Lecture 12 Linkage And Recombination PDF

10/19/2022 UNIVERSITY OF SOUTHERN MINDANAO Gen 211a Lecture 12 PRINCIPLES OF GENETICS LINKAGE AND RECOMBINATION GWEN IRIS D. EMPLEO Subject Professor...

10/19/2022 UNIVERSITY OF SOUTHERN MINDANAO Gen 211a Lecture 12 PRINCIPLES OF GENETICS LINKAGE AND RECOMBINATION GWEN IRIS D. EMPLEO Subject Professor Definition of Terms Definition of Terms Linkage - condition in which 2 or more non-allelic genes Complete linkage – genes on the same chromosome tend to be inherited together are very close together so that they are always - loci of the genes are on the same chromosome transmitted together - genes do not assort independently - genes can be separated through crossing-over Incomplete linkage – genes are on the same chromosome - genes are far from each other Linkage group - group of genes that have their loci in the same chromosome - recombinant types are produced by crossing-over - number of linkage groups = number of chromosomes in a haploid cell Types of Linkage Types of Linkage 1. Complete linkage - two loci are so close that they 2. Incomplete linkage – due to crossing-over are always inherited together Crossing over within a region is rare (so crossing-over products are rare). Most of the time it will not occur, so Dihybrid cross AABB x aabb AaBb parental types will be the most common gametes produced Test cross: AaBb x aabb 50% AB 50% ab 0% Ab 0% aB 1  10/19/2022 Gametes produced when genes assort Gametes produced when genes are linked independently Gametes produced when genes are linked Linkage Map A genetic map of a chromosome based on recombination frequencies Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency Recombination frequency is determined by observing the number of offspring showing recombinant phenotypes Map units indicate relative distance and order, not precise locations of genes Determination of Linkage Testcross progeny Phenotype Genotype Crossover type No. The strength of linkage is expressed as linkage value. 1. Normal V G1 Va Parental 235 v g1 va 2. Virescent, glossy, variable sterile v g1 va Parental 270 Linkage value = Total number of crossover x 100 v g1 va Total no. of population 3. Glossy, variable sterile V g1 va Single crossover region I 60 v g1 va 4. Virescent v G1 Va Single crossover region I 62 Linkage values involving three genes v g1 va 5. Variable sterile V G1 va Single crossover region II 40 Make a cross between an F1 and the recessive parent v g1 va 6. Virescent, glossy v g1 Va Single crossover region II 48 v g1 va V G1 Va x v g1 va v g1 va v g1 va 7. Glossy V g1 Va Double crossover 7 v g1 va 8. Virescent, variable sterile v G1 va Double crossover 4 v g1 va 2  10/19/2022 Computation of linkage values Interference and coefficient of coincidence The farther apart two genes are, the higher the The strength of interference can be measured by the probability that a crossover will occur between them, coefficient of coincidence (C.C.): and therefore higher the recombination frequency C.C. = actual DCO Region I = 60 + 62 + 7 + 4 x 100 = 18.3% expected DCO 726 e.g. the coefficient of coincidence is: Region II = 40 + 48 + 7 + 4 x 100 = 13.6 % 726 C.C. = 0.0152 = 0.61 0.0249 DCO = 7 + 4 x 100 = 1.52% 726 Interference and coincidence Interference and coincidence Crossing over does not occur uniformly along a This lack of independence is called interference chromosome. Results in the observation of fewer double Fewer crossovers occur in the area around the crossover types than would be expected according centromere than in other areas of the chromosome to true map distance. Making the loci appear closer together than they actually are The formation of one chiasma typically makes it less likely that a second chiasma will form in the immediate vicinity of the first Linkage Mapping is the Basis for Determining the proper gene order Chromosome Mapping A cross is made between homozygous wild-type Each gene has a particular, well-defined locus in the female Drosophila (a+a+b+b+c+c+) and triple-mutant males (aabbcc) (the order here is arbitrary). chromosome The F1 (a+ab+bc+c) females are testcrossed back to Linkage values would indicate the definite serial order the triple-mutant males and the F2 phenotypic ratios are for the genes in a chromosome and would serve as as follows: bases for mapping distance relations among linked genes a+b c 18 Chromosomes can be mapped effectively in terms of a b+c 112 crossover percentage obtained from genetic abc 308 experiments a+b+c 66 1% crossing-over is equivalent to 1 map unit or a b c+ 59 centiMorgan. a+b+c+ 321 a+b c+ 102 a b+c+ 15 1000 3  10/19/2022 Determining the proper gene order Determining the proper gene order 1. The gene order can be determined by examination of the relative frequencies of the F2 phenotypes. c. Then, to determined the physical order of loci, compare a. Because linked loci tend to stay together, the the parental and double crossover phenotypes. The non-crossover (NCO) or parental