Reproduction in young sheep: some genetic and environmental sources of variation

"The extent of genetic and environmental variation" in the development of reproduction in sheep is illustrated by examples with particular reference to variation among breeds and to the effects of photoperiod. The interactions between genetic and environmental effects are introduced; these may be so great that genetic groups may reverse their ranking for rate of development in different environments. The "physiology of puberty" is then discussed. The difficulty of separating puberty from seasonal variation is stressed, and a possible contrast is drawn between the physiological characteristics of genetic variation and those of environmental variation in reproductive development. Finally the physiological factors associated with sterility in young females are discussed; most studies, however, have been conducted during the time of year when adult females would also be expected to be sterile, so that conclusions are difficult and a "missing link" cannot be identified.