Linkage - Linkage and Chromosome Mapping(Chapter 7...

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Linkage and Chromosome Mapping (Chapter 7) Suggested problems : 1-10, 14, 16-19, 22-24, 26, see pp 193-196. Question: Humans have approx. 20,500 genes. How many chromosomes should humans have for the alleles of these 20,500 genes segregate independently of one another? Terminology Linkage . The presence of two or more genes in the same chromosome, which leads to a tendency for parental traits to be inherited together or travel together. Genes are units of heredity, whereas chromosomes are units of transmission. Genetic Linkage is rule rather than exception. It may be complete , incomplete , or absent (not detectable), depending on the distance between linked genes in a chromosome. Alfred H. Sturtevant discovered linkage and constructed the first genetic map. Example. Genes " a " and " b " are in the same chromosome: three possible scenarios (below). Linkage " a " and " b " are too close to each other. " a " and " b " are separated to allow crossing-over between them in up to 100% of meiocytes. " a " and " b " are too far apart to allow crossing- over between them in 100% of meiocytes. a b complete incomplete b a < 50 Not detectable a b > 50
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4 types of gametes No Linkage 2 types of gametes Complete Linkage Parental Parental Recombinan t Recombinan t Incomplete Linkage 4 types of gametes but not equal in frequency Example: two linked characters in tomatoes Vine height (tall vs. dwarf) and fruit shape (round vs. pear) P Tall x Dwarf Round x Pear F 1 Tall Round F 2 3 Tall : 1 Dwarf 3 Round : 1 Pear ! Both characteristics show monogenic inheritance when one studies them separately.
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Linkage violates Mendel’s Second Law A dihybrid testcross yields results that are not consistent with the expected 1:1:1:1 ratio. P Tall, Round x Dwarf, Pear (+ + / + +) (dd / pp) F 1 Tall, Round (+ d / + p) Testcross + d / + p x dd / pp Cont… Observed Expected Tall, round 81 ( 4 ) 49.75 ( 1 ) Tall, pear 22 ( 1 ) 49.75 ( 1 ) Dwarf, round 17 ( 1 ) 49.75 ( 1 ) Dwarf, pear 79 ( 4 ) 49.75 ( 1) 199 199 If linkage If independent assortment Cont… Hypothesis. This is a dihybrid testcross ratio of 1:1:1:1 for two genes segregating independently. X 2 = 73.85 ; X 2 for 3 df. at 1% is 11.34 . There is not even one in thousand chance that the hypothesis is true. The hypothesis must be rejected . Alternative Hypothesis . The vine height and the fruit shape genes are on the same chromosome; they are not segregating independently. How to detect linkage when you see it? Parental traits tend to be inherited together. Testcrosses produce parental types in excess of >50%. Alleles of two linked genes may be arranged in two different ways in the chromosome. Coupling and repulsion refer only to F 1 hybrids .
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