Explain how a combination of sex chromosomes determine the sex of a person  


Gametes are the sex cells that are produced by the male and female of a species.  Let us consider human beings as an example.  Normal chromosome count in every human cell is 46 chromosomes, which are paired together in 23 homologous pairs.  That 23rd pair of chromosomes is what is known as the "sex chromosome", the chromosome pair that genetically dictates whether you are male or female.  In this chromosome pair is either an "xx" combination for female, or an "xy" combination for male.  The gametes, or sex cells, are produced by a process known as meiosis, which halves the number of chromosomes.  It also, in males, produces some sperm which have the x part of their sexual makeup, and some sperm that have the y part of their sexual makeup.  The female egg only has the x part, since the female sexual makeup is xx.  When a couple has sexual intercourse, the first sperm that reaches the egg establishes the sexual identity of the zygote.  If the sperm is carrying the x chromosome, it will be female;  if the sperm is carrying the y chromosome, the zygote will be male.

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