Thus, the short answer to the question is: In his paper, Mr Gellatly demonstrates that it is likely men carry two different types of allele, which results in three possible combinations in a gene that controls the ratio of X and Y sperm; -- Men with the first combination, known as mm, produce more Y sperm and have more sons. It is a simplified example, in which men either have only sons, only daughters, or equal numbers of each, though in reality it is less clear cut. This would explain why the men that survived the war were more likely to have male children, which resulted in the boy-baby boom. This is often expressed as the percentage of boys among all births, or about He only passes on the m allele, so his children are more likely to have the mm combination of alleles themselves. How does the gene work? The year after World War I ended, an extra two boys were born for every girls in the UK, compared to the year before the war started.
Workers producing 1,2-dibromochloropropane DBCP , a chemical used to kill worms in agriculture, experienced even larger decreases in the number of male babies they welcomed into the world. This would explain why the men that survived the war were more likely to have male children, which resulted in the boy-baby boom. Men determine the sex of a baby depending on whether their sperm is carrying an X or Y chromosome. On the opposite end of the spectrum, most research shows that the share of baby boys increases during and after wartime. Therefore it is unlikely that the declining SSR in many countries results solely from large-scale changes in such personal factors. It shows that although the gene has no effect in females, they also carry the gene and pass it to their children. It is a simplified example, in which men either have only sons, only daughters, or equal numbers of each, though in reality it is less clear cut. It is difficult to discern how much of the decrease in sex ratio since the s arises from contaminants in the environment. These dramatic changes resulting from extreme exposures raise the concern that chemicals in the environment at lower concentrations may also change the SSR by exposing people over longer periods of time. While there are still slightly more boys born than girls in these places — which are centered in sub-Saharan Africa — the sex ratios are nonetheless much lower than average. With regard to environmental factors, improved prenatal and obstetrical care during the first part of the 20th century is largely responsible for an increased SSR over this period in many countries. In the second tree B the grandfather is ff, so all his children are female, they have the ff combination of alleles because their father and mother were both ff. Furthermore, for men who go on to develop testicular cancer, both their semen quality and the SSR of their children are significantly reduced, suggesting a possible biological link between these male reproductive characteristics. The chance of having a boy appears to decline with the mother's age, the father's age and the number of children the family already has. That male determines the sex of the children, so the grandchildren are all male. In the first family tree A the grandfather is mm, so all his children are male. As the odds were in favour of men with more sons seeing a son return from the war, those sons were more likely to father boys themselves because they inherited that tendency from their fathers. Further evidence of this association between maternal stress and the increased share of girl babies emerges from an analysis of births in the vicinity of the Kobe earthquake in Japan, which showed that the share of females born nine months after that event was significantly higher than would have been expected under normal circumstances. Some research suggests that the share of newborn boys declines with older parents , and that the high share of girls in Sub-Saharan Africa may be linked to the practice of polygamy multiple wives. So what explains these differences in the shares of baby boys and girls? This means that a man with many brothers is more likely to have sons, while a man with many sisters is more likely to have daughters. Effects of DBCP on sperm quality were discovered incidentally when male workers found that they were unable to father children. The male fetus is more susceptible to loss in the womb than is the female fetus, so with more conceptions reaching term, proportionally more males are born. In some cases, though, wartime is associated with a higher share of girl babies. Researchers hypothesize that both situations are associated with less frequent intercourse. In the UK and US, for example, there are currently about males born for every females. The decreases were most extreme among the children of fathers who were exposed at earlier ages:
Video about 2 girls and 1 boy sex:
1 Room 2 Girls 1 Boy
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