Chap 4 - Notes - GENETICS (BIO 325) _ Chapter 4 (Mapping of...

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GENETICS (BIO 325) ________________________________________________________________ Chapter 4 (Mapping of Eukaryote Chromosomes by Recombination) 1. Linkage When two or more genes reside in the same chromosome, they are said to be linked. They may be linked together on one of the autosomes or connected together on the sex chromosome. Genes on different chromosomes are distributed into gametes independently of one another (Mendel’s Law of Independent Assortment). Genes on the same chromosome, however, tend to stay together during the formation of gametes. Thus the results of testcrossing dihybrid individuals will yield different results, depending upon whether the genes are linked or on different chromosomes. Genes on different chromosomes Parents A/a; B/b x a/a; b/b Gametes AB Ab aB ab ab F 1 ¼ A/a; B/b: ¼ A/a; b/b: ¼ a/a; B/b: ¼ a/a; b/b Genes on the same chromosome Parents AB/ab x ab/ab Gametes AB ab ab F 1 ½ AB/ab: ½ ab/ab Large deviations from a 1:1:1:1 ratio in the testcross progeny of a dihybrid could be used as evidence for linkage. Linked genes do not always stay together, however, because homologous non-sister chromatids may exchange segments of varying length with one another during meiotic prophase. During meiosis, homologous chromosomes pair with one another in a process called “synapsis” and the points of genetic exchange, called “chiasmata”, produce recombinant gametes through crossing over. 2. Crossing Over
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B b A A b B B b A B b During meiosis, each chromosome replicates, forming two identical sister chromatids; homologous chromosomes pair (synapse) and crossing over occurs between non-sister chromatids. This latter process involves the breakage and rejoining of only two of the four chromatids at any given point on the chromosome. In the following figure, a crossover occurs in the region between the A and B loci. Two of the meiotic products (AB and ab) have the genes linked in the same way as they were in the parental chromosomes. These products are produced from chromatids that were not involved in crossing over and are referred to as non- crossover
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This note was uploaded on 04/03/2009 for the course BIO 325 taught by Professor Saxena during the Spring '08 term at University of Texas at Austin.

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Chap 4 - Notes - GENETICS (BIO 325) _ Chapter 4 (Mapping of...

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