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Mendelian Gene[Cs & the Chromosome Theory of Inheritance

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Mendelian Gene[Cs & the Chromosome Theory of Inheritance Mendelian Genecs & the Chromosome Theory of Inheritance • Mendel 1840s-1860s – Ignored Completely • Darwin 1860 – Theory incompable with blending model of inheritance. • Microscopy techniques constantly improving • Mitosis discovered in 1870s • Meiosis discovered in 1890s • In 1900s Mendel’s work rediscovered! • Walter Suon & Theodor Boveri recognized the parallels between Mendel’s work and how chromosomes behaved during meiosis. Meiosis & Chromosomes 1. Chromosomes present in 1. Genes present in pairs in diploid cells. pairs in diploid cells. 2. Homologous chromosomes 2. Alleles segregate separate during meiosis. during meiosis. 3. Ferlizaon restores paired 3. Ferlizaon restores condion for chromosomes. paired condion for genes. Figure 15.2a P Generation Yellow-round Green-wrinkled seeds (YYRR) seeds (yyrr) Y y × r R R r Y y Meiosis Fertilization R Y y r Gametes Figure 15.2b All F1 plants produce yellow-round seeds (YyRr). F1 Generation R R y y r r Y Y LAW OF INDEPENDENT Meiosis LAW OF SEGREGATION ASSORTMENT Alleles of The two alleles for each R r r R genes on nonhomologous gene separate during chromosomes assort gamete formation. Metaphase I independently during gamete Y y Y y formation. 1 1 R r r R Anaphase I Y y Y y R r Metaphase r R 2 II 2 Y y Y y y Y Y Y y Gametes Y y y R r R r r r R R 1 1 1 1/ /4 YR /4 yr /4 Yr 4 yR Figure 15.2c LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F2 Generation An F1 × F1 cross-fertilization 3 Fertilization 3 Fertilization results recombines the in the 9:3:3:1 R and r alleles 9 : 3 : 3 : 1 phenotypic ratio in at random. the F2 generation. Chromosomal Theory of Inheritance • May seem obvious now, but was controversial at the me. • E.g. couldn’t explain quantave traits. • Some of the (in hindsight) greatest genecists of the day didn’t see the connecon. • Including… Chromosomal Theory of Inheritance • Thomas Hunt Morgan (Columbia University) • Mendel was all the rage because of the rediscovery of his laws AND the histological phenomena of meiosis and mitosis. • Morgan set out to prove him wrong. Chromosomal Theory of Inheritance • Morgan developed the fruit fly, Drosophila melanogaster for working on genecs. • Why??? What traits make this a good organism? Drosophila • But they weren’t ideal. • What would do you NEED to test how variaon was inherited? • VARIATION! • Hunt couldn’t find any. • “Two years’ work wasted. I have been breeding those flies for all that me and I’ve got nothing out of it”. Finally… • Aer 50 generaons of growing flies and looking at hundreds of thousands under a microscope… • One white-eyed male. • All others had red eyes. • These were termed “wild type”. • The allele AND the mutaon were called “white eye”. • So he began looking at this loss of funcon mutaon. So he did the cross • Red-eyed female X white-eyed male • Resulted in 100% red eye • Notaon he used was different. • Red eye = wild type = w+ • White eye = mutant = w • Small ‘w’ because mutaon is recessive Then he did the hybrid cross • F1 red eye female X F1 red eye male • 75% red eye: 25% white eye • What does this remind you of? BUT! • 100% of the females have red eyes, 50% of the males do! • Clearly, something to do with sex determinaon. • The traits were SEX- LINKED. Sex determinaon • Sex determinaon chromosomal (like us) • Autosomes: 3 pairs of chromosomes, indisnguishable under microscope. • Sex chromosome: 1 pair, quite different under microscope in males, idencal in females. • Female: XX; Male: XY What chromosomes will each sex pack into the gametes? Female Gametes X X Male X Gametes Y Sex determinaon • Y-chromosome much smaller than X, lacks many genes. • Morgan concluded that the gene for eye color is on the X-chromosome, no allele on the Y- chromosome. • Why? The crosses again P: Xw+ Xw+ female x Xw Y male 100% red-eye The crosses again w+ w w F1: X X female x X Y male Females 100% red-eye Males 50% red-eye Sex-linkage! Chromosomes • Morgan set out to disprove the chromosome theory of inheritance but provided the best evidence to date for it. • Showed that eye-color gene was associated with that odd sex- chromosome. Linkage • Morgan discovered a • NO INDEPENDENT more general ASSORTMENT phenomenon known as chromosome linkage. • Linkage: genes for different characters that are on the same chromosome. • What Mendelian Law does this mean does not always hold? Back to Drosophila • Over the years, LOTS of new mutants have been isolated. • Mutaon rates increased using mutagens – Chemicals – X-rays – Ultraviolet radiaon – Etc. • Let’s look at linkage now. Figure 15.9-4 EXPERIMENT P Generation (homozygous) Double mutant Wild type (black body, (gray body, normal wings) vestigial wings) b+ b+ vg+ vg+ b b vg vg F1 dihybrid Double mutant (wild type) TESTCROSS b+ b vg+ vg b b vg vg Testcross offspring Eggs b+ vg+ b vg b+ vg b vg+ Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal b vg If this were pleiotropy! Sperm b+ b vg+ vg b b vg vg b+ b vg vg b b vg+ vg PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 RESULTS 965 : 944 : 206 : 185 PREDICTED RATIOS If genes are located on different chromosomes: 1 : 1 : 1 : 1 If genes are located on the same chromosome and parental alleles are always inherited together: 1 : 1 : 0 : 0 RESULTS 965 : 944 : 206 : 185 • Where do these others come from? – Doesn’t fit with independent assortment. – Doesn’t fit with pleiotropy. • Only 83% are the parental type. • The other 17% are from crossing over. • They are on the same chromosome. • They therefore do not sort completely independently. • Only assort independently if crossing over makes them independent. The test cross • ANY test cross that produces more than 50% of the parental phenotypes indicates that the genes are linked. • What were the parental phenotypes in the previous result? Figure 15.10a Testcross Gray body, normal wings Black body, vestigial wings parents (F1 dihybrid) (double mutant) b+ vg+ b vg b vg b vg Replication Replication of chromosomes of chromosomes b+ vg+ b vg b+ vg+ b vg b vg b vg b vg b vg Meiosis I b+ vg+ Meiosis I and II b+ vg b vg+ b vg Meiosis II Recombinant chromosomes b+vg+ b vg b+ vg b vg+ b vg Eggs Sperm Figure 15.10b Recombinant chromosomes b+vg+ b vg b+ vg b vg+ Eggs Testcross 965 944 206 185 offspring Wild type Black- Gray- Black- (gray-normal) vestigial vestigial normal b vg b+ vg+ b vg b+ vg b vg+ b vg b vg b vg b vg Sperm Parental-type offspring Recombinant offspring Recombination 391 recombinants × 100 = 17% frequency = 2,300 total offspring Mapping Chromosomes • Can use this informaon to map chromosomes: Determine the relave posions of genes on chromosomes. • HOW? • Take parallel linear structures, allow them to cross over at random. • The probability of an event happening between any two points on that line is dependent upon the distance between those two points. Mapping Chromosomes • The farther apart 2 genes are on a chromosome, the more likely they are to be separated by a crossing over event. • Sturtevant developed a system where one map unit = 1% recombinaon frequency (RF). • So, how many map units apart are b & vg? Mapping Chromosomes • Let’s take a 3rd gene and see how we can use RF to map the relave posion of genes on chromosomes. • Add Cinnebar eye color cn+. • Have three opons. Mapping Chromosomes • Perform the crosses: Trihybrid test cross. • Know that B+vg+ is 17. • Do the second test cross: – B+ b cn+ cn x b b cn cn • RF = 9% • Can be: Mapping chromosomes • Now, do the third test cross. – cn+ cn vg+vg x cn cn vg vg • Get RF = 9.5% • Does not add up to 8 units expected! Why not? • There is a 1.5% chance that there are two crossing over events between the two, restoring the parental condion. Mapping Chromosomes • Does a 1:1:1:1 test cross rao mean that the two traits are definitely on different chromosomes? • NO. Why not? • Could be more than 50 map units apart—50% chance of crossing over effecvely shuffles them enrely. Chromosomes • How were chromosomes originally discovered and named? • Why was it debated whether or not chromosomes carried the genec material? • What evidence did Morgan and his students use to support the hypothesis that chromosomes carry the genec material? A liNle more… • Of what are chromosomes made? • If you were around before Watson & Crick, which would you think would carry the genec material? • What did Watson & Crick discover? DNA • Explained a fundamental mystery about living organisms. • Showed that a simple molecule could carry complex instrucons. • How? • Unique base pairing. Sex Chromosomes A wee bit more • About 1500-2000 genes on human X chromosome. • 78 genes coding for 25 proteins on the Y chromosome. • What are these responsiBle for? Gene Dosage & the X-chromosome • Note that men only have one copy of genes on the X-chromosome. • Are they heterozygous or homozygous? • NO. They are hemizygous. Gene dosage and the X-chromosome • Most X-linked genes are expressed EQUALLY in males and females. • How are autosomal genes expressed? • So, how does this work? • X-inacvaon & Barr bodies. Barr bodies • Barr bodies are determined randomly at me of X-inacvaon during embryonic development.
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