We have seen that Mitosis produces genetically identical cells for growth of an organism. There is a another type of cell division, called Meiosis, that is used for cells involved in sexual reproduction. Cells produced by meiosis are called gametes. Prior to Meiosis, the cells replicate their DNA during the S-stage, just as they do before Mitosis.

We saw earlier that in almost all cells of most organisms, there exists two of each type of chromosome, called homologs. This type of chromosome complement is described as diploid (2n or two of each type). Almost all Human cells contain 46 chromosomes, the 46 chromosomes can be arranged into 23 pairs of homologous chromosomes. In 22 of the pairs, the chromosomes match each other perfectly in appearance: these are called autosomes. The remaining pair of chromosomes are the sex chromosomes, designated X and Y.

Meiosis reduces the number of chromosomes in the cells. Gametes (e.g. sperm or egg cells) contain haploid numbers of chromosomes (called n) that is exactly half the diploid number. These gametes only contain one of each of the homologous pairs. In humans this haploid number is 23, representing +22 autosomal chromosomes plus one of the sex chromosomes. Fusion of the haploid male and female gametes restores the normal chromosome complement to the diploid number of 46 (n + n = 2n). Many of the mechanics of the processes of mitosis and meiosis are similar, but the outcomes are very different. To reduce the number of chromosomes, Meiosis uses two separate cell divisions (cytokinesis), in contrast to the single division in mitosis.

During the two divisions of Meiosis, the homologs separate. Either Homolog can wind up in any one gamete, the orientation is random, This means that there is a 50-50 chance for the daughter cells (gametes) to get either of the parent's homologs for each type of chromosome.

If a gene you receive from your parents is abnormal, you can end up with an abnormal protein or an insufficient amount of a normal protein. This could cause a Genetic Disorder. However, since the chromosomes are paired, there are 2 copies of each gene. If only one of these genes is defective, the other may code for sufficient protein so that the disorder is not clinically apparent. This is called a recessive disease gene. If one abnormal gene somehow produces disease, this is called a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from mom or dad, the child will show the disease. In the case of a recessive disease, if just one abnormal gene is inherited, the child will not show clinical disease, but they will pass the abnormal gene to 50% (on average) of their offspring.

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