(i) The structure and genetic history of the pedigree Hereford breed in Australia are analysed by pedigree sampling methods. (ii) The pattern of the breed structure is generally similar to that found in other breeds, but it is extremely dynamic owing to the present rapid expansion of the breed. Changes are taking place in the herd composition of the major breeders' groups and there are many new herds yet to find their level in the structure. (iii) Considerable emphasis has been placed on the use of imported animals in the development of the breed. Of all herds registering in Volume 24 of the herd book, 31.6% used imported sires, and the percentage of genes in the breed in 1949 derived from animals imported since 1880 was 97.7, and from animals imported since 1930, 53.9%. (iv) The most important herd in 1949 made a genetic contribution to the breed of 33.1%, while the contributions of the four next most important herds were 17.4, 10.1, 7.4, and 6.1%. In the four-generation pedigrees from which these figures were derived, the contribution of imported animals was 56.1%. (v) The animal with the highest relationship to the breed was Free Town Director (Imp.), with direct relationships of 5.6 and 9.2% to the 1941 and 1949 samples respectively. Of the 16 sires and one dam whose direct relationships are 3.0% or more in any of the three sample years, 12 sires were imported. (vi) The total inbreeding in 1949 (base year 1880) was 2.6%. This comprised 0.2% current inbreeding, 1.6% long-term inbreeding, and 0.8% strain inbreeding. There was no evidence of subdivision of the breed into separate strains. (vii) The effective generation length has decreased since 1930 to about 5.5 years in the 1949–50 sample. Nearly 50% of the animals in this sample were sired by bulls 4 years old or younger, while about 33% were from dams 4 years old or younger. (viii) Bulls from major breeders' herds are used widely throughout the breed, while about 90% of sires and 45% of dams were bred in herds other than the one in which they were used. Therefore, it is unlikely that there is much genetic variation between herds.
No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births
