<P> The same study also identified 18 rare alleles, which generally have a weaker glycosylation activity . People with weak alleles of A can sometimes express anti-A antibodies, though these are usually not clinically significant as they do not stably interact with the antigen at body temperature . </P> <P> Cis AB is another rare variant, in which A and B genes are transmitted together from a single parent . </P> <P> The distribution of the blood groups A, B, O and AB varies across the world according to the population . There are also variations in blood type distribution within human subpopulations . </P> <P> In the UK, the distribution of blood type frequencies through the population still shows some correlation to the distribution of placenames and to the successive invasions and migrations including Norsemen, Danes, Saxons, Celts, and Normans who contributed the morphemes to the placenames and the genes to the population . </P>

Explain how codominance and multiple alleles function in the abo blood typing system found in humans