With the exception of the mother’s egg cells or the father’s sperm cells, every cell in the normal human body contains 23 pairs of chromosomes, totaling 46 in each cell called the diploid number. According to The Human Genome project there are over 30,000 genes in every cell, spread unevenly across the chromosomes. Of the 23 pairs of chromosomes, one member is inherited from the mother, and one from the father. Members of each pair are called homologous. The first 22 pairs are numbered 1-22, and the 23rd pair is labeled X & Y and called the sex chromosomes. Males have one X and one Y, while females have 2 X and no Y-chromosomes. During meiosis, the number of chromosomes is reduced and genetic information is shuffled allowing for variations, such as eye or hair color. Occasionally however, humans are born with an abnormal number of chromosomes because they incorrectly separate during meiosis. This is called nondisjunction, and can happen during meiosis I when both members of the same homologous pair go into the same daughter cell or II, if the sister chromatids fail to separate and both chromosomes go into the same gamate. If an egg with an extra chromosome (24) is fertilized with normal sperm it is called a trisomy, where one type of chromosome is present in 3 copies. An egg having only 22 chromosomes, results in a monosomy, having only one type of chromosome present in a single copy. Down syndrome called trisomy 21 is the most well known type of mutation occurring in 1 in 800 births, causing eyelid folds and a round head as well as mental disabilities. The odds increase in mothers over age 40. Nondisjunctions involving X and Y-chromosomes can also cause syndromes, such as Turner syndrome, where a female is missing a sex chromosome, characterized by a webbed neck and underdeveloped breasts and ovaries. Scientists called Cytogeneticists can now examine a persons chromosomes called a Karyotype, for defects. Usually they analyze the white...
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