Probability theory turns out to have applications in biology and genetics.
Example: Human blood types are very complicated, but some simple aspects can be understood with reasonable accuracy.
83% Rh+, 17% Rh- in Caucasians. Nearly 100% Rh+ in other populations. At a high level, there are two types of ``alleles'', one that produces Rh factor, and one that doesn't. Each individual has two alleles. Two negatives make an Rh-, at least one positive makes an Rh+.
From some simple equilibrium assumptions: 65.5% +- (heterozygous), 17.5% ++ (homozygous positive), 17.0% - (homozygous negative).
Knowing the Rh status of a kid is important because an Rh- mom can make antibodies against an Rh+ kid, presenting major difficulties for later Rh+ kids.