Information is presented on some ways in which spermatozoa vary within a mammalian species, and on the relative importance of non-genetic factors affecting this variation. The work is basic to current researches in this laboratory on the genetics of mammalian spermatozoa. Five characteristics of rabbit spermatozoa were examined in permanent nigrosin-eosin preparations: the incidence of stained spermatozoa and the incidence of spermatozoa lacking the acrosomal cap, both related to the fertility of semen; the length, breadth and area of the spermatozoan head. Various factors have been studied that potentially affect these characteristics, with emphasis on variation attributable to technique and observation. The effects of the factors have been recorded in quantitative form and, in the light of the present work, suggestions have been made for the design of future experiments. Experimental and statistical procedures are described. Observations on variation due to biological causes include a separation of the variance between the males of a litter from the variance between the litter means. It has also been shown that the average length of the spermatozoan head in the vas deferens remains virtually unchanged in the ejaculate. Mensuration characteristics (length, breadth and area) are better adapted to objective analysis than are enumeration characteristics (proportions of abnormal spermatozoa).

The study of genetic effects on the phenotype of gametes may be called the genetics of gametes. As evidence of such genetic effects, marked inter-strain differences have been demonstrated in characteristics of rabbit spermatozoa examined in nigrosin-eosin preparations. Strain differences have been found in the size and shape of the spermatozoan head, and in a measure of spermatozoan viability—the incidence of stained heads. A first comparison of the spermatozoa of specific breeds is given. The size of the spermatozoan head is to some extent independent of the body weight.

In animals, the expression of genetic factors has been studied mainly after fertilization, in the embryo or the adult. The study of genetic effects on the gametes themselves has been called the genetics of gametes. As evidence of such genetic effects on gametes, numerous differences have been found in the characteristics of spermatozoa from eight inbred strains of mice. The spermatozoan characteristics studied are mainly dimensional, but also relate to the physiological maturation of spermatozoa and possibly to semen fertility. The dimensional characteristics of spermatozoa are insulated, to an unusual degree, from the effects of environmental and other factors. The genetics of gametes can be regarded as the study of the genetics of the carriers of genes from one generation to the next; this has relevance to general genetics, and also to experiments for controlling the transmission of hereditary factors from parent to offspring.

The expression of heritable variation in mammals is usually studied among the attributes of the adult; for example, in the body weight, milk yield, or colour of the pelt. A relatively new subject of study is that of heritable variation in the characteristics of gametes. It is of particular interest, because such variation in gametes may affect their fertility, which in turn might have repercussions on both fundamental and applied genetics. Current research is directed mainly to surveying the ways in which spermatozoa are subject to genetic influences. In the present work, heritable variations in the spermatozoa of mice are demonstrated, in the form of differences between inbred strains. The results are in virtually complete agreement with those of a previous study. It has also been found that variation in first generation crosses made from the pure strains can be largely accounted for by a blending inheritance of the characteristics of the parental types, together with an effect of heterosis. This appears to be the first demonstration in gametes of the phenomenon of heterosis, which is nearly but not quite the same as “hybrid vigour”.

The possible effect of genes on the spermatozoa carrying them can be investigated by searching for two kinds of spermatozoa in the semen of heterozygous males. During the development of appropriate biometrical methods, it became desirable to specify more exactly the basic form of variation in size between spermatozoa sampled from the same ejaculate and mounted on the same nigrosin-eosin preparation. The present study of the head length of rabbit spermatozoa indicates that apart from rare spermatozoa with exceptional dimensions the variation of spermatozoa on a preparation follows almost completely a “normal” distribution, the variance being independent of the mean and homogeneous from one preparation to another. The coefficients of skewness (g1) and of kurtosis (g2) and the statistic k4, average close to zero, g2 and k4 being homogeneous between preparations. There is evidence of heterogeneity in skewness between preparations, but the reality of the heterogeneity is not quite certain. A potential dimorphism between spermatozoa bearing X- and Y-chromosomes is inherent in the material, but analysis showed no significant evidence of any dimorphism in head length.

Genetic transformation in the rabbit has been attempted by the artificial insemination of semen incubated with deoxyribonucleic acid. No evidence was obtained of any transformation at the albino or rex loci, nor of sex. The experiments comprised twenty inseminated does, and 133 offspring of several categories.