San José State University
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The Explanation of the Sex Ratio of Human Births

The natural ratio of male births to female births is about 1.05, which means that about 51.22 percent of babies born are males. The literature on the explanation for the sex ratio not being 1.00 is filled with a multitude of abstruse and obscure explanations. Most and perhaps all of this is nonsense. The explanation for the unequal proportion is utterly simple. The X chromosome is longer and hence heavier than the Y chromosome. The sperm carrying X chromosomes, which will conceive females, are laboring under a heavier burden than those carrying Y chromosomes, which will conceive males. This heavier burden delays the swimming toward the ovum where the journey is uphill. However on a downhill run the heavier burden might possibly speed the journey, although not necessarily because as Galileo discovered heavier objects in free fall travel at the same speed as lighter objects.

A rough estimate of the weight ratio of the DNA materal carried by the two types of sperm may be obtained by assuming that an X-chromosome is of average weight compared to the other 22 non-sex chromosomes and that the Y-chromosome is one third the length and weight of X-chromosome. Thus an X-laden spermatozoa carries 23 units of weight whereas the Y-laden spermatozoa carries 22+0.33=22.33. The ratio of 23 to 22.33 is 1.03.

A more precise estimate requires some detailed meaurements. The X chromosomes contain 153 million base pairs whereas the Y chromosomes contain only 58 million. Sperm carry another 22 chromosomes as well as the sex chromosome so the differential burden is much less than 2.64 ratio of the weights of the X and Y chromosomes. The literature on this topic says that X chromosomes represents 5 percent of the DNA material of a female and 2.5 percent of the male. The Y chromosome represents 0.4 of 1 percent of the DNA material of a male. To get the differential burden start with 100 units of DNA material for a female. Five units of this is X chromosomes so there are 95 units of non-sex chromosomes. An X-laden spermatozoa would then contain 47.5 units plus 2.5 units for the X chromosome. A spermatozoa carrying a Y chromosome would be carrying 47.5 units of non-sex chromosome material plus 0.4 units for the Y chromosome. Thus the spermatozoa carrying a Y chromosome would be carrying 47.9 units of DNA material altogether compared to the 50 units being carried by the X chromosome carrying spermatozoa. The ratio of 50 to 47.9 is 1.044. This is the ratio of the payloads of the two types of sperm. The hull of the spermatzoa would be about the same so the ratio of the burdens of the two types of sperm would be less than the 1.044 ratio. But the ratio of burdens does not translate into a corresponding reciprocal ratio of conceptions. Nevertheless the above argument indicates that the sex ratio should be greater than 1.0.

An examination of the statistics of competitions indicates that the closeness of the sexratio, 1.05, to the ratio of the burdens, 1.044, is merely a coincidence. The ratio should depend upon the difference in burdens rather than their ratio. And it would depend upon many other things besides the difference in burdens.

If the above explanation for the sex ratio is correct then couples can influence the probabilities for a male or female conception by altering the length and difficulty of the journey of the sperm to the ovum. A longer and more uphill journey would increase the chances of a male conception. It is notable that the positioning of the uterus enhances the chances for the conception of a female in quadrupeds. The practice of humans of sleeping on their backs make the arrangement of female reproductive organs an enhancement for male conception.

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