phenotypes marker that appears to “switch places” is in the middle should be most frequent (and equal in number). [technically, this marker is said to be “out of phase”]. In this case, Here, the a+b+c+ NCO and ab+c+ DCO phenotypes indicate that the a locus falls between the b and c loci. a+b+c+ (321) and abc (308) The correct order of the loci is bac. [Note that this order is equivalent to cab, and that the order of the b. Because simultaneous crossovers between the outside markers is arbitrary]. outside and middle loci are unlikely, the double- crossover (DCO) genotypes should be the least frequent. We observe a+bc (18) and ab+c+ (15) 3. The percent recombination between two markers 2. The remaining two pairs of phenotypes correspond indicates the map distance between them: to single-crossovers (SCO) events in the region between either b and a, or between a and c. 1% recombination = 1 map unit (m.u.). a. b+ac (112) and ba+c+ (102) phenotypes indicate To determine the map distance between a pair of loci, crossovers between b and a. count the number of SCO and DCO events, and use b. b+a+c (66) and bac+ (59) phenotypes indicate the following formula crossovers between c and a. Map distance = % recombination SCO phenotypes + DCO phenotypes x 100 = total progeny Map distance = % recombination 4. We can now draw a map segment showing order and distances among loci. Again, note that the orders =SCO phenotypes + DCO phenotypes x 100 bac and cab are equivalent and that the left/right the total progeny orientation of this map is arbitrary. Map distance (ba) = 112 + 102 + 18 + 15 x 100 = 24.7% = 24.7 m.u. b a c 1000 | 24.7 | 15.8 | Map distance (ac) = 66 + 59 + 18 + 15 x 100 = 15.8% = 15.8 m.u. 1000 | 40.5 | Map distance (bc) = 24.7 m.u. + 15.8 m.u. = 40.5 m.u. 4  10/19/2022 USING A GENE MAP USING A GENE MAP The gene map can be used as a table of There is a certain probability that a crossover will probabilities to predict the expected amount of form between a and b loci (equivalent to the map recombination between certain loci. distance between a and b) and another In a test cross the male contributes only recessive independent probability that a cross-over will occur alleles. between b and c loci (equivalent to the map Recombination occurs in the formation of the distance between b and c). female gametes. Therefore, whatever alleles present in the female The probability of a double crossover is the gamete will be expressed in the phenotype of the PRODUCT of these two independent probabilities. offspring. Example 2: Given the map segment Expected SCO (cn-vg) cn vg sm | 9.5 | 24.5 | From the gene map 9.5% of the gametes would be expected to have crossovers between cn and vg, however, this In a testcross of cn+vg+sm+ // cn vg sm includes the 2.3% of double crossovers. Expected DCO = (% recomb. cn-vg) (% recomb. vg-sm) Therefore, 9.5-2.3 = 7.2% of the female gametes should = 0.095 x 0.245 = 2.3% have single crossovers: Therefore , we expect to find 2.3% of the female gametes to 3.6% cn vg+ sm+ and 3.6% cn+ vg sm be the results of double crossovers 1.15% cn+vg sm+ 1.15% cn vg+ sm Factors Affecting Recombination Frequencies Expected SCO (vg-sm) 1. Sex. In general, the heterogametic sex of a species has From the gene map 24.5% of the gametes would be lower crossover frequencies, e.g. Drosophila melanogaster expected to have crossovers between vg and sm, this – crossing-over in males is completely supressed includes the 2.3% of double crossovers. 2. Maternal Age. Crossing-over tends to decrease when the maternal age increase. Therefore 22.2% of the female gametes should have single 3. Temperature. In Drosophila, crossing-over tends to crossovers increase when the temperature is cooler or warmer than 11.1% cn+vg+sm and 11.1% cn vg sm+ 22oC. 4. Cytoplasmic effect. Some female Drosophila had reduced a recombination frequency that passes through their daughters, indicating that the crossover factors are carried in the cytoplasm. 5  10/19/2022 Factors Affecting Recombination Frequencies Factors Affecting Recombination Frequencies 5. Nutrition. Young Drosophila females on high calcium diet 9. Chromosome Structure. Crossing-over is reduced in the showed deceased crossing-over. Larval starvation at heterozygotes of structural rearrangements of the certain ages generally increased crossing-over. chromosome. Sturtevant (1926) observed that 6. Chemical effects. Crossing-over increased upon aberrations in one pair of chromosomes increased the injection of antibiotics, such as mitomycin C and frequency of crossing-over in other non-homologous actinomycin D. chromosomes. 7. Radiation. When Drosophila exposed to X-ray irradiation, crossing-over increases. 10. Centromere. Genes located close to the centromere 8. Genotype. Certain genes affected the preconditions for tended to show reduced crossing-over, but the physical exchange. e.g. chromosome pairing. Others acted after distance between them was not changed. pairing had been completed 6