1. A random bred population of mice was subjected to inbreeding and the changes of litter size, measured as the number of live young in first litters, were followed.2. The mean litter size declined at a rate of 0·56 young per 10% increase of the inbreeding coefficient.3. Selection for large litters within the lines during the inbreeding did not effectively reduce the rate of decline.4. Out of twenty lines at the beginning of the inbreeding seventeen were lost by the time the inbreeding coefficient reached 76%. Two more were lost later and one survived indefinitely. The three lines that survived longest started at a level below the mean and did not decline in litter size. The one that survived indefinitely reached 99% inbreeding without dropping below the non-inbred control.5. The three lines surviving at 81% inbreeding were crossed and the litters produced by the crossbred progeny were larger than the non-inbred control by about two young per litter. This gain from heterosis is attributable to selection among the lines on their performance as inbreds. A second and third cycle of inbreeding and crossing yielded no further progress, and the level of the first cross was never regained. This is attributable to the ineffectiveness of the selection applied and to the previous restrictions of the population size.6. The behaviour of the lines in the inbreeding and crossing point to simple dominance rather than over-dominance at the loci causing variation of litter size.7. This experiment suggests that, as a means of improvement of farm animals, cyclical inbreeding and crossing does not look very hopeful.

SUMMARYRecords of 178 593 monoparous births between 1964 and 1970 in Israeli-Friesian dairy cattle were analysed for information on environmental and genetic factors affecting calving difficulties and perinatal mortality. The incidence of difficult calvings (DC) and peri-natal calf mortality (PM) were 6·9 and 9·1% in heifer calvings, and 1·6 and 4·1% in cow calvings. Calving characters were not related t o heifer age nor cow parity, but there was a seasonal trend in heifer calvings, with both DC and PM being high in winter and low in summer calvings.Heritability estimates for calf-genotype effects on PM were 0·042 in heifer calvings and 0·013 in cow calvings. Heritability estimates for dam-genotype effects on PM were 0·018 and 0·004 respectively. Heritability estimates for DC were similar t o those for PM. The genetic correlations between DC and PM in the same class of calvings ranged between 0·8 and 0·9, while the environmental correlations were considerably lower, about 0·3. The genetic correlations between the calf-genotype effects in heifer and cow calvings were about 0·5, indicating that different factors may affect heifer and cow calvings.It is proposed that sires already proven for milk yield be tested for their calf effects in heifer calvings, using contemporary comparisons within herds and seasons. Results will be available when the sires are 6 to 6½ yr old. Sires causing fewest calving difficulties would be nominated for heifer matings, so that most heifer inseminations would be by sires proven for both yield and calving characters. Some long-term progress might be obtained if sires with a high incidence of difficulties in their heifer mates were not used to produce young sires.

1970). Photoperiodism in the ewe: 2. The effects of various patterns of decreasing daylength on the onset of oestrus in clun forest ewes. SUMMARYIn six light-proofed buildings experiments were conducted with Clun Forest ewes to determine whether the date of first oestrus was affected by the absolute size of reduction in the case of an abrupt decrease in daylength and by the rate of reduction in the case of a given total decrease in daylength. The mean number of days from the onset of the light treatments to first oestrus (the reaction interval) for ewes receiving gradual decreases in daylength of 3-5, 7-2 or 10-9 min/day from 1 July were 66-4, 53-7 or 33-6 days, respectively. The reaction interval for ewes receiving abrupt decreases in daylength of 3-75, 7-75 or 11-75 hr on 1 July were 59-5, 44-8 or 33-6 days, respectively. A control group of ewes at pasture on natural daylength had a mean reaction interval of 66-2 days. It is concluded that the various light treatments applied significantly affected the date of onset of oestrus in Clun Forest ewes.

SUMMARYAn estimate of the frequency of twin calvings was obtained from the analysis of over 38 000 calvings of Friesian cows in M.M.B. milk-recorded herds. The incidence of twinning increased from 0·54 % in the first parity to 3·37% in the fifth parity. A significant excess of cows above expected produced either more than one set of twins or no twins at all. Repeatability ranged from 0·0 for cows with three calvings to 0·063 for cows with five calvings.Data on lactation yield and duration, butterfat percentage and calving interval showed no significant effect of twinning. However, consistent trends were noted showing that dams which calve twins have longer subsequent lactations, higher whole lactation yields and lower fat percentages than contemporary dams with single calves. The possible causes for these trends are discussed and are considered to be the lower breeding efficiency and longer calving intervals of dams of twins.

A Robertsonian translocation in the heterozygous condition was observed in a phenotypically normal Friesian bull. This chromosomal abnormality was probably transmitted by the dam, which also displayed heterozygosity. It is suggested that this cow could have been the original carrier. Chromosome studies on forty-six animals in the pedigree of the heterozygous bull are described.Karyotypically normal cows (139) were inseminated with frozen semen from the bull and ninety-one calves were born. The distribution of the translocation and the sex ratio in the progeny group did not significantly deviate from 0\m=.\5.One heterozygous male calf died from a septal cardiac defect but the association between the deformity and heterozygosity was considered coincidental. The fertility of the bull was normal and meiotic studies were undertaken after slaughtering.