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A Canterbury University geneticist, working with a colleague in the United Kingdom, has identified an unusual biological phenomenon in the genes linked to the fertility of domesticated sheep.
Associate Professor Neil Gemmell (Biological Sciences) and Dr Jon Slate at the University of Sheffield have just had a paper published in PLoS ONE, an open access online journal published by the Public Library of Science. The paper describes a new example of heterozygote advantage, the situation where individuals with two different copies of a genetic variant (allele) are fitter than individuals that possess two copies of the same gene variant.
Dr Gemmell said the discovery was significant in that heterozygous advantage is one of several explanations for the existence of genetic variability in natural populations. However, despite being first proposed more than 80 years ago only one or two other examples of heterozygote advantage have held up under scrutiny, the classic example being variations in the blood protein haemoglobin that cause sickle cell anaemia.
“What we've done is highlight a suite of mutations that appear to indicate heterozygote advantage that have not been previously recognised as such by the evolutionary genetic community or by the biological community at large.
“We've recognised the significance of these mutations, so other people can explore them further.”
The phenomenon discovered by Dr Gemmell and Dr Slate was found in a variant gene, called the Inverdale gene, associated with the elevated fertility of domesticated sheep.
“Under normal circumstances an average or wildtype sheep will produce 1.8 lambs per breeding attempt but sheep heterozygous for this genetic mutation, possessing one mutant and one normal gene copy, will produce an average of about 2.8 lambs per breeding attempt. That's an increase of about 50 per cent.
“The really interesting thing is that if a sheep has two copies of the mutant gene it will be sterile and have what's called streaked ovaries.”
Dr Gemmell said the Inverdale gene was isolated by scientists at AgResearch about seven years ago but the significance of the discovery for the evolutionary biology and genetics community was not recognised at that time.
“That's where we came in,” said Dr Gemmell. “We noticed that the data they had strongly suggested a heterozygote advantage situation. What we tried to do was work out under what conditions heterozygote advantage would be maintained or persist, and what the frequency of these mutations would be in natural populations.”
The paper contains a close analysis of the data collected by AgResearch and also reviews the evidence surrounding other suggested examples of heterozygote advantage.
Dr Gemmell and Dr Slate plan to take their research further by investigating whether the phenomenon can be detected in wild populations of sheep and other hoofed mammals such as cows and goats.
Editor's Note: Original news release can be found here.
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