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Chapter 12 Meiosis

2 at least some offspring may be able to fight off

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2. At least some offspring may be able to fight off new strains of disease-causing agents. Mistakes in Meiosis When homologous chromosomes separate during meiosis I, a complete set of chromosomes is transmitted to each daughter cell. What happens if there is a mistake and the chromosomes are not properly distributed? Down syndrome is a disorder associated with the presence of an extra copy of chromosome 21. This situation is called a trisomy (in this case, trisomy-21) because each cell has three copies of the chromosome.
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How Do Mistakes Occur? For a gamete to get one complete set of chromosomes, two steps in meiosis must be perfectly executed. 1. The chromosomes in each homologous pair must separate from each other during the first meiotic division, so that only one homolog ends up in each daughter cell. 2. Sister chromatids must separate from each other and move to opposite poles of the dividing cell during meiosis II. If both homologs or both sister chromatids move to the same pole of the parent cell, the products of meiosis will be abnormal. This meiotic error is referred to as nondisjunction, because the homologs fail to separate or disjoin. Monosomy is a situation in which a gamete has only one copy of a particular type of chromosome. Cells that have too many or too few chromosomes are said to be anueploid. Mistakes in meiosis are the leading cause of spontaneous abortion in humans. Why Do Mistakes Occur? The leading hypothesis to explain the incidence of trisomy and other meiotic mistakes is that they are accidents- random errors that occur during meiosis. Most of the trisomies and monosomies observed in humans involve the sex chromosomes. Klinefelter syndrome, which develops in XXY individuals occurs in about 1 in 1000 male births. Trisomy X occurs in about 1 in 1000 live births and results in females who may or may not have symptoms such as impaired mental function and sterility. Turner sydrome develops in XO individuals, where "O" stands for lack of a second X. Individuals with this syndrome are female, but sterile.
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