Haldane's Law

Dan Weinreich dmw at MCZ.HARVARD.EDU
Tue Aug 2 08:01:46 EST 1994


In diploid individuals, offspring get one of each chromosome from each
parent, giving it a total of two of each chromosomes.  In the case of the
sex chromosome, of which there are generally two easily distinguishable
forms (Carothers described sex chromosomes in male grasshoppers as
"heteromorphic" pairs in 1913), offspring again get one from each parent,
but homogametic individuals get similar chromosomes while heterogametic
individuals get dissimilar chromosomes.

The foregoing applies to (almost) all diploid species, and has nothing to
do with hybrid crosses.

In mammals and among most insects, heterogamous individuals are males
(that is, they're the ones who produce sperm) and homogamous individuals
are females.  But in butterflies as well as in birds, it is the homogamous
individual which are male and the heterogamous which are female.

In some hybrid crosses, all offspring are "sick," i.e. dead, rare or
sterile, but in many such crosses, only one sex is affected.  Haldane's
Rule states that in such cases, it is always the heterogametic sex which
is so affected.  If you're interested in the genetics of this phenomenon,
you might read Jerry Coyne, e.g. Nature 369:189 May 19, 1994, or the
chapter he and H. Allen Orr wrote in _Speciation_and_its_Consequences_.

Regards,
Dan.

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