Monday, October 5, 2015

A network of blood types

The relationship between phenotypes and allele frequencies is often introduced in textbooks using the example of human blood type. There are three alleles for the blood-type gene (IA, IB, IO), and these produce four phenotypes (A, B, AB, and O) since IA and IB are co-dominant and IO is recessive.

The proportions of these phenotypes vary among human ethnic groups, and this variation provides one of the simplest demonstrations that human inter-breeding does not occur at random — that is, Hardy-Weinberg equilibrium is not maintained at a global scale. This can be pictured using a phylogenetic network.

The data come from Racial and Ethnic Distribution of ABO Blood Types. As usual, the phylogenetic network is being used as a form of exploratory data analysis. I first used the manhattan distance to calculate the similarity of the ethnic groups, based on the frequencies of the four blood phenotypes. This was followed by a Neighbor-net analysis to display the between-group similarities as a phylogenetic network. So, ethnic groups that are closely connected in the network are similar to each other based on the relative frequencies of their blood types, and those that are further apart are progressively more different from each other.

You will note that very few of the ethnic peoples that are either geographically or historically closely related to each other have similar distributions of blood types. Indeed, only the Irish and the Scots are closely related both in history and in the network. So, at a global scale, breeding occurs almost entirely within ethnic groups and not between them. Widespread modern migration has not yet obscured this pattern.

There is, however, a broad range of phenotypic variation in blood type. For example, the bottom right-hand part of the network shows those ethnic groups that are dominated by the O phenotype, the top-right is dominated by type A, and the bottom-left by type B.

Of particular interest are those groups for which the B allele is not been recorded in the dataset (ie. the B and AB phenotypes are absent), which includes the Australian Aborigines, the Bororo and Peruvian Indians from South America, the Shompen Nicobars from the Indian Ocean, and the Blackfoot and Navajo peoples from North America. The Maoris and Mayans also have a very low frequency. The Bororo, Peruvian Indian and Shompen peoples also seem to lack the A allele; and it is extremely rare in the Mayans. No group lacks the O allele, but it is lowest in the people from the Grand Andaman islands in the Indian Ocean.

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