Hi.
The difference between expected and observed heterozygosity gives you the f coefficient which says you whether your population experiences inbreeding or not, right?
But since these differences between observed and expected heterozygosities are deviatons from H-W equilibium, are they also useful for assesing the effect of other evolutionary forces other than non-random mating? What other kind of information can you get from this difference? Does the wahlund effect deals with this difference?
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In general, a deficiency in heterozygotes is an indication of inbreeding, as inbreeding increases the frequency of homozygotes at the expense of heterozygotes. For this to continue to "fixation," where the population is composed only of homozygotes and no heterozygotes (we are only talking about a single locus here) means that the two homozygous phenotypes only mate among themselves since any mating between the two will produce nothing but heterozygotes. If you see this in a captive population at a fish hatchery, it is probably a good indicator of inbreeding. You will seldom go wrong if you assume that a captive fish hatchery population is inbred. Few hatchery managers make any effort to prevent inbreeding, let alone know what causes inbreeding or what inbreeding does to the population.
In the wild, a deficiency of heterozygotes can be caused by what is termed the Wahlund effect, and it is likely due to a mix of two sub-populations that mate mostly among themselves but overlap. In unusual situations, it could be caused by a decrease in fitness for the heterozygotes, but for you to make that assessment you would need to show that this was the case.
Thanks for your answer! I will pay attention on Wahlund effects. So, is it expected that the number of observed heterozygotes will be low when you have a clear population structure?
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No. That can occur if and only if you happen to have a mix of two sub-populations and the "opposite" homozygous genotypes dominate in each.
You also have to consider one other reason for a lack of heterozygous genotypes--sampling error. This is a real possibility. It is not an explanation that most biologists like to propose about their data, but sampling error is always a possibility.
Lack of non-additive genetic effects and variance, no specific combining ability effects between the populations involved in the cross, history and selection practices followed are the plausible causes. Estimation of the magnitude of the type of genetic variances in the lines crossed will give a clue.
lnathgubbi
Further to my reasoning provided earlier, the role of Natural Selection in relation to artificial selection, number of generations of selection undergone by the lines involved in the cross for exploiting heterosis will throw some insight to the situation encountered.
lnathgubbi
Dear Ringo
The decrease in the heterozygous in the population results from inbreeding which increase the homozygous genotypes and decrease the heterozygous.
By comparing the number of heterozygotes observed to the number expected for a population in H-W equilibrium, we can estimate the degree of inbreeding.
A measure of inbreeding in the population is “inbreeding coefficient”, F.
F = 1 - (obs. hets) / (exp. hets).
The higher F is the more inbreeding in the population.
If the F = 0, the observed heterozygotes are equal to the expected number, meaning that the population is in H-W equilibrium.
But in case of F = 1, there are no heterozygotes, indicates a completely inbred population.
Good Luck
@Douglas Tave: Thanks a lot for your answer... So (if I am understanding this well) is the mix of those two subpopulations with opposite homozygous genotypes who is explaining the deficiency of heterozugotes ?.
@Loknath G.R. : Thanks for your answer... So, can I considerer selection (natural and artificial) even tough I am assessing neutral genetic variation?
@Khalid Hassan: Thanks for your answer... You are right, according to F coefficient calculations I have a lot of inbreeding in my population. Anyways, likelihood approaches found that inbreeding is lower than I thought. But now that you mentioned it, I remembered that my species has a high rate of autopolinization. So, do you think autopolinization may be considered as an extreme case of inbreeding, which be explaining the high F values I am getting?
So much is said and done about this. One more last opinion. Rest assured that the observed heterozygosity though present is not to the extent of expected. The latter is more of a theoretical consideration than the actual outcome. This could be the result of drainage of non-additive genetic variance (dominance/epistasis and other genetic interactions), which do not continue to exist consistently in long term selection experiments. Hence, within- line genetic study is desirable to know the nature of actual genetic variances and the magnitude of such variances.
Thanking you,
lnathgubbi
If you find a deficiency of heterzygotes for a locus in the wild the Wahlund effect is a possible explanation and that is likely due to the mixture of two subpopulations. But until you actually show that this is the case, it is only a possible answer. One thing you must always remember when you sample a population is your data may be influenced by sampling error. Even when you take an adequate sample, sampling error can occur.
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