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Unformatted text preview: Linkage and mapping Linkage So far  we have stressed how alleles assort independently Possible gametes from an AaBb individual are AB/Ab/aB/ab with equal likelihood This is due to the assumption that during meiosis, each trait is on a separate chromosome 2 But what if genes A and B are on the SAME chromosome? 3 B A a b a b B A B A B A a b a b Now, they do not randomly distribute but rather, they make only two gametes: AB and ab Alleles on different chromosomes, assort randomly and in addition produce gametes Ab and aB What would the ratio of progeny be in AaBb self cross where A and B are linked on the same chromosome (Punitt square from data on previous slide)? A) 9:3:3;1 B) 15:1 C) 4:8:4 D) 4:4:4:4 4 AB AB ab ab AB AABB AABB ABab ABab AB AABB AABB ABab ABab ab ABab ABab abab abab ab ABab ABab abab abab What would the ratio of progeny be in AaBb self cross where A and B are linked on the same chromosome (Punitt square from data on previous slide)? A) 9:3:3;1 B) 15:1 C) 4:8:4 D) 4:4:4:4 4 AB AB ab ab AB AABB AABB ABab ABab AB AABB AABB ABab ABab ab ABab ABab abab abab ab ABab ABab abab abab Some terms Genes close together on the same chromosome are linked Genes that are linked are in the same linkage group Genes on the same chromosome are in one linkage group 5 Crossing over or recombination 6 Chromo 1 Chromo 2 Chromo 2 Chromo 1 Homologues pair in meiosis I  red is ♂ blue is ♀ Crossing over (cont’d) 7 After crossing over two gametes are parental two are NOVEL What’s happening at the “crossover” point (synaptonemal complex)  a double crossover 8 Note there are NEW combinations as a result 9 B A b a b a B A b A B A B a b a Gametes possible WITHOUT recombination and AB linked: AB, ab Gametes possible WITH recombination: AB, ab aB, Ab Genetic variability Chromosomal reassortment alone  variability due to random assortment of 23 chromsome pairs? 2 23 = 8x10 6 Not too may possibilities really Allow 2 crossovers/chromosome which would yield 9 possible pairs of chromosomes so there would be 9 23 = 8.9x10 23 combos  Avogadro would be proud! 10 1 2 3 1 2 3 Recombination frequency As a review, lets first consider two genes A and B on different chromosomes (i.e. undergo independent assortment) and each has two alleles A/a and B/b. Now test cross the heterozygote AaBb with aabb. What fraction of the progeny have either AaBb or aabb genotypes? a)1.0 b)0.75 c)0.5 d)0.25 11 Progeny with either AaBb or aabb Answer Lets represent genes like this where progeny get 1 from column a and on from like b: 12 A a Parent 1 genotype: AaBb All gametes from homozygote contain ab Gametes from heterozygote are ab, Ab, Ba, AB Parent 2 genotype: aabb Parent 1 gametes: B b a a b b Parent 1 gametes: split line means different chromosome above or below line are alternate alleles a a b b A a B b Progeny with Aabb or aaBb a a B b A a b b AB Ab aB ab ab ABab Abab aBab abab ab ABab Abab aBab abab Progeny with either AaBb or aabb Answer...
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 Spring '06
 WINEY,MARK
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