My experience with estimating heritability of pig traits on behalf of the
Animal Breeding Research Organisation in Edinburgh was that it was not easy
to get precise estimates even with large samples (thousands rather than
hundreds of animals). The method involved hierarchical analysis of variance
(between sires, between dams within sires, and within litters), and the
genetic components of variance were estimated by differencing, and since
each sum of squares was distributed propotionally to chi-squared one
frequently had apparently negative components of variance even when the
expectations were positive. If the measured traits were not normally
distributed (e.g. binomial presence-absence only) the interpretation of the
estimated heritabilities was even more problematic. The estimation of
genetic correlations was even more difficult, and only possible if estimated
heritabilities were positive.
The purpose of estimating heritability in animal breeding is to know
which traits can be improved in a reasonable number of generations. Genetic
correlations predict how breeding for one trait may affect other traits,
possibly undesirably. Under inbreeding schemes heritability declines with
successive generations. Heritability was more easily studied with small
animals (mice and flies) where numbers were greater and more intense
inbreeding could be used.
I am not up to date with how they do it now at Roslin, say, where it is
all GM nowadays rather than old-fashioned selection of parents. My old
textbook, Introduction to Quantitative Genetics by D.S. Falconer is still
very useful on the basic statistics.
How this relates to the current discussion of human heritability is not
clear, but the data here is presumably even less susceptible to proper
design.
Gavin Ross
----- Original Message -----
From: "Daniel Davies" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, June 17, 2007 6:48 PM
Subject: Re: Hereditability
>I really didn't expect to see the heritability debate being carried out at
> such a low level on a specialist statistics list. Heritability refers to
> the ratio of the within-family variance of a particular characteristic of
> interest to the between-family variance. It does not mean "the percentage
> which is genetically determined". The philosopher Ned Block piquantly
> observed that "number of legs" has very low heritability, since almost all
> the variance in observed leg number is a result of amputations or
> accidents,
> while "speaking Chinese" has comparatively high heritability, since most
> people who speak Chinese have parents who also speak Chinese.
>
> besst
> dd
>
> -----Original Message-----
> From: email list for Radical Statistics
> [mailto:[log in to unmask]]On Behalf Of Paul Spicker
> Sent: 17 June 2007 17:09
> To: [log in to unmask]
> Subject: Re: Hereditability
>
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