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R. A. BEATTY, G. H. BENNETT, J. G. HALL, J. L. HANCOCK and D. L. STEWART

Summary.

Friesian cows were inseminated with semen mixtures containing equal numbers of spermatozoa from a Friesian and a Hereford bull. The five bulls of each breed gave twenty-five possible combinations. The paternity of calves was established by inspection of colour and conformation. Heterospermic indices were calculated to express the relative ability of sires to father offspring after mixed insemination. There were significant differences between the heterospermic indices of bulls, the maximum observed difference being twenty-one-fold. The indices were consistent over two series. The homospermic index was defined as the 16-week non-return rate after normal single first inseminations. The heterospermic index established differences between bulls more efficiently than the homospermic index; one estimate showed that the heterospermic method needed less than 1/170th the number of inseminations required by the homospermic method. The homospermic index was predictable from the heterospermic index, the regression coefficient having a significance level of 0·05 > P > 0·025. The initial spermatozoan concentration of a bull's semen (before dilution) was highly correlated with the heterospermic index. Measures of semen quality based on the morphology and staining affinity of spermatozoa predicted heterospermic and homospermic indices non-significantly but in the right direction.

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D. L. STEWART, R. L. SPOONER, G. H. BENNETT, R. A. BEATTY and J. L. HANCOCK

Summary.

Spermatozoa from four Friesian bulls were mixed in equal numbers. After insemination of the mixture, the paternity of calves was scored by blood-typing. Consistent results were obtained over replicate experiments conducted in each of 6 successive weeks. After insemination of `fresh' mixed semen, the number of calves sired per bull were in a ratio close to 1:1:1:1. Normal conception rate tests with fresh semen from one bull at a time (4- and 16-week non-return rates) also showed no significant differences in bull fertility. After inseminating mixtures subjected to deep-freezing at −196° C, the numbers of offspring were no longer nearly equal; one bull now sired ~50% of the progeny. It is concluded that bulls vary in the ability of their spermatozoa to withstand deep-freezing. No significant prediction of semen fertility based on staining affinity of spermatozoa could be realized, but the experiment was of limited size for this particular purpose. In two sets of twins, the co-twins were sired by different fathers. The sex ratio of progeny was unaffected. Experiments of this type offer no apparent risk to cooperating farmers, since overall conception rate was not diminished. Stocks of deep-frozen semen have been laid down for testing in subsequent years.

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H. J. Stewart, D. S. C. Jones, J. C. Pascall, R. M. Popkin and A. P. F. Flint

Page Introduction 2 Structures of genes and mRNA molecules 2 Structure of genes 2 Sequences of promoters and enhancers 3 Signals for terminating RNA synthesis 5 Structure of mRNA molecules 5 Structure of genes coding for reproductive polypeptide hormones and hypothalamic releasing factors 7 Structure of gonadotrophin genes 7 Structure of placental lactogen, prolactin and growth hormone genes 10 Structure of the LHRH gene 12 Inhibin, activin and Müllerian duct-inhibiting hormone 13 Localization of gene products by in-situ hybridization 16 Pre- and post-translational processing 18 Pretranslational processing: differential splicing of RNA 18 Post-translational processing 18 Steroidogenic enzymes and steroid sulphatase 20 Steroid hormone receptors 22 Steroid receptor primary structure: amino acid sequences derived from cloned DNA 23 Steroid hormone receptors and oncogenesis 26 Mechanism of binding of receptors to DNA—the finger hypothesis 27 DNA sequences recognized by steroid receptors in eukaryotic cells 28 Fertilization and early development 30 Expression of