Vadoma
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The Vadoma, also Wadoma (singular Mudoma) are a tribe living in the west of Zimbabwe, especially in the Urungwe and Sipolilo districts on the Zambezi river valley. They have few contacts with the Bantu majority.
A substantial minority of this tribe has a condition known as ectrodactyly in which the middle three toes are absent and the two outer ones are turned in, resulting in the tribe being known as the "two toed" or "ostrich footed" tribe. This is an autosomal dominant condition resulting from a single mutation on chromosome number seven. It is reported that those with the condition are not handicapped and well integrated into the tribe. Indeed it may help in tree climbing. The Kalanga of the Kalahari desert also have a number of members with ectrodactyly, and may be related.
These Vadoma, known as the "ostrich people" or the "two-toed tribe," are a popular example of the genetic effects of small population size on genetic defects and mutation. Due to the Vadoma tribe's isolation, they have developed and maintained ectrodactylyl, and due to the comparatively small gene pool, the condition is much more frequent than elsewhere.
[edit] Population genetics of Vadoma ectrodactyly and Tay-Sachs syndrome
Another example of genetic effects in small populations is Tay-Sachs disease. This is a rare inherited disease found much more frequently in certain human populations, such as Ashkenazi and French Canadians, which were small and close-knit during a period of their history. As opposed to the condition in the Vadoma, Tay-Sachs disease inevitably leads to an early death. As the mode of inheritance is autosomal recessive, however, the condition will not "purge" itself from the gene pool.
Comparing the Vadoma and Tay-Sachs disease, it is obvious that for a dominantly inherited mutation to survive its effects must have a net beneficial or neutral effect, or at least decrease the odds of survival and successful reproduction less than other diseases affecting the population. In the first case (beneficial), the mutation will spread, in the second one it will initially remain at a low frequency and in time either die out from chance events or due to changing environmental conditions gain a net beneficial effect. The situation is more complicated with slightly deleterious mutations; if the overall mortality from all causes is high in then population, they can be "drowned in background noise" for enough time that their effect can change into an advantage due to change of environmental conditions. Alternatively, they may even confer an advantage in certain situations, such as with sickle cell anemia in malaria-ridden regions.
In contrast, a recessively inherited mutation will only cause its effect to manifest when all copies of the gene where it is encoded are of the mutant form. Thus, even if the consequence would be spontaneous abortion of the developing embryo - the most drastic deleterious effect imaginable -, the mutant would persist in individuals where it is suppressed by the dominant unmutated allele. Thus, while the frequency of a recessively-inherited harmful mutation won't exceed a certain threshold (which varies somewhat depending on the circumstances), it will on the other hand not completely disappear from the population, barring chance or sophisticated assortative mating strategies (like genetic counseling in the case of Tay-Sachs disease):
[edit] References
- NCIB abstract The two-toed Wadoma--familial ectrodactyly in Zimbabwe
- The two toed tribe
- Ripley's believe it or not (with photo).
- Not Just Making Babies Photo half way down.
