STUDY HINTS

The rules of Mendelian inheritance are largely based upon the probabilities of inheriting a given allele or combination of alleles from a specified genotype. With the exception of sex-linked traits, each individual carries at most two alleles of each gene, and the number of possible combinations is limited. Expectations can be calculated easily, since the probability of getting a certain allele from a known heterozygous diploid is p=q=.5.

In a population the focus changes from the genome to the gene pool and to estimates of allele frequency, yet the rules of probability still apply. Allele frequencies p and q are determined by pooling all of the genes carried by all of the individuals in the population. Working problems that require estimating allele and genotype frequencies from the Hardy–Weinberg relationship are fairly straightforward if you keep in mind that you are dealing with probabilities and if you are consistent with your use of symbols.

For example, if we let the frequency of the A allele be p and the frequency of the a allele be q in a population in Hardy–Weinberg equilibrium, p+q=1, as long as A and a are the only alleles at that locus segregating in the population. In other words, the probability of picking an A allele at random plus the probability of picking an a allele at random account for all possible events. The genotypes that can be produced and their probabilities are shown in the following table.