No abstract

Different polymorphic elements of the enzyme glucose-6-phosphate dehydrogenase (G6PD) are favoured under selection for long versus short life span. Replicate independently selected populations of short-lived individuals exhibit a more rapidly migrating and less actively staining allozyme, while long-lived populations have a slower migrating and more active allozyme. These correspond to the common ZwA and ZwB variants of the G6PD locus Zw. In vitro measurements show G6PD activity varies with allozymes and life span. Long-lived males have 64 per cent greater activity in G6PD, while females of long-lived strains are 108 per cent higher than those of short-lived strains. Previous studies of these strains have repeatedly demonstrated additivity of life span in F1 crosses. Activity of G6PD in reciprocal F1 populations is additive and intermediate between parents.

No abstract

A family is described in which the tendency to bear twins is expressed in the offspring of males as well as females. All of the twins born in this family show marked discordance in birth weight and gestational age. In 5 of the 6 pairs, one twin was normal while the other was either a macerated fetal mass, stillborn, or died of prematurity in the neonatal period. The one pair in which both twins did survive is known to be DZ and showed a difference in maturity and a 21% discrepancy in birth weight. Thus, twinning in this family appears to be transmitted as an autosomal dominant trait which is expressed in the offspring of both female and male carriers. Of all possible genetic mechanisms which could explain this familial aggregation of markedly discordant twins, supcrfetation seems most consistent with the genetic transmission and expression of the trait in the offspring of both males and females. The most plausible explanation of the pedigree is that a dominant gene is segregating in the family which is expressed in the fetal placenta where it acts to reverse the normal hormonal inhibition of ovulation. Since both the father and the mother contribute to the genotype of the placenta, superfeiation could occur among offspring of both males and females who carry the gene.