Mendelian Gene*cs & the Chromosome Theory of Inheritance

Transcription

1 Mendelian Gene*cs & the Chromosome Theory of Inheritance Mendel 1840s- 1860s Ignored Completely Darwin 1860 Theory incompa*ble with blending model of inheritance. Microscopy techniques constantly improving Mitosis discovered in 1870s Meiosis discovered in 1890s In 1900s Mendel s work rediscovered! Walter SuNon & Theodor Boveri recognized the parallels between Mendel s work and how chromosomes behaved during meiosis.

2 Meiosis & Chromosomes 1. Chromosomes present in pairs in diploid cells. 2. Homologous chromosomes separate during meiosis. 3. Fer*liza*on restores paired condi*on for chromosomes. 1. Genes present in pairs in diploid cells. 2. Alleles segregate during meiosis. 3. Fer*liza*on restores paired condi*on for genes.

3 Figure 15.2a P Generation Y Y R R Yellow-round seeds (YYRR) y y r r Green-wrinkled seeds (yyrr) Meiosis Gametes R Y Fertilization y r

4 Figure 15.2b All F 1 plants produce yellow-round seeds (YyRr). F 1 Generation LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. R Y r y R R y r r Y Y Meiosis Metaphase I y r Y R y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. 1 R r r R 1 Y y Anaphase I Y y R r r R Metaphase 2 II 2 Y y Y y Gametes Y Y y y Y Y y y R R r r r r R R 1 / 4 1 / 4 1 / 4 1 / 4 YR yr Yr yr

5 Figure 15.2c LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F 2 Generation An F 1 F 1 cross-fertilization 3 Fertilization recombines the R and r alleles at random. 9 : 3 : 3 : 1 3 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation.

6 Chromosomal Theory of Inheritance May seem obvious now, but was controversial at the *me. E.g. couldn t explain quan*ta*ve traits. Some of the (in hindsight) greatest gene*cists of the day didn t see the connec*on. Including

7 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.

8 Chromosomal Theory of Inheritance Morgan developed the fruit fly, Drosophila melanogaster for working on gene*cs. Why??? What traits make this a good organism?

9 Drosophila But they weren t ideal. What would do you NEED to test how varia*on 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.

10 Finally Afer 50 genera*ons 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 muta*on were called white eye. So he began looking at this loss of func*on muta*on.

11 So he did the cross Red- eyed female X white- eyed male Resulted in 100% red eye Nota*on he used was different. Red eye = wild type = w + White eye = mutant = w Small w because muta*on is recessive

12 Then he did the hybrid cross F 1 red eye female X F 1 red eye male 75% red eye: 25% white eye What does this remind you of?

13 BUT! 100% of the females have red eyes, 50% of the males do! Clearly, something to do with sex determina*on. The traits were SEX- LINKED.

14 Sex determina*on Sex determina*on chromosomal (like us) Autosomes: 3 pairs of chromosomes, indis*nguishable under microscope. Sex chromosome: 1 pair, quite different under microscope in males, iden*cal in females. Female: XX; Male: XY

15 What chromosomes will each sex pack into the gametes? Female Gametes X X Male Gametes X Y

16 Sex determina*on 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?

17 The crosses again P: X w+ X w+ female x X w Y male 100% red- eye

18 The crosses again F 1 : X w+ X w female x X w Y male Females 100% red- eye Males 50% red- eye Sex- linkage!

19 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.

20 Linkage Morgan discovered a more general 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? NO INDEPENDENT ASSORTMENT

21 Back to Drosophila Over the years, LOTS of new mutants have been isolated. Muta*on rates increased using mutagens Chemicals X- rays Ultraviolet radia*on Etc. Let s look at linkage now.

22 Figure EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings) b + b + vg + vg + Double mutant (black body, vestigial wings) b b vg vg F 1 dihybrid (wild type) b + b vg + vg TESTCROSS Double mutant b b vg vg Testcross offspring Eggs b + vg + b vg b + vg b vg + Wild type (gray-normal) Blackvestigial Grayvestigial Blacknormal b vg If this were pleiotropy! Sperm PREDICTED RATIOS If genes are located on different chromosomes: b + b vg + vg b b vg vg b + b vg vg b b vg + vg 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

23 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.

24 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?

25 Figure 15.10a Testcross parents Gray body, normal wings (F 1 dihybrid) b + vg + Black body, vestigial wings (double mutant) b vg Replication of chromosomes b vg b vg Replication of chromosomes b + vg + b vg b + vg + b vg b vg b vg Meiosis I b vg b vg b + vg + b + vg Meiosis I and II b vg + Meiosis II b vg Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + b vg Sperm

26 Figure 15.10b Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Testcross offspring 965 Wild type (gray-normal) 944 Blackvestigial 206 Grayvestigial 185 Blacknormal 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 frequency = 391 recombinants 2,300 total offspring 100 = 17%

27 Mapping Chromosomes Can use this informa*on to map chromosomes: Determine the rela*ve posi*ons 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.

28 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% recombina*on frequency (RF). So, how many map units apart are b & vg?

29 Mapping Chromosomes Let s take a 3 rd gene and see how we can use RF to map the rela*ve posi*on of genes on chromosomes. Add Cinnebar eye color cn +. Have three op*ons.

30 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:

31 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 condi*on.

