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the law of segregation

Understanding the genetics of most diploid organisms requires that you first have a good foundation in the way chromosomes move and interact during the production of gametes. For simplicity, the following hypothetical example (Fig. 28) has two pair of homologous chromosomes, a pair of autosomes and a pair of sex chromosomes, and crossovers between homologous pairs are not considered. By carefully following these chromosomes from the parents’ somatic cells, through meiosis to form gametes, then fertilization to the resulting offspring; one can see the connection between chromosome movement and several of the laws Mendel observed. Note the sex chromosomes in the male and female nuclei represented in the example. One of the male sex chromosomes is ‘crooked’. This is similar to the X-Y sex determination system seen in a wide variety of organisms, including fruit flies, horses, and humans. In the male of this example, one homologue of the sex chromosome pair is ‘crooked’ (the Y) and the other is ‘straight’ (the X). The female has two ‘straight’ (X) homologues. Each sperm cell has either one ‘straight’ sex chromosome or one ‘crooked’ sex chromosome in addition to one autosome, while all eggs have one ‘straight’ sex chromosome plus one autosome. All the gametes are haploid (n). Union of eggs and sperm produce male or female diploid (2n) offspring, depending on which sex chromosome the sperm cell was carrying. The fundamentals of the meiotic divisions can be summarized as follows: 1) during interphase I of meiosis, the chromosomes are duplicated so that, 2) the duplicated chromosomes, duplicates attached at the centromere, can be seen during prophase I, 3) the homologous chromosomes, with their attached duplicates, line up at the cell’s equatorial plane, during metaphase I, 4) followed by a reduction division during anaphase I through telophase I, where the homologues separate TABLE 26. Loci of some fruit fly traits. Traits beginning with a capital letter denote the allele is dominant. Trait Chromosome Position (Map Units) yellow body 1 0.0 Curly wings 2 6.1 purple eyes 2 54.5 Wrinkled wings 3 46.0 eyeless 4 0.2 Mendelian Genetics: Lessons from the Fruit Fly 82 BS/LBS 158H into different cells, 5) the second part of meiosis proceeds with the now unpaired chromosomes lining along the cell’s equatorial plane by their centromeres during metaphase II, then 6) the duplicates separate during anaphase II through telophase II—an equatorial division—to form the gametes. Mendel proposed two laws, based on his work with garden peas, that he believed governed the inheritance of traits. The first was the law of segregation, which states that if different forms of a trait (alleles) are present in the parent, they separate during gamete formation; and the second, the law of independent assortment, states that if more than one pair of alleles are present in the parent, they will separately independently during gamete formation. These laws hold true only for special cases, where none of the traits examined are linked, but they do explain the movement of chromosomes. Mendel was fortunate none of the traits he was studying in garden peas occurred on the same chromosome as another; otherwise, he would have had extremely confusing results.