Little Mouse inheriting a vam37 allele from each parent is 0.5 x 0.5 = 0.25.
What if you’re not fond of probability equations? That’s ok in fact, most of the time you wouldn’t use a probability calculation to figure this out. Instead, you would use a Punnett Square. Remember, the Punnett Square is just a way to combine two sets of options. So, one set of options are “possible alleles from Mum”, and the other set is “possible alleles from Dad”.
Specifically, put the mother’s possible egg types on the top, and the father’s possible sperm types on the side. The kid gets one egg from Mum and one sperm from Dad, and that’s what will go in the center of the Punnett Square.
Or, you could do this as a Punnett Square. This time the mother’s possible egg types go on the top, and the father’s possible sperm types on the side. The kid gets one egg from Mum and one sperm from Dad and that’s what will go in the centre of the Punnett Square.
Here is a Punnett Square. See if you can fill it in — remember, both Mum’s and Dad’s genotypes are Tt. Click the “check” buttons at the bottom which, will tell you if you’ve filled it in correctly. (Also, notice that the top and left cells are color-coded: pink for Mum and baby-blue for Dad!)
If you count, you will find 3 normal mice and 1 with fangs. This leads to a classic ratio, namely 3:1, or three to one. Applied to our mice, that means,if Mr and Mrs Mouse have a copy of each allele, they will produce (approximately) 3 normal looking mice for every 1 vampiric baby. Looking at the Punnett Square, you can also deduce that, on average, 2 of the normal-looking offspring will have the vampire allele but not show it, while one will have no vampire allele at all.
It’s very important to realise that these numbers are approximate. The actual production of eggs and sperm and their union are random processes, so the exact number of each phenotype is only approximate. And of course, if Mr and Mrs Mouse have 5 or 7 or 19 children, there is no way that they can exactly reproduce a 3:1 ratio.