Introduction to Genetic Analysis 75

Introduction to Genetic Analysis 75 - with, there are two...

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74 Chapter 3 The Chromosomal Basis of Inheritance CHAPTER OVERVIEW T he elegance of Mendel’s analysis is that to assign alleles to phenotypic differences and to predict the outcomes of crosses it is not necessary to know what genes are, nor how they control phenotypes, nor how the laws of segregation and independent assortment are accomplished inside the cell. We simply represent genes as abstract, hypothetical factors using symbols, and move these around in crosses without any concern for their molecular structures or their locations in a cell. Never- theless, our interest naturally turns to the location of genes in cells and the mechanisms by which segregation and independent assortment are achieved at the cellular level. We shall see in this chapter that the key components to answering our questions about the cellular basis of heredity are the chromosomes. In eukaryotic cells, most of the chromosomes are worm-shaped structures found in the nuclei. In most of the larger organisms we are familiar
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Unformatted text preview: with, there are two complete homologous sets of chro-mosomes in each nucleus, a condition known as diploidy . Genes are functional regions in the long, continuous coiled piece of DNA that constitutes a chromosome. There are other DNA segments between genes, and this intergenic material differs in its extent and nature in dif-ferent species. In higher organisms much of this intergenic DNA is repetitive and of unknown function. Since a diploid cell has two sets of chromosomes, genes therefore are present in pairs. However, although Figure 3-1 Transmission of genes and chromosomes at mitosis and meiosis. CHAPTER OVERVIEW Figure MITOSIS MEIOSIS Centromeres Premitotic DNA replication Premeiotic DNA replication Single division Two divisions A AA a Two daughter cells also of genotype / Four haploid meiotic products , Heterozygous diploid ( / ) 1 2 1 2 44200_03_p73-114 3/3/04 1:47 PM Page 74...
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