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7 Cards in this Set
- Front
- Back
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meiosis
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Meiosis is a reduction division in terms of diploid and haploid numbers of chromosomes.
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homologous chromosomes
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• Homologous chromosomes are two chromosomes that correspond in proportion, value, and structure meaning that they contain the corresponding genes for the same traits.
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Outline the process of meiosis, including pairing of chromosomes followed by two divisions, which results in four haploid cells.
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• Meiosis can be divided into two segments, meiosis I and II. In meiosis I, the the chromosomes meet in homologous pairs. Each chromosome consists of 2 identical "sister" chromatids, therefore each homologous pair is a group of 4 chromatids, called a tetrad. The first division occurs by each of these chromosome pairs segregating, or seperating onto different sides of the cell. This produces two cells with the diploid number of chromosomes. Then, the second division occurs in both new cells when the sister chromatids are separated, pulling apart the chromosome. This produces four cells with the haploid number of chromosomes.
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Explain how the movement of chromosomes during meiosis can give rise to genetic variety in the resulting haploid cells.
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• The arrangement of chromosomes at metaphase I of meiosis is a matter of chance. This arrangement determines which chromosomes will be packaged together in the haploid daughter cells. Also, crossing over of alleles between homologous chromosome pairs gives rise to new combinations of DNA. Thus, genetic variety results.
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Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down's syndrome (trisomy 21).
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Non-disjunction is when certain homologous chromosomes or sister chromatids fail to separate. This results in one gamete receiving two of the same type of chromosome and another gamete receiving no copy. An example is Down's syndrome which results from trisomy of chromosome 21. This means the individual with the syndrome has received three, rather than two, copies of chromosome 21.
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State Mendel's law of segregation.
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• Two alleles for a character are packaged into separate gametes and then randomly re-form pairs during fusion of gametes at fertilization.
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Explain the relationship between Mendel's law of segregation and meiosis.
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• In meiosis I, the chromosome pairs are separated. However, the two alleles for a character are still together and not separated. They are only separated in meiosis II when the sister chromatids separate and are packed into separate gametes.
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