It is well-documented that more males die in childhood and before they are old enough to have children. On a larger scale, the number of men with more X sperm compared to the number of men with more Y sperm affects the sex ratio of children born each year. The research involved a study of family trees containing information on , people from North America and Europe going back to In contrast, men with more daughters may have lost their only sons in the war and those sons would have been more likely to father girls. It's in the father's genes. 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. The grandsons have the mf combination of alleles, because they inherited an m from their father and an f from their mother. A Newcastle University study involving thousands of families is helping prospective parents work out whether they are likely to have sons or daughters.
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