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Duration: 11min 31sec Views: 952 Submitted: 22.01.2020
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In anthropology and demography , the human sex ratio is the ratio of males to females in a population. More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult. Like most sexual species, the sex ratio in humans is close to In humans, the natural ratio between males and females at birth is slightly biased towards the male sex, being estimated to be about 1. Sex imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants, [5] [6] war casualties, sex-selective abortions , infanticides, [7] aging, gendercide and problems with birth registration.

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According to popular theory, men live shorter lives than women because they take bigger risks, have more dangerous jobs, drink and smoke more, and are poor at seeking advice from doctors. But research by scientists at UNSW Sydney suggests the real reason may be less related to human behaviour and more to do with the type of sex chromosomes we share with most animal species. In a study published today in Biology Letters , researchers from UNSW Science's School of Biological, Earth and Environmental Sciences analysed all available academic literature on sex chromosomes and lifespan -- and they tried to establish whether there was a pattern of one sex outliving the other that was repeated across the animal kingdom. Specifically, they wanted to test the 'unguarded X hypothesis' which suggests that the Y chromosome in heterogametic sexes -- those with XY male sex chromosomes rather than XX female sex chromosomes -- is less able to protect an individual from harmful genes expressed on the X chromosome. The hypothesis suggests that, as the Y chromosome is smaller than the X chromosome, and in some cases absent, it is unable to 'hide' an X chromosome that carries harmful mutations, which may later expose the individual to health threats. Conversely, there is no such problem in a pair of homogametic chromosomes XX , where a healthy X chromosome can stand in for another X that has deleterious genes to ensure those harmful genes aren't expressed, thus maximising the length of life for the organism. First author on the paper and PhD student Zoe Xirocostas says that after examining the lifespan data available on a wide range of animal species, it appears that the unguarded X hypothesis stacks up.

Human sex ratio

The ratio between the number of males and females in a society is referred to as the gender ratio. This ratio is not stable but instead shaped by biological, social, technological, cultural, and economic forces. And in turn the gender ratio itself has an impact on society, demography, and the economy. In this entry we provide an overview of the variation and the changes of the gender ratio across the world. We study how it changes from birth to late life; the forces that change the ratio of men to women.
Sex chromosomes influence physical differences between male and female animals, such as feather patterns in mandarin ducks. In most animals, sex chromosomes help determine whether an individual develops as a male or female. Sexes of some animals, such as most male arachnids, lack a second sex chromosome entirely.