Kan^ofi Academii of Science. 



243 



INHERITANCE OF FERTILITY IN SHEEP. 



Edward N. Wkstwcjrtii. 



THE inheritance of fertility in mammals is difficult of in- 

 vestigation because of the large number of environmental 

 influences which may limit the full expression of the inborn 

 ability of the animal with reference to gamete production. 

 Sheep are particularly difficult animals to study in this par- 

 ticular, because of their extreme sensitiveness to changes in 

 climate and food supply. Evvard has shown that ewes gaining 

 lightly at time of mating produced only 1.44 lambs; ewes gain- 

 ing a medium amount, produced 1.59 lambs; while ewes gain- 

 ing rapidly produced 1.8 lambs. Hence if the basis of selection 

 for high fertility be numbered of offspring at a birth, it is 

 possible to make rather serious mistakes due to the failure 

 of the number actually produced by a ewe to correspond closely 

 with the inborn fecundity of the animal. 



The figures quoted in the following paragraphs are derived 

 from a study of 3,715 Southdown births recorded in pedigrees 

 started from single births, and 5,313 matings recorded in pedi- 

 grees started from twin births. Table I shows the results for 

 the first group and Table II the result for the second group. 



Table I. — Pedigrees started from single births. 



RELATIVE INFLUENCE OF SIRE AND DAM ON BIRTH NUMBER. 



Table II. — Pedigree started from twin births. 



RELATIVE INFLUENCE OF SIRE AND DAM ON BIRTH NUMBER. 



