Studies on the sex-ratio and related phenomena

Summary

• 1. There is an apparent discrepancy between the facts pointing to the male as the sex heterozygous parent in mammals, and the facts which show that the mother has some influence on the sex-ratio at birth. Chief among the latter are those relating to the influence on the sex-ratio of the age of the mother and the number of the pregnancy.

• 2. The only sex-ratio, however, which we know to be affected by the mother is the ratio at birth, whereas the facts of sex determination are, concerned solely with the ratio at the time of conception, or when sex is first determined.

• 3. It is possible, therefore, that changes in the sex-ratio between conception and birth may account for the apparent anomaly, and this would reconcile the two opposing sets of facts.

• 4. As changes in the sex-ratio after sex determination can only go on as a result of unequal elimination of the sexes the source of any change in the sex-ratio during gestation must be sought in a sexually differential pre-natal mortality.

• 5. Other authors have made it clear that there is a considerable amount of pre-natal mortality which falls more heavily on the males than on the females, and the material here presented shows that pre-natal mortality occurs preponderatingly with older mothers and in later pregnancies.

• 6. These facts substantiate the possibility that the decrease in the sex-ratio with older mothers and at higher parities is brought about as the result of increased pre-natal wastage of males, and not as the result of an increased proportion of females at sex determination.

• 7. Influences exerted by the mother, therefore, have no relation to sex determination, and offer no contradiction to the chromosome theory of sex determination, which implies that the male controls determination.