Just considering one locus (a specific location on a DNA strand), evidence at a crime scene shows a genotype of 5, 9 STR repeats at this locus. The general population frequencies for the alleles 5 and 9 are 0.100 and 0.300, respectively. How frequent would the genotype 5, 9 be expected to be in the general population

0.1 x 0.3 = 0.03 = 3%

Explanation:

Assuming a process of random mating in the population, it is possible to infer that the genotype frequencies will be the product of the allele frequencies. The random mating is one of the postulates of the Hardy-Weinberg Law, which is widely used in population genetics to estimate genotypic frequencies when populations are expected to be in an equilibrium state (it is also expected by default in human populations).  In this case, the genotype frequencies will be certainly low (3%), thereby this locus may be useful to investigate a police case.