Overview

Genetics is a quantitative science. Many genetic processes (mutation, recombination, assortment of genes) are not deterministic but are based on random events. The outcome of a random process cannot be known; its frequency of occurrence is predicted by its probability, or expected frequency. Thus, one needs statistics, the mathematical analysis of random variables. To design an experiment in genetics and to interpret the results correctly – or at all – requires knowledge of probability and statistics.

This chapter introduces a few rules of probability and gives the bare minimum of statistics for describing genetic phenomena and for testing genetic hypotheses. It is just a little toolkit, to be hauled out when you need it.

Rules of Probability

A probability is a number between 0 and 1 that predicts the frequency of a random event; the sum of the probabilities of all possible outcomes is 1. During meiosis in a heterozygote A1/A2, a gamete is equally likely to receive A1 or A2, and the probability of each is ½. In general:

If event E happens in m of n equally likely events, then the probability of event E is P[E] = m/n.

Solved Problem. In a cross between two heterozygotes, A1/A2 × A1/A2, what is the probability of a heterozygous offspring? Because alleles segregate 1:1, half the eggs and sperm are A1 and half are A2.