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Animation: The Chromosomal Basis of Independent Assortment

by Pearson
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These identical diploid cells have just begun meiosis. Their progress through the cell divisions will illustrate the principle of independent assortment. Each cell is heterozygous for the shape and color genes. One chromosome contains the dominant R allele, and the homologous chromosome contains the recessive r allele. Another pair of chromosomes contains the dominant Y and recessive y alleles of the color gene. During meiosis, the homologous chromosomes pair up and migrate to the middle of the cell. Notice that in one cell, the chromosomes containing dominant alleles are lined up on the same side of the cell; whereas in the other cell, they are on opposite sides. This arrangement at metaphase I determines which chromosomes segregate to which pole of the cell during division. In the first case, the dominant R and dominant Y alleles end up in the same cell. In the second case, the dominant R and dominant Y alleles end up in different cells. The alleles of the shape gene have segregated from each other independently of the alleles of the color gene. This is the basis of Mendel's principle of independent assortment. In meiosis II, the cells divide again. Notice that there are equal proportions of gamete genotypes. One-fourth are dominant R, dominant Y; one-fourth are recessive r, recessive y; one-fourth are dominant R, recessive y; and one-fourth are recessive r, dominant Y. On average, half the cells that undergo meiosis will produce the genotypes shown on the left, and the other half will produce the genotypes shown on the right. Genes located on different chromosomes, as in our example, always assort independently. It is important to note, however, that genes located on the same chromosome generally do not.
These identical diploid cells have just begun meiosis. Their progress through the cell divisions will illustrate the principle of independent assortment. Each cell is heterozygous for the shape and color genes. One chromosome contains the dominant R allele, and the homologous chromosome contains the recessive r allele. Another pair of chromosomes contains the dominant Y and recessive y alleles of the color gene. During meiosis, the homologous chromosomes pair up and migrate to the middle of the cell. Notice that in one cell, the chromosomes containing dominant alleles are lined up on the same side of the cell; whereas in the other cell, they are on opposite sides. This arrangement at metaphase I determines which chromosomes segregate to which pole of the cell during division. In the first case, the dominant R and dominant Y alleles end up in the same cell. In the second case, the dominant R and dominant Y alleles end up in different cells. The alleles of the shape gene have segregated from each other independently of the alleles of the color gene. This is the basis of Mendel's principle of independent assortment. In meiosis II, the cells divide again. Notice that there are equal proportions of gamete genotypes. One-fourth are dominant R, dominant Y; one-fourth are recessive r, recessive y; one-fourth are dominant R, recessive y; and one-fourth are recessive r, dominant Y. On average, half the cells that undergo meiosis will produce the genotypes shown on the left, and the other half will produce the genotypes shown on the right. Genes located on different chromosomes, as in our example, always assort independently. It is important to note, however, that genes located on the same chromosome generally do not.