Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Fig. 4-1Chapter 4overviewGenetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different fromthe combinations received from parents.• Independent assortment of homologous chromosomes (Anaphase I). Genes on non- homologous chromosomes (unlinked genes) assort independently.Fig. 4-6Fig. 4-7Using a testcrossto distinguish gamete genotypesFig. 4-850% = independent assortment(genes are not linked)Genetic recombination: mixing of genes during gametogenesis that produces gametes with combinations of genes that are different fromthe combinations received from parents.• Independent assortment of homologous chromosomes (Anaphase I). Genes on non- homologous chromosomes (unlinked genes) assort independently.• Crossing over (recombination among linked genes)Fig. 4-2cis linked: both dominant alleles on the same homologtrans linked: dominant alleles on different homologsFig. 4-3Crossing over• Physical exchanges among non-sister chromatids; visualized cytologically as chiasmata• Typically, several crossing over events occur within each tetrad in each meiosis (chiasmata physically hold homologous chromosome together and assure proper segregation at Anaphase I)p. 115Fig. 4-4Crossing over occurs at the four-strand stage(pre-meiotic G2 or very early prophase I)Fig. 4-5Crossing over can involve 2, 3, or 4 chromatids in a single meiosisCrossing over• Physical exchanges among non-sister chromatids; visualized cytologically as chiasmata• Typically, several crossing over events occur within each tetrad in each meiosis (chiasmata physically hold homologous chromosome together and assure proper segregation at Anaphase I)• The sites at which crossing over occur are random• The likelihood that a crossover occurs between any two particular sites (genes) is a function of the physical distance between those two sitesFig. 4-9Crossing over usually affects a minority of chromatids in a collection of meioses – recombinants are typically a minority of productsFig. 4-10<50% = linked genesA.H. Sturtevant (1911-3): frequency of crossing over between two genes is a function of their distance apart on the chromosome; created the first genetic mapnumber of recombinantsRecombination frequency = total number of progenyOne map unit = one centimorgan = 1% recombinantsFig. 4-11 Rationales:• Crossover events are random• Greater separation, greater likelihood that crossover will occur• Map distance should be sum of smaller intervals• Construct entire chromosome maps by mapping intervals• Linear map correlates with linear chromosomeMarkers used in trihybrid testcross in Drosophilav = vermilion eyes (red eyes; v+ are red-brown)cv = crossveinless (cv+ wings have crossveins)ct = cut wing (ct+ wings have regular margins)Data from three-point testcrossv+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)Progeny phenotypesv cv+ ct+580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 51448Steps in solving three-point testcross problem1. Anticipate and identify eight types of products (23)2. Identify pairs of reciprocal productsData from three-point testcrossv+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)Progeny phenotypesv cv+ ct+580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 51448Steps in solving three-point testcross problem1. Anticipate and identify eight types of products (23)2. Identify pairs of reciprocal products3. Identify parental types as the most frequent pair of products4. Identify double crossover products as least frequent pair of productsData from three-point testcrossv+/ v cv+/ cv ct+/ ct X v / v cv / cv ct / ct (trihybrid) (tester)Progeny phenotypesv cv+ ct+580v+ cv ct 592v cv ct+ 45v+ cv+ ct 40v cv ct 89v+ cv+ ct+ 94v cv+ ct 3v+ cv ct+ 51448 Parental types - ncodcoscoscoSteps in solving three-point testcross problem1. Anticipate and identify eight types of products (23)2. Identify pairs of reciprocal products3. Identify parental types as the most frequent pair of products4. Identify double crossover products as least frequent pair of products5. Compare the parental and double crossover products to deduce the order of the three gene lociFig. 4-12In dco products, the central marker is displacedrelative to the parental typesFig. 4-13Steps in solving three-point testcross problem1. Anticipate and identify eight types of products (23)2. Identify pairs of reciprocal products3. Identify parental types as the most frequent pair of products4. Identify double crossover products as least frequent pair of products5. Compare the parental and double crossover products to deduce the order of the three gene loci6. Compute map distances by breaking down the results for each interval85 + 8 1448(0.064)183 + 8 1448 (0.132)RF =Fig. 4-1285 + 8 1448(0.064)183 + 8 1448 (0.132)RF = 13.2 m.u. 6.4 m.u. Fig. 4-12 v ct cvInterference: crossing over in one region interferes with simultaneous crossing over in adjacent regionsExpected frequency of dco = product of frequency crossovers in two regions0.132 X 0.064 = 0.00840.084 X 1448 = 12 expected (if two sco are independent events)Interference: crossing over in one region interferes with simultaneous crossing over in adjacent regionsExpected frequency of dco = product of frequency crossovers in two regions0.132 X 0.064 = 0.00840.084 X 1448 = 12 expected (if two sco are independent events)Coefficient of coincidence = observed dco / expected dco8 / 12 = 0.667 Interference = 1 – coefficient of coincidence1 – 0.667 = 0.333Fig. 4-14Tomato karyotype (n=12)Fig. 4-14Tomato linkage map(1952)p. 136Typical phenotypic ratios for a variety of crosses(complete allele