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Herein lie the "Lost" Boreas Files by Rockessence

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Author Topic: Herein lie the "Lost" Boreas Files by Rockessence  (Read 7546 times)
Janna Britton
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« Reply #165 on: November 16, 2008, 03:49:36 am »

Two different mutations
Most studies for practical reasons have focused on lactase persistence in Europe, but lactase persistence is also common in certain tribes in Africa that have a history of dairying. Is lactase persistence in these people caused by the same mutation - as would seem likely - and has it been under positive selection as well? The first part of this question has been answered by Mulcare et al.13 Their paper shows that the putative causative allele 14 kb upstream from the lactase gene is not at frequencies high enough for it to be the causative allele in Africa, even when the inherent errors in lactose tolerance testing are taken into account. There could be two reasons for this - either the allele is not causative at all and is merely strongly associated with the causative allele, or in Africans lactase persistence is due to another mutation. The first reason is possible, especially given the high LD across the region - many polymorphisms within this region will be strongly associated with lactase persistence just by virtue of being on the same huge haplotype. But functional studies from two groups show that the putative causative allele is a gain-of-function mutation increasing the expression driven from the lactase promoter in reporter gene assays in a human intestinal cell line.14, 15 So what about the second reason - a different causative mutation in Africans? Intuitively, this seems unlikely, but given the powerful selective advantage of being lactase persistent any mutation is very unlikely to be lost by genetic drift. It is possible that another mutation in the same regulatory element, a different element, or even in a trans-acting transcription factor may be responsible for lactase persistence in Africans. The answer will only be found by further genetic analysis of this locus in Africans.

Mendelian composits
As well as examining the role of this polymorphism in human evolution, this work provides an interesting case study for those concerned with finding alleles that confer susceptibility to common disease. In this case, we have a clear clinical phenotype (lactose tolerance) with a very strong well-defined Mendelian genetic component (lactase persistence/nonpersistence polymorphism), and a well-defined 'candidate' gene (LCT, lactase). Despite these factors, the causative polymorphism has proved difficult to discover, and the most likely causative polymorphism is located 14 kb away in an L2 repeat within an intron of another gene. Added to this, if this polymorphism is causative, then it is not the causative polymorphism in all populations. If there is a lesson to be learned from this, it is that the genetics of complex disease are likely to be very complex indeed.


European Journal of Human Genetics (2005) 13, 267-269. doi:10.1038/sj.ejhg.5201297 Published online 15 December 2004.
E-mail: ed.hollox@nottingham.ac.uk

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