This might be because there's more to eat in the rainy season and sons need more food to grow, so it's better to have girls when there's less food. The identification of an individual as either biologically male or female can have legal ramifications for marriage licenses, spousal support and eligibility for parenthood. Buffalo in Africa have mostly sons when it rains and more daughters when the climate is dry. In a coin flip, it doesn't matter what came before. In families that have all boys or all girls, it turned out to be the same one each time. And yet, we all know families that are all boys or all girls.
For example, more boys are born each year than girls. There are many more examples of this in nature, and a lot of it depends on survival of the species. However, the animal world is a whole different story. In families that have all boys or all girls, it turned out to be the same one each time. Although the details remain controversial, the idea is a sound one. Since most alligator eggs are hatched at cooler temperatures, the natural ratio is 1 male to 5 females! All sorts of things can happen so that more boys are born than girls and vice versa. Tissues of affected individuals are unresponsive to male hormones androgens yet respond to estrogens. One fish called the bluestreak cleaner wrasse hangs out in groups of about 8 females and 1 male. Because X-chromosome genes spend twice as much of their evolutionary history riding in female bodies rather than male ones because mammalian females have two Xs and males only one , X-chromosome genes are selected to benefit females twice as often as they are selected to benefit males. The Y chromosome is small, carries few genes, and has abundant repetitive sequence, while the X chromosome is more autosome-like in form and content. The issue of sexual classification, however, is complicated by factors such as chromosomal complement, external genitalia, gender identification and surgical alteration. The truth is that evolution is ultimately a question of some genes getting into the future at the expense of others, and consequently genetic conflict, not simple sex-chromosome determinism, is what explains sex-determination. Affected individuals are usually taller than average and have slender builds. Many reptiles also have sex ratios that depend on the environment. According to her theory, more dominant women with higher levels of testosterone are more likely to conceive sons, and less dominant ones with lower levels, daughters. To the extent that such homosexual males may be feminized, the evolutionary insight explains the apparent paradox: One problem is the fact that the Y-chromosome is tiny by comparison with the X and only produces odd proteins, mostly concerned with highly male-specific functions like sperm-production. Affected males are typically fertile and many are unaware that they have a chromosomal abnormality. This is an X-chromosome gene that competes for control of sexual development with SRY, the male Y-chromosome sex-determining gene in mammals which develop as females if SRY is not expressed. Men usually have one X and one Y chromosome, while women have two X's. The X, by contrast, has almost genes, with at least implicated in intelligence and cognition. Indeed, as I argue in The Imprinted Brain , genetic conflicts—including those related to sex-determination—almost certainly explain both mental health and illness—and arguably do explain the striking sex differences in the incidence of psychiatric illness. The idea is that sperm that carry a Y are slightly speedier and so are a bit more likely to make it to the egg first. How mom or dad lives can affect the sex of their baby too. Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. As a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics.
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