Jacobson et al in the journal Human Reproduction conducted a study in 1999 that showed that the sex ratios of children born in a study group had a schewed sex ratio. Normally a 1 to 1 ratio of boys to girls is expected, since there is a 50 percent chance that a male parent will pass on either one of the sex determining chromosomes. When this ratio is schewed, explanations are needed. Jacobson showed that the sex ratio decreased with increased number of chiren per plural birth and with paternal age, for example. This meant that there were more girls born than expected.
Sex allocation theory hypothesises that parents will increase their fitness by controlling the sex ratio of their offspring, schewing it torwards producing more of the rarer sex. Lumma et al are an adaptive feature and can change as the adaptive environment requires. Lummaa et al showed that this occured in a set of pre-industrial humans living between 1175-1850 in Finland. Where males were rare, more sons were produced. Mackenzie et al studied a population of Canadian aboriginees and showed that their sex ratio had declined from a stable ratio to a very low one, and hypothesised that this was due largely to the influx of industrial pollutants in the river that the aboriginees lived in close association with.
When considering what affects the human sex ratio, there are pre-fertilization factors to consider, and post-fertilization factors to consider.
Rex-Kiss in Act.Biol.Hung. found that fetal-maternal blood group incompatibility will lead to a higher newborn sex ratio, iow more males than expected. That researcher felt that the blood group incompatibility has an effect on the X chromosome, and that whatever this effect is the elimination rate of the zygotes fertilized by Y chromosome-carrying sperm will decrease .
Rex-Kiss also found that incompability within the Rh-factor system also increased the sex ratio.
Andersson and Bergstrom foundd that short maternal stature and obesity in African populations of the C.A.R. were related to a lower sex ratio at birth, meaning that more females were produced than males. Experiments in animals indicated that this was a result of maternal malnutrition on the male fetuses. TM Allan in the journal Reproduction studied, along with many others, the effects of blood type upon the at birth sex ratio. Allan found that AB mothers tend to have boys, and that if a baby is type A, it will tend to be a girl. More specifically, he found that the ratio is low for AB babies of AB mothers, and also for A babies of A mothers. The ratio was high for O babies of O mothers B babies of B mothers. He hypothesised that the ratio schewing is caused by "sex-differential mortality caused by interaction of the ABO genes, and some o the sex-determining genes, with oestrogen and progesterone".
Environmental factors such as pollutants can affect the sex ratio by affecting the already formed zygote, or they can also affect the parents in such a way, pre-fertilization, as to change the ratio. Moracelli et al in Lancet in 2000 studied the paternal concentrations of the highly dangerous industrial poison Dioxin on this. The 2000 study was a continuation of a previous study examing a population exposed to a dioxin release in Italy in 1976. They showed that there was an increasing probability of producing female offspring with increasing concentrations of dioxin in the fathers. Furthermore, fathers exposed when they were less than 19 years old produced a sgnificantly greater number of girls than boys (with a ratio of 0.38, as opposed to 0.5). They also quantized their study and found that the median concentration of dioxin that produced this effect was similar to doses that induced epididymal impairments in rats. Many people have hypothesized that this might explain the sex ratio schewing that is occuring in industrialized nations, however they found that the media dose in their study was twenty times the average estimated concentration in people in industrialized nations.
Another important pre-fertilization factor to consider is meiotic drive. This is where whole chromosomes will have one effect or another and cause themselves to be represented in the gametes in higher than normally expected amounts. This can occur with autosomal chromosomes, but also with sex chromosomes. Jaenike in Evolution in 1999, for example, noted that a particular species of Drosophila is polymorphic for X-chromosome meiotic drive, and that matings with males who have a 'sex-ratio' X chromosome (designated XSR) result in the production of strongly female-biased ratios, and also that there was variation amoung the species for suppression of this drive. In humans, Jaenike in Annual Review of Ecology and Systematics showed in 2001 the presence of driving sex chromosomes can reduce fitness within a population, and even bring about intragenomic conflict between the X, Y, and autosome chromosomes.
A theoretical study conducted by Norberg in 2004 and published under the NBER Working Paper Series found that sex determination can actually be affected by the pressence or absence of two-parent care. Norberg found that, from a sample population, 51.5% of respondants who were living with a partner before the child's conception or birth reported male births. 49.9% of those who were not reported male births.
And finally Grech et al in BMJ reported on "unexplained differences in sex ratios at birth in Europe and North America", noting that mammals in general have more males than females born, and that in humans the actual ratio is expected to be 0.515. In their study it also was found that statistically more boys were born in soutern countries than in central europe or the nordic countries. A low ratio was found in mexico, higher in teh US, and even higher in Canada. In a reply however, Voracek and Fisher reanalysed the data and found that the ratio was varying with latitude, and by virtue of that varying seasonal changes in the climate and varying photoperiods. Sheilds et al also responded to the Grech study, noting that maternal infection with the cytomegalovirus also influenced the sex ratio torwards more males, and that this infection was also associated with "social deprivation and unmarried status"
So studies have shown that the sex ratio is offset from the expected 50:50, in many animals, by adaptive and non-adaptive traits, and by pre and post zygotic factors, ranging from biochemical incompatibility to environmental pollutants and climate.
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