Final answer:
The lower than expected frequency of AO blood type offspring in OO x AO matings can be attributed to genetic drift, which causes random fluctuations in allele frequencies that can deviate from Mendelian expectations.
Step-by-step explanation:
The observed frequency of AO blood type offspring in OO female x AO male mating in human populations is lower than the expected frequency of 50% because of a concept known as genetic drift. In genetics, genetic drift is a mechanism of evolution that refers to random fluctuations in the frequencies of alleles from one generation to the next. These fluctuations can result in alleles becoming more or less common purely by chance, and in small populations, the effects can become more pronounced. Therefore, the frequency of the AO blood type may be reduced in the population because alleles can fluctuate in distribution due to random sampling, rather than being strictly a result of Mendelian inheritance patterns, where a 50% frequency would be expected.
The ABO blood group system explains how inherited alleles determine an individual's blood type. In this system, alleles IA and IB are codominant to each other and both are dominant over the allele i. Thus, a genotype of IAi or IAIA would result in blood type A, and a genotype of IBi or IBIB would result in blood type B. An individual with both dominant alleles, IAIB, has blood type AB expressing both antigens equally. Lastly, an individual with two recessive alleles (ii) has type O blood.