Introduction to Genetic Analysis 76

Introduction to Genetic Analysis 76 - 75 3.1 Historical...

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Unformatted text preview: 75 3.1 Historical development of the chromosome theory in diploid organisms genes and chromosomes come in sets of two, the chromosomes are not physically paired in the cells of the body. When body cells divide, each chromosome also divides in an accompanying nuclear division called mitosis. Homologous chromosomes physi- cally pair only during the nuclear division that occurs during gamete formation—the two nuclear divisions called meiosis. At mitosis and meiosis duplicated chromosomes are partitioned into the daughter cells by molecular ropes called spindle fibers . These fibers attach to specialized re- gions of the chromosomes called centromeres . When a heterozygote ( A/a ) undergoes mitosis, the A and the a chromosomes replicate, and the copies are pulled into opposite daughter cells, which then both have a genotype identical with the original ( A/a ). This process is summarized in Figure 3-1. Meiosis is two consecutive nuclear divisions, starting with a pair of replicated chromosomes. When a cell un- dergoes meiosis, the pulling apart of the replicated ho- mologous chromosomes by the spindle fibers leads to the creation of four haploid cells. When a heterozygote A / a undergoes meiosis, half the haploid cells are A and half a (see Figure 3-1). This separation of alleles into dif- ferent haploid cells is the basis for Mendel’s first law of equal segregation....
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This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.

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