Chapter 15 The Chromosomal Basis of Inheritance PowerPoint Lectures for Biology Eighth Edition Neil Campbell and Jane Reece Fig 15 2 P Generation Yellow round seeds YYRR Y R Y r R Green wrinkled seeds yyrr y r y Meiosis Fertilization R Y Gametes y r All F1 plants produce yellow round seeds YyRr F1 Generation R R y r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation y r LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation Meiosis R r Y y r R Y y Metaphase I 1 1 R r Y y r R Y y Anaphase I R r Y y Metaphase II r R Y y 2 2 Y Y Gametes R R 1 F2 Generation 4 YR y r r r 1 4 Y Y y yr r 1 4 Yr y y R R 1 4 yR An F1 F1 cross fertilization 3 3 9 3 3 1 Morgan first observed and noted Wild type or normal phenotypes that were common in the fly populations Traits alternative to the wild type Are called mutant phenotypes Figure 15 3 Morgan determined That the white eye mutant allele must be located on the X chromosome EXPERIMENT Morgan mated a wild type red eyed female with a mutant white eyed male The F1 offspring all had red eyes P Generation X F1 Generation Morgan then bred an F1 red eyed female to an F1 red eyed male to produce the F2 generation RESULTS The F2 generation showed a typical Mendelian 3 1 ratio of red eyes to white eyes However no females displayed the white eye trait they all had red eyes Half the males had white eyes and half had red eyes F2 Generation Figure 15 4 CONCLUSION Since all F1 offspring had red eyes the mutant white eye trait w must be recessive to the wild type red eye trait w Since the recessive trait white eyes was expressed only in males in the F2 generation Morgan hypothesized that the eye color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome as diagrammed here P Generation W X X X W W W Ova eggs F1 Generation X Y Sperm W W W W W Ova eggs F2 Generation Sperm W W W W W W W W Recombination of Unlinked Genes Independent Assortment of Chromosomes When Mendel followed the inheritance of two characters He observed that some offspring have combinations of traits that do not match either parent in the P generation Gametes from yellow round heterozygous parent YyRr YR Gametes from greenwrinkled homozygous recessive parent yyrr yr Yr yR Yyrr yyRr yr YyRr yyrr Parentaltype offspring Recombinant offspring Morgan crossed flies That differed in traits of two different characters P Generation homozygous EXPERIMENT Morgan first mated true breeding Wild type wild type flies with black vestigial winged flies to produce gray body heterozygous F1 dihybrids all of which are wild type in normal wings appearance He then mated wild type F1 dihybrid females with b b vg vg black vestigial winged males producing 2 300 F2 offspring which he scored classified according to F1 dihybrid phenotype wild type gray body normal wings CONCLUSION If these two genes were on different chromosomes the alleles from the F1 dihybrid would sort into gametes independently and we would expect to see equal numbers of the four types of offspring If these two genes were on the same chromosome we would expect each allele combination B vg and b vg to stay together as gametes formed In this case only offspring with parental phenotypes would be produced Since most offspring had a parental phenotype Morgan concluded that the genes for body color and wing size are located on the same chromosome However the production of a small number of offspring with nonparental phenotypes indicated that some mechanism occasionally breaks the linkage between genes on the same chromosome Figure 15 5 Double mutant black body vestigial wings Double mutant black body vestigial wings x b b vg vg Double mutant black body vestigial wings b b vg vg Double mutant TESTCROSS black body x vestigial wings b b vg vg RESULTS b vg b vg b vg 965 944 Wild type Black gray normal vestigial b vg b vg 206 Grayvestigial 185 Blacknormal Sperm b b vg vg b b vg vg b b vg vgb b vg vg Parental type offspring Recombinant nonparental type offspring Linked genes Exhibit recombination frequencies less than 50 Testcross parents b vg Gray body normal wings b vg F1 dihybrid Replication of chromosomes b vg Meiosis I Crossing over between b and vg loci produces new allele combinations b vg b vg vg b b vg vg b b vg b vg Meiosis II Segregation of chromatids produces recombinant gametes with the new allele combinations Gametes Black body vestigial wings b vg double mutant Replication of chromosomes b vg Meiosis I and II Even if crossing over occurs no new allele combinations are produced Recombinant chromosome Ova Sperm b vg b vg b vg b vg b vg Testcross offspring b vg Sperm b vg Figure 15 6 b vg 944 965 BlackWild type gray normal vestigial b vg b vg b vg b vg b vg 206 Grayvestigial b vg b vg b vg Ova 185 BlackRecombination normal b vg frequency b vg Parental type offspring Recombinant offspring 391 recombinants 2 300 total offspring 100 17 A linkage map Is the actual map of a chromosome based on recombination frequencies APPLICATION A linkage map shows the relative locations of genes along a chromosome TECHNIQUE A linkage map is based on the assumption that the probability of a crossover between two genetic loci is proportional to the distance separating the loci The recombination frequencies used to construct a linkage map for a particular chromosome are obtained from experimental crosses such as the cross depicted in Figure 15 6 The distances between genes are expressed as map units centimorgans with one map unit equivalent to a 1 recombination frequency Genes are arranged on the chromosome in the order that best fits the data RESULTS In this example the observed recombination frequencies between three Drosophila gene pairs b cn 9 cn vg 9 5 and b vg 17 best fit a linear order in which cn is positioned about halfway between the other two genes Recombination frequencies 9 5 9 17 Chromosome b cn vg The b vg recombination frequency is slightly less than the sum of the b cn and cn vg frequencies because double crossovers are fairly likely to occur between b and vg in matings tracking these two genes A second crossover Figure 15 7 would cancel out the first and thus reduce the observed b vg recombination frequency Many fruit fly genes Were mapped initially using recombination frequencies I Y II X IV III Mutant phenotypes Short aristae Black body 0 Figure 15 8 Long aristae appendages on head Cinnabar Vestigial eyes wings 48
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