Gametes

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A Gamete is a haploid (1n) cell that when fused with another gamete cell during fertilisation becomes a diploid (2n) cell in organisms that reproduce sexually. Gametes will carry half the genetic information of that person. Each sex produces one type of gamete, in humans the male gamete is the sperm cell which is much smaller than the female gamete the ovum. These cells will be composed of 22 different autosomal chromosomes and one sex determining chromosome, either X (ovum and sperm cell) or Y (sperm cell only). If the zygote inherits two X chromosomes then it will be female, if it inherits an X and a Y then it will be male.

Gametes are developed by the process of meiosis in germ cell lines. Sperm cells are produced by spermatogenesis whilst ovum cells first divide in the embryo but stop at prophase 1. Meiosis recomences prior to ovulation.   

When meiosis in germ cell lines goes wrong and nondisjunction occurs the results are aneuploid gametes, which means the gamete cells have incorrect chromosome numbers, whether it be for autosomal or sex chromosomes. When it occurs for autosomal chromosomes the zygote can be either monosomic (have one of the required chromosomes) or trisomic (have an extra chromosome). One example of autosomal trisomy is Down Syndrome, which is the presence of an extra chromosome 21.  

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