Q:

Distribution of genotypes after adverse selection

I am doing some research into Mendelian genetics and came across this phrase called adverse selection.
I found a few questions but I am unsure of the calculations. For example given a distribution 

[0.36 0.48 0.16]

it is stated that 25% of individuals with Aa(middle) genotype dies. The distribution changes to 

[0.4091 0.4091 0.1818].

I initially thought to just take off 25% of the middle becoming 0.36 and equally distributing the 0.12 change to the other two geneotypes. Can someone explain how they did this computation.

A:

If you distribute the 0.12 equally to the two other genotypes you change the relative frequencies. You have to withdraw 0.12 to 0.48 and then set the three frequencies so that they sum up to 1. Therefore, the correct calculation is the following:
If we set $fAA = 0.36$, $fAa = 0.48$ and $faa = 0.16$ and then simulate the death of 25% of the Aa individuals, the same variables at the next time step (without reproduction) will be...
$$faa = \frac{0.16}{0.36 + 0.36 + 0.16} ≈ 0.1818$$
$$fAa = \frac{0.36}{0.36 + 0.36 + 0.16} ≈ 0.4091$$
$$fAA = \frac{0.36}{0.36 + 0.36 + 0.16} ≈ 0.4091$$

