Mutation is one of the basic phenomena of life. Without mutation, the gradual development of life from inorganic material would have been impossible, and the evolution of living beings from the first groups of molecules in which a primitive, information-carrying unit cooperated with an energy gaining device59 up to the present diversity of highly refined living organisms could not have occurred. Therefore, one would expect that mutation, and especially “spontaneous” mutation, i.e., that occurring naturally and without any detectable external reasons, would attract the research activity of many biologists, especially geneticists. Surprisingly, however, this is not the case. In most experimental studies—for example, on microorganisms, Drosophila or the mouse—research on spontaneous mutation is being carried out more or less as a sideline of other work, and the results are widely scattered.

Spontaneous mutation in man.

This paper reviews recent literature on the determinants of the sex ratio at birth. The first part provides an overview of the major findings on the biological determinants, and their implications on the racial differentials and the wartime increases in the sex ratio at birth. The second part discusses the sociodemographic factors of the sex ratio at birth (age of parents, birth order, and socioeconomic status). About thirty studies based on samples of millions of births drawn from national vital registration statistics in various countries are reviewed and compared. The results indicate that birth order and paternal age appear to be negatively correlated with the sex ratio at birth. Parents of higher socioeconomic status may be more likely to bear sons than parents of lower socioeconomic status. Some possible causal mechanisms are suggested for the effects of the sociodemographic factors.

Determinants of the sex ratio at birth: review of recent literature.

The risk of preterm delivery in a recent sample (1990-1994) of Italian liveborns was examined, taking into account child birth order, and maternal age and education in addition to the fetal gender. Univariate analyses showed that a higher risk was associated with male than female babies, with first- than second-born children, with older mothers, and with less educated mothers. The relative weights of the factors examined were evaluated through logistic regression analyses and the highest and the lowest risks were found to be associated with advanced maternal age and male fetal gender respectively. Our findings therefore suggest that biological factors associated more with advanced maternal age than with the male gender of the fetus may influence premature onset of labour.

/pdf/risks-of-preterm-delivery-and-association-with-maternal-age-ov2qberd66.pdf

Risks of preterm delivery and association with maternal age, birth order, and fetal gender

In industrialized countries, male excess is generally found in early deaths, despite the overall decrease in mortality. We studied the association between sex and some factors generally considered crucial for babies' survival, such as mother's age and education, birth order, and gestational age, in order to gain insight into the causes underlying the persisting higher vulnerability of male sex in early life. The analysis was performed on babies dying during the perinatal period. These were subdivided into those who were stillborn and those who died during the first week of extrauterine life. A higher male excess among babies dying during the neonatal period than among those who were stillborn was always found in all classes of all factors. The finding of such generalized male overmortality in the early extrauterine period of life, together with the patterns shown by the temporal sex ratio in stillbirths and in early deaths, supports the hypothesis of a postponement of male risk from late fetal into neonatal life.

Sex differential patterns in perinatal deaths in Italy.

Several studies have shown that the human secondary sex ratio is affected by a wide range of biological and environmental factors. Here, we describe a partitioning of the sex ratio variability as observed in the Italian population over the last two generations. This period has seen drastic changes in the environmental conditions of Italy. As a consequence, demographic and biological variables that can affect the sex ratio have also changed dramatically. In an attempt to isolate any specific effect, we used a stepwise multiple regression to analyze the covariation over time of the sex ratio and of relevant parameters, such as stillbirth rate, maternal age, firstborn proportion, and birth order. The results show that a quadratic function of the firstborn proportion and mother's age is a fairly good predictor of sex ratio values.

H M Cann

Papers

Effects of grandparental and parental age, birth order, and geographic variation on the sex ratio of live-born and stillborn infants.