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the homologous chromosomes

The chromosomes in the example are differentially shaded to make it easy to follow their movement through various meiotic phases. The black chromosomes originally can from the male parent’s father and the dark-shaded chromosomes were from the male’s mother. Similarly, the white chromosomes originally came from the female parent’s father, and the lightshaded chromosomes were from the female’s mother. Mendel’s law of independent assortment is evident at metaphase I of the example, where the duplicated homologues are lined up on the cell’s equatorial plane. At this point, there is an equal probability that the male’s dark-shaded autosome and black sex chromosome will end up on the left side of the equatorial plane (Fig. 28a) as there is of the dark-shaded auto some and sex chromosomes ending up on the left side (Fig. 28b). The same pattern is seen for the female’s white and light-shaded chromosomes during metaphase I (Fig. 28g and 28h). Because organisms may have hundreds of cells undergoing meiosis during gametogenesis, nearly equal numbers of both possible chromosome combinations would be expected. Mendel’s law of segregation is also the result of chromosome movement during meiosis. Comparing the parent’s chromosomes with those of its gametes shows the homologues are segregated though meiosis so that exactly half of the resulting gametes end up each homologue. Segregation can be illustrated by following the male parent’s sex chromosomes (the black ‘straight’ and dark-shaded ‘crooked’ chromosomes) through the meiotic stages. These chromosomes are lined up on opposite sides of the cell’s equatorial plane during metaphase I, then separate into different cells during telophase I. The subsequent equatorial division of meiosis II results in one-half the sperm cells obtaining the black ‘straight’ chromosome and the other half having the dark-shaded ‘crooked’ chromosome