I’m doing a piece about an incredibly inbred pack of wolves, and I want to be sure I understand the COI properly. Can anybody explain it in simple terms? Some of the sites I looked at got pretty esoteric pretty quickly.
My interpretation is that it’s an estimate of the percentage of shared genes, from 0.0 (completely unrelated) to 1.0 (identical twin). Is that a fair statement?
In one example, one wolf bred with his daughter, who in turn bred with their son. What is the COI of the son? What is the COI of that last litter?
And just to cover my bases, what is the difference or relationship between COI and the coefficient of relationship?
IIRC, COI is the measure of the increased chance of homozygosity due specifically to inbreeding. That is, let’s say that two random individuals in a population have a 5% chance of producing an offspring that’s homozygous at some locus, but two specific individuals have a 25% chance because they’re closely related. The COI would then be 25-5=20%. That’s digging up some long, long-buried memories, though, so I could be wrong.
(I’ve taken the liberty of labeling the four “players,” G, D, S, L.)
Assuming that G is completely unrelated to D’s mother and neither is themselves inbred (rash assumptions, I’ll guess) then S’s inbreeding coefficient is 0.25 and L’s is 0.375. If, instead, G and D were themselves the results of inbreeding and each had inbreeding coefficient 0.1, I think that S’s and L’s coefficients would be 0.275 and 0.4125, i.e. 10% higher. I think.
I think Smeghead’s comment is essentially correct. On a blackboard I might be able to show you how to calculate these coefficients and convince you it satisfies Smeghead’s criterion, but I won’t try without the blackboard.
The COI applies to an individual, and is related(*) to the coefficient of relationship between that individual’s parents. So the COI of an identical twin depends solely on the parents, not the existence of the twin. (One could ask what the COI of offspring would be if the twins mate but identical twins are same-sex and I don’t know if “Wright’s Inbreeding Coefficient” is usually applied to hermaphroditic species. )
It may be interesting to note that if father and mother are unrelated, their children have COI of zero no matter how inbred father and mother themselves are. This follows directly from Smeghead’s homozygosity definition.
(* - I don’t know if “coefficient of relationship” (COR) has a meaning as standardized as Wright’s COI, but as Wikipedia defines it, a COI will be identical to the COR between parents if common ancestors are not themselves the result of inbreeding. The COI has a correction factor since homozygosity of the common ancestor increases homozygosity likelihood of the descendant.)
In the particular case I’m looking at, D’s mother would have been incredibly inbred (researchers estimated the COI of some animals at .81) and G was the first new blood in decades. The COI dropped enormously at first, but has started to rise again thanks to G’s and D’s offspring.
How’s this for a layperson explanation?
“The measure of how closely two animals are genetically similar is called the coefficient of inbreeding. Offspring get 50% of their genes from each parent, so the pup of two unrelated wolves has a coefficient of 0.5 with its parents and littermates.”
As for twins not mating, wasn’t there a theory awhile back that some “fraternal” twins were identical but for a “broken” X chromosome, making a YX pair in what was supposed to be an XX? I ought to look that up. . .
COI (I=Inbreeding) applies to a single individual. COR (R=Relationship) applies to a pair. COI is almost (but not quite) the same as one-half the COR between the individual’s parents. (I think I neglected the “1/2” in the earlier post.)
It sounds like you’re using a different measure than Wright’s COI. In Wright’s system the COI of the offspring of unrelated non-inbred parents is zero. A COI above 0.5 requires very extreme inbreeding. (An individual with COI=1 is homozygous in every gene.)
The COR between an individual and one of its parents or siblings is 0.50 so if they mate (and there is no other inbreeding source) the offspring’s COI will be 0.25.
Here is the argument that proves the offspring of siblings has COI=0.25 : One allele comes from one of father’s parents. The other allele comes from one of mother’s parents, but since father and mother are siblings, those parents are the same. There is a 50% chance both alleles come from the same grandparent. In that case, since the grandparent has two alleles, there is a 50% chance the same allele is inherited twice. 50% x 50% = 25%, the chance of homozygosity at any gene.
Ah, no. I don’t know where you heard that, but that’s not biologically possible. A Y chromosome is not simply a shortened X. They’re very, very different things.
Interestingly*, though, it could sort of work the other way around. The Y chromosomes contains the SRY genetic locus, which is sort of the master switch for maleness. If you have the SRY, you head down the path that leads to being male; if you don’t have it, you stay with the default female configuration. There have been instances of XY female humans where the Y chromosome has had a deletion that included the SRY region. So if you have a shortened Y chromosome, you could get a female. The odds of this happening in one twin but not another, though, are astronomical.
*I originally wrote “ironically,” but these days using that word is like putting a target on your forehead.
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