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Unformatted text preview: 44200_03_p73-114 3/3/04 1:48 PM Page 102 102 Chapter 3 • The Chromosomal Basis of Inheritance Meiotic recombination between unlinked genes by independent assortment A Interphase. Chromosomes are unpaired. a B 1 b A Prophase. Chromosomes and centromeres have replicated, but centromeres have not split. A a a B B b b 2 A B B b b Prophase. Homologs synapse. A a 3 a A B B b b Anaphase. Centromeres attach to spindle and are pulled to poles of cell. A a a The other, equally frequent, alignment A A a a b B b B 4
A B B b b 4´
A A a b b B B www. ANIMATED ART Telophase. Two cells form. A a 5 a 5´ a A B B b b A A a b b B B Second anaphase. New spindles form, and centromeres finally divide. A a 6 a 6´ a A B
1 4 1 4 A b End of meiosis. Four cells produced from each meiosis. A B A b a b
1 4 1 4 a B a b
7´ a B 7 Figure 3-39 Meiosis in a diploid cell of genotype A/a ; B/b. The diagram shows how the segregation and assortment of different chromosome pairs give rise to the 1 : 1 : 1 : 1 Mendelian gametic ratio. Now we leave Neurospora to consider the law of independent assortment. The situation is diagrammed in Figure 3-39. The ﬁgure illustrates how the separate behavior of two different chromosome pairs gives rise to the 1 : 1 : 1 : 1 Mendelian ratios of gametic types diagnostic of independent assortment. The genotype of the meiocytes is A /a ; B/b, and the two allelic pairs, A /a and B/b, are shown on two different chromosome pairs. The hypothetical cell has four chromosomes: a pair of homologous long chromosomes and a pair of homologous short ones. Parts 4 and 4 of Figure 3-39 show the key step in understanding the effects of Mendel’s laws: they illustrate both equal segregation and independent assortment. There are two different allelic segregation patterns, ...
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