Sex Determination in Humans
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Sex determination in humans is a genetic process that occurs at conception and is dictated by the 23rd pair of chromosomes.
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Humans have a total of 46 chromosomes, presented in 23 pairs. The first 22 are called autosomes, they determine non-sex traits such as eye colour. The 23rd pair are the sex chromosomes which determine the gender of the individual.
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Females have two of the same kind of sex chromosome (XX), while males are the ones who have two distinct sex chromosomes (XY).
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During reproduction, each parent contributes one chromosome to the 23rd pair. Females can only contribute an X chromosome since they have two, while males can contribute either an X or a Y chromosome because they have one of each.
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Therefore, it is the male’s sperm that ultimately determines the sex of the offspring. An X chromosome will lead to a female offspring (XX), while a Y chromosome will lead to a male offspring (XY).
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The Y chromosome carries the SRY gene, or sex-determining region Y. This gene triggers male development if present.
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For an embryo to develop as a male, the SRY gene on the Y chromosome must be present and functioning. If an embryo has no Y chromosome, or the SRY gene isn’t working correctly, it’ll develop as a female.
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These patterns of genetic transmission are called sex-linkage. This linkage can impact the presentation of certain traits or diseases (like colour blindness and haemophilia) more frequently in one sex than another.