Section 14 — Animal genetics and breeding 



crosses. In addition, matings between abnormal 

 half-sibs produced normal and abnormal calves. 

 Thus, the factor appears to be monofactorial, 

 autosomal, dominant with incomplete pene- 

 trance. 



The viability of the abnormal heterozygotes is 

 reduced up to adult age. 



(1) Out of 9 "Tete de mouton" calves kept 

 under special nutritional conditions only 

 4 lived more than 2\ years. 



(2) The perinatal mortality of Charmant's 

 calves was 32 per cent ± 12.2 among the 

 "Tete de mouton" calves and only 4 per 

 cent ±1.4 among the normals. 



(3) The embryonic mortality — estimated in- 

 directly by the proportion of returns for 

 service among inseminated cows, between 

 30 and 120 days after insemination — was 

 higher in the case of Charmant (8.62 

 per cent ± 0.44) than in the total of all 

 other Limousin bulls used in the A.I. 

 Center during the same period (6.02 

 0.14). 



Considering all degrees of expressivity (early 

 embryonic mortality, perinatal mortality of the 

 abnormals and viable "Tete de mouton"), 

 penetrance of the factor can be estimated at 21 

 per cent for heterozygous fertilized eggs. 



Further information on this study will be pub- 

 lished in Annates de Zootechni. 



14.42. Sex Chromosome Mosaicism in Unlike 

 Sexed Cattle Twins. N. S. Fechheimer, M. S. 

 Herschler and L. O. Gilmore (Columbus, 

 U.S.A.). 



Female calves born co-twin to males exhibit 

 maldevelopment of the reproductive apparatus 

 when there has been vascular anastomosis of 

 the twins' circulatory systems in utero. Such 

 twin pairs also exhibit an erythrocyte mosaicism 

 and a tolerance to reciprocal skin grafts. This 

 fact has led previous investigators to postulate 

 that cells other than erythrocyte precursors may 

 be exchanged. 



This hypothesis has been examined using the 

 leucocyte culture method In spite of the large 

 diploid chromosome number in cattle (60), the 

 X and Y chromosomes are morphologically 

 unique. All autosomes are acrocentric. The X is 

 large and submediocentric, while the Y is medio- 

 centric and much smaller than the X. Only the 

 expected XX or XY sex chromosome constitu- 

 tion was observed in intact cells from seven 

 single born females and six single born males 

 respectively. 



Among 13 animals from 11 sets of unlike 

 sexed multiple births 7 females (presumed free- 

 martins) and 4 males possessed cells of both 

 male and female origin. Sufficient numbers of 

 cells from the remaining two calves were not 

 examined. Mosaicism of erythrocytes for anti- 

 genic properties was manifested by the cells of 

 9 of 10 animals for which a test was made. 



These findings raise the possibility that the 

 intergrade sexuality of females co-twin to males 

 is caused not by a humoral agent produced by 

 the male but may be a function of the sex 

 chromosome mosaicism. 



14.43. Heterosis from Crosses among British Breeds 

 of Beef Cattle. R. C. Carter, W. H. McClure, 

 J. A. Gaines and D. W. Vogt (Blacksburg, 

 U.S.A.). 



This experiment was designed to estimate 

 heterosis in beef calves resulting from various 

 levels of breed crossing, not confounded with 

 heterosis in maternal traits of their dams. Pure- 

 bred or high grade Aberdeen-Angus, Hereford 

 and Shorthorn cows, here designated as 

 "straightbreds", were mated to purebred bulls of 

 these three breeds to produce straightbred and 

 reciprocal two-breed cross calves. Similar 

 straightbred cows were bred to crossbred bulls 

 (Fi) to produce three-breed and back-cross calves. 

 This design permitted comparisons among 

 straightbred, two-breed, three-breed, and back- 

 cross calves, born in the same year, and all from 

 straightbred dams. Five calf crops were produced, 

 1957 through 1961 , with a total of 504 calves born 

 alive and 480 weaned. 



The most important differences were in fert- 

 ility and viability. Cows in crossbred matings 

 had 8 per cent more calves born and 1 1 

 per cent more calves weaned than those 

 bred to a bull of the same breed. There was 

 evidence for heterosis in birth weights, growth 

 from birth to weaning, and postweaning growth. 

 Average birth weights, adjusted for sex differ- 

 ence, were, in pounds: straightbreds, 68.4; two- 

 breed crosses, 71.4; three-breed crosses, 70.4; 

 and back-crosses, 69.3. Average weaning weights 

 for the four groups were respectively: 402, 426, 

 433 and 413 lb. There were significant deviations 

 from mid-parental means for birth weight in two 

 breed and back crosses; and in growth from birth 

 to weaning and weaning weights, in two-breed 

 and three-breed crosses. Differences in post- 

 weaning growth rates were generally smaller 

 than in growth to weaning, but crossbreds were 

 heavier at all periods. Differences in feeder 



265 



