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Four strains of mice were used to found a heterogeneous stock from which one line was selected for high adrenal weight/100 g body weight and another line for low adrenal weight/100 g body weight. By the eighth generation of selection, two stocks of mice were produced, one with adrenals whose absolute weight was greater than any of the parental strains. The realized heritability estimates were 75% for females and 67% for males during the period of selection, showing that a large component of variation in adrenal weight was genetic in origin.Asymmetry in response to selection was also noticed and attributed to possible variation in adrenal structure between sexes. Estimates of variances in each generation, particularly the eighth, did not show any decline which suggested that further responses to selection would be possible with further selection.

Adrenal weight and body weight were measured for 100 mice of each sex of the strains: A/Cam, CBA/FaCam and SF/Cam. This was also done for reciprocal F1 hybrids between A and CBA, for reciprocal backcrosses, and for F2 hybrids. Using regression analysis, adrenal weights were corrected for differences in body weights. Sexes and strains were found to be significantly different with respect to both absolute and corrected adrenal weight. The males of the F1 cross between the A and CBA strains showed partial dominance of the A genotype. Reciprocal F1 females were found to differ significantly. This may be explained in terms of parent-genotype: offspring-genotype interactions.The variances of the heterogeneous stocks: F2, backcross of F1 to A and backcross of F1 to CBA, were found to be significantly higher than those of the homogeneous stocks: A, CBA and F1's. This indicated that at least some of the variation in relative adrenal weight between the strains was genetically determined. The frequency distributions of the ratio of adrenal weight to body weight in F2 and reciprocal backcrosses conformed to the classical pattern of a metrical character.

The karyotype and quantitative characteristics of a wild population of rats, Rattus rattus, were studied. Individuals of the population were classified into three distinct groups, each with a characteristic chromosome number of 38, 42 and ± 54 respectively. The frequency distribution of the three groups of rats in the sample studied was as follow: group I with 38 chromosomes formed 14%, group II with 42 chromosomes formed 54% and group III rats have had chromosome numbers varying between 50–60 formed 32%. The rats with 38 chromosomes had two pairs of marker chromosomes (2 long metacentric pairs). Those of group III were characterised by having a marked decrease or complete absence of short metacentric chromosomes with a simultaneous increase in the frequency of short telocentric chromosomes. Group II rats had more or less the chromosomal characteristics established for laboratory rats studied by previous workers. The total chromosomal length of somatic cells in either group I and II were found to be similar. The notable chromosomal polymorphism in number was explained in terms of centromeric fusion or dissociation.

The growth in weight of the adrenal gland has been followed in three normal strains of mice: A/Cam, CBA/FaCam, and Peru.CBA mice have the heaviest adrenals and A mice the lightest. Female mice have larger adrenals than male mice in all three strains. This sex-difference is significant in Peru mice before puberty and becomes more pronounced in all three strains as development proceeds.Adrenal weight bears a linear relation to body weight in the female mice. The heaviest adrenals, relative to body weight, are found in Peru females and the lightest in A females.The relative growth of the adrenals of male mice can be divided into two phases; an early, rapid, one, and a slower, later, one. The ranking of the strains according to the relative weight of the adrenals is different in the two phases. Peru mice have the heaviest adrenals during the first phase while those of CBA mice are the heaviest in the second. The two phases of growth are separated by a transition phase. An absolute fall in adrenal weight occurs during the transition phase in Peru males, but not in A or CBA males. Histological observations show that degeneration of the X zone coincides with the transition phase in all three strains. The three strains differ in the age and body weight at which X zone regression takes place in male mice.

THERE have been many studies of the effects of ethyl alcohol on reproductive performance and of the genetic aspects of this relationship1–10. But in spite of extensive investigations of the genetic determination of alcohol preference in experimental animals, there has been little research on other genetic aspects of alcohol consumption11–13. It has been assumed for a long time that alcohol presents no genetic hazard. Using the dominant lethal technique, we have now found that in mice ethyl alcohol can be mutagenic when given under certain conditions.

Eighty-one Egyptian workers in an asbestos-cement pipe factory, who exhibited either the clinical or the radiological signs of asbestosis, or both, were classified into the blood groups A1, A2, B1, A1B, A2B and O. A group of 97 healthy workers in the same factory, with no signs of chronic bronchitis, were classified in the same way as controls. There were no significant differences between the phenotype and genotype frequencies of the control group and the diseased group of workers taken as a whole. There was, however, a significantly higher proportion of individuals in group O among the workers showing clinical signs of asbestosis than in the controls, and this was more marked in patients showing both clinical and radiological signs. A calculation of the relative incidence has shown that group O individuals have a greater likelihood of contracting asbestosis than do individuals in other blood groups. This association between blood groups and asbestosis is discussed in relation to the possible implication of genetic factors in the susceptibility of individuals to inhaled dusts.