Re: Where can i find info on Zimbabwe tribe Wadomo that have ectrodactyly

Thanks for the interesting question. It sounds like you already have quite a lot of information about Ectrodactyly, an hereditary defect in which the hands and feet develop into large, two-membered "claws" (e.g. the inserted image), so I think that I will be brief on the subject, and will only offer a few comments. Some information about the Kalanga people of Botswana and Zimbabwe is also available on the Ethnologue web site.

You can find a comprehensive review of the "stats" of ectrodactyly, at this page on the Online Mendelian Inheritance in Man (OMIM)web site. This page includes numerous references detailing the studies and populations in which this syndrome has been observed. However, while one defect that contributes to Ectrodactyly does map to chromosome 7 as described on the OMIM web site, another defect has been mapped to chromosome 19 .

This brings us to an interesting issue regarding the relationship between genetics and phenotype when we think about the potential causes of ectrodactyly. First of all, I want to point out that inbreeding does not cause genetic defects. Now, the development of an organism is very complicated, and we really can’t go into great length here, but "normal" development depends on the correct expression of a very large number of genes and the proper function and interaction of their protein products. As you can see on this comprehensive page of digit deformations, there are many ways for the development of the "normal" five-fingered hand to go awry. Ectrodactyly results when the middle "rays" (structures that will eventually develop into bones) of the hand and foot are not created during development. As a result, there is nothing to build the middle digits from. The number of different ways in which this development can go awry (all of those different dactylies) tells us something about the number of different genes and proteins involved in the development process. Looking at the list, it is obviously a large number. The fact that ectrodactyly maps to at least two places in the genome (chromosomes 7 and 19), suggests that there are at least two gene/protein pairs involved in the creation of the middle rays of the developing limb terminus. I also want to point out that there are numerous other deformations associated with each of these dactylies, and that these deformations have nothing to do with limb development, so these genes and proteins clearly have multiple functions.

In addition, ectrodactyly syndrome results from the activity of a dominant gene. This means that you only have to inherit one ‘defective’ copy of the gene in order to have the ectrodactylous genotype. I put defective in quotes, because the ectrodactylous pheontype does not really seem to have a large negative impact on the lives of ectrodactylous individuals. I have not been able to determine if any of the ectrodactylous individuals have two copies of the defective gene or only one, although I suspect that two copies might be lethal. Because numerous examples of ectrodactyly have been seen in unrelated populations and families around the world, I suspect that independent mutations occurred around the world, and gave rise to the groups of ectrodactylous people that we see today. Because this mutation is relatively uncommon, it seems likely that there are many possible defects which can be made in the genes responsible, but that most of them are extremely deleterious, and lethal. Modern ectrodactylous people are lucky enough to have inherited a gene which resulted in an interesting phenotype, but did not distrupt their development enough to be lethal.

Many hundreds of millions of years ago, the first four-legged animals (tetrapods) were evolving. The early tetrapods were much like us in many ways, but the number of digits they had varied dramatically from what we see today. Modern tetrapod species have between one and five digits on the end of each limb, but some of our ancient tetrapod ancestors had at least six, seven or even eight digits. It seems that in the long run, the tetrapods with five digits were the most successful of the lot, but evolution’s experiments with digit number did not end with the success of five-digited tetrapods. It seems likely that changes in the number of digits, both in those ancient tetrapods and in modern organisms with fewer than five digits, have had their basis in mutations in the genes that are responsible for the various dactylies we see today. That is a long-winded way of saying that ectrodactyly is not a new phenomenon – these sort of mutations have been occurred many times over hundreds of millions of years, and they are part of what makes a species adaptive and strong.

OK, that wasn’t quite as brief as I had originally imagined, but I hope it helps.