Which Process Can Occur In Both Mitosis And Meiosis - T-Tees - Your Answers Await, Life's Questions Unraveled

Chromosome Disorders

Changes in Chromosome Number

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What would happen if an entire chromosome were missing or duplicated? What if a human had only 45 chromosomes? Or 47? This real possibility is usually due to mistakes during meiosis; the chromosomes do not fully separate from each other during sperm or egg formation. Specifically, nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number. Nondisjunction may occur during meiosis I or meiosis II Most human atypical chromosome numbers result in the death of the developing embryo, often before a woman even realizes she is pregnant. Occasionally, a zygote with an extra chromosome can become a viable embryo and develop.

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Trisomy is a state where humans have an extra autosome. That is, they have three of a particular chromosome instead of two. For example, trisomy 18 results from an extra chromosome 18, resulting in 47 total chromosomes. To identify the chromosome number (including an abnormal number), a sample of cells is removed from an individual or developing fetus. Metaphase chromosomes are photographed and a karyotype is produced. A karyotype will display any abnormalities in chromosome number or large chromosomal rearrangements. Trisomy 8, 9, 12, 13, 16, 18, and 21 have been identified in humans. Trisomy 16 is the most common trisomy in humans, occurring in more than 1% of pregnancies. This condition, however, usually results in spontaneous miscarriage in the first trimester. The most common trisomy in viable births is Trisomy 21.

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Figure (PageIndex{3}): Nondisjunction in Meiosis: Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number. Nondisjunction may occur during meiosis I or meiosis II. If nondisjunction occurs in meiosis I, the homologous pair of chromosomes remain bound. This results in an extra chromosome (n + 1) in two of the gametes and a missing chromosome (n – 1) in two of the gametes. If nondisjunction occurs in meiosis II, two cells enter meiosis II with the correct number of chromosomes. Nondisjunction occurs in one of these cells when one of the sister chromatids does not separate. This results in a cell with an extra chromosome (n +1), a cell missing a chromosome (n – 1), and two cells with the expected number of chromosomes (n).

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