title: Modelling sub-binomial variation in the frequency of sex combinations in litters of pigs.
abstract: In some samples of pig litters, the number of males per litter exhibits significant sub-binomial dispersion: there are fewer unisexual and more "sex-balanced" litters than expected under a binomial model. It is argued here that the explanation is that the sex of zygotes may be subject to hormonal control with the consequence that, within a litter, the zygotes do not all have the same probability of being male. This variation in probability is approximated by a quadratic function of the time during oestrus that a zygote is formed so as to parallel the variation in maternal hormone levels. Other explanations are examined briefly, but each seems to have serious weaknesses.
authors.affiliation: Department of Statistical Science, University College London, England.
authors.forename: R J
authors.initials: RJ
authors.lastname: Brooks
authors.forename: W H
authors.initials: WH
authors.lastname: James
authors.forename: E
authors.initials: E
authors.lastname: Gray
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D016010:Binomial Distribution; D001699:Biometry; D005260:Female; D008098:Litter Size; D008297:Male; D015233:Models, Statistical; D016012:Poisson Distribution; D011247:Pregnancy; D012744:Sex Ratio; D013552:Swine