Sex ratio is the ratio of males to females in diclinous population.
Sex ratio along with sexual dimorphism is an important characteristic of diclinous population. Usually it expresses the number of males per 100 females, the proportion of male animals, or as a percentage. Depending on the stage of ontogenesis primary, secondary, and tertiary sex ratio can be distinguished. The primary sex ratio is established in zygote after fertilization; secondary sex ratio — at birth and, finally, the tertiary sex ratio is determined in mature, capable of breeding individuals of a population.
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It is now commonly acknowledged that in most species of animals and plants the main mechanism determining sex is chromosomal. Since during the gametogenesis the gametes containing X- or Y-chromosomes are produced in equal numbers, it was believed that this mechanism provides a roughly equal proportion of sexes during conception. However, the secondary sex ratio depends not only on the proportion of heterogamete, but also many other factors, such as the speed of ageing and elimination of X-and Y-bearing sperm in the male body, on their ability to reach the egg and fertilize it, from the egg affinity to X-or Y-sperm, and, finally, the viability of male and female embryos at different stages of embryonic development.
Long ago, it was observed that in many animal species, secondary sex ratio is slightly different from the aspect ratio 1:1 towards an excess of males is approximately 105-106 males to 100 females. The most reliable data are collected on human species. The average value of the secondary sex ratio in every human population is around 106. Taking into account various (differential) mortality among opposite genders during the embryonic stage of development, shifts the extrapolated value of primary sex ratio further from 1:1 aspect ratio. All available data on the gender composition of miscarriages and stillbirths in humans show that boys die before birth 2-4 times more often than girls. Thus, the primary sex ratio in humans seem to deviate from the ratio of 1:1 in the direction of male zygotes excess, and it seems more than the secondary. The human sex ratio at birth can be artificially changed due to abortion and infanticide.
Between the secondary and tertiary sex ratio there is a direct relationship — the better the fertility of male animals, the more of them can live to their adulthood.
Many studies also indicate that the secondary sex ratio is dependent on the tertiary. Eight plants and animals species (silene dioica, guppy, ticks (3 species), drosophila, mouse, rat, North American forest marmot, and humans) showed increased tertiary sex ratio leading to decreasing in the secondary.
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