32 Mapping Chromosomes Does a 1:1:1:1 test cross ra*o 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 effec*vely shuffles them en*rely.

33 Chromosomes How were chromosomes originally discovered and named? Why was it debated whether or not chromosomes carried the gene*c material? What evidence did Morgan and his students use to support the hypothesis that chromosomes carry the gene*c material?

34 A linle more Of what are chromosomes made? If you were around before Watson & Crick, which would you think would carry the gene*c material? What did Watson & Crick discover?

35 DNA Explained a fundamental mystery about living organisms. Showed that a simple molecule could carry complex instruc*ons. How? Unique base pairing.

36 About genes on human X chromosome. 78 genes coding for 25 proteins on the Y chromosome. What are these responsible for? Sex Chromosomes A wee bit more

37 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.

38 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- inac*va*on & Barr bodies.

39 Barr bodies Barr bodies are determined randomly at *me of X- inac*va*on during embryonic development. There are already quite a few cell lines established. What is the result? Mosaicism! How did they figure out? Can you tell the sex of this cat?

40 Sex- linked Gene*c Disorders Defec*ve gene on which chromosomes? Which do you think would be more common? X or Y? Which sex would express these more ofen?

41 Gene*c disorders Why would they therefore be much more common as recessive alleles? No*ce that they MUST be inherited from the mother. Why? You therefore have differen*al expression between the sexes. Female X A Male Y X A X A X A X A Y X a X A X a X a Y

42 Sex- linked disorders

43 Sex- linked disorders & Barr bodies Why is this a complica*on? How can females be carriers? Shouldn t some of their cells express the deleterious trait?

44 Other Chromosomal disorders Quick review of meiosis: When do homologous chromosomes separate? When do sister chroma*ds separate? What if they fail to do so? Non- disjunc*on

45 Non- disjunc*on Aneuploidy Monosomy Trisomy

46 Non- disjunc*on & gene*c disorders Most are fatal Probably a large number never diagnosed, false pregnancies, and miscarriages There are some viable autosomal aneuploid disorders E.g. trisomy 21: Down s Syndrome Only trisomies 13, 18, 21, X, & Y Why both sex chromosomes?

47 Aneuploidy of sex chromosomes What are the possible combina*ons? Y = lethal (of course) XXY XYY XXX X Female Female Male XY 0 X XXY X X XX X Male 0 YY X X XYY X X XYY Male X Y Female XX XXX XXY 0 X Y

48 XXY Klinefelter syndrome 1:500 to 1:1000 live male births Testes abnormally small, man is sterile. Why viable? What does this say about X- inac*va*on?

49 Normal aneuploids XYY No obvious effects Some increase in height No evidence of behavioral issues, despite junk science to the contrary XXX No syndrome, nearly no effects. Both 1:1000 births

50 X The only viable monosomy known. Why? Turner syndrome 1:2500 female births Sterile sex organs Estrogen therapy effec*ve

51 What if gametes fail to separate the genome? Diploid sperm/diploid egg. Fer*liza*on results in polyploidy Triploid/Tetraploid Quite common in plants Rare in vertebrates Fishes/amphibians excep*ons Failure of meiosis

52 Muta*on Most gene*c diseases are caused by mutadons. Imperfect duplica*on of genome. But turn this around, what are alleles? Where must they have come from? Ul*mately, the source of genedc variadon is muta*on. Why is this important?

53 Gene*cs overview GENOME the complete collec*on of gene*c material that is passed down from genera*on to genera*on. Packed into chromosomes: stretches of DNA.

54 Gene*cs overview Deoxyribonucleic acid Complex macromolecule. 2 strings of bases arrayed in a double helix ladder. The pairing of these bases permits copying.

55 Gene*cs overview Some stretches of DNA code for proteins. Some are regulatory regions. A lot is junk * DNA. Humans have 3.2 X 10 9 (3.2 billion) base pairs in our genome, about 95% is junk. * Junk DNA in some ways belies our arrogance- - we currently do not know whether it has func*on.

56 Gene*cs overview Gene to protein: DNA to mrna via DNA transcriptase (transcrip*on). mrna out of nucleus. mrna translated into protein using rrna and trna (transla*on)

57 Transla*on: 20 amino acids, trnas specific to each amino acid. mrna read in mul*ples of 3. Each 3- lener code codes for an amino acid. trna reads these and uses rrna to add each amino acid to the protein chain. Gene*cs overview

58 Again, change in DNA. Muta*on Not all muta*ons are harmful or beneficial. If they change the amino acid sequence: nonsynonymous muta*ons. Most are neutral: silent or synonymous muta*ons. Junk DNA. Redundancy in triplet codon. 64 possible combina*ons with 4- leners available, only 20 amino acids.

59 Types of muta*ons Simplest: point mutadon or subsdtudon. One base pair inserted in the place of another. Ofen silent (synonymous). But many gene*c diseases are point muta*ons.

60 Other types of muta*ons These ofen have more severe effects on fitness. Can throw a protein- coding region out of frame (frameshif muta*ons). When in mul*ples of 3 do not cause frameshif: e.g. cys*c fibrosis.

61 Muta*ons Ul*mately, muta*on is the source of gene*c varia*on. It is this varia*on that makes evolu*on possible. It was the merging of the science of gene*cs with evolu*onary biology that permined us to understand the gene*c basis of evolu*on.

62 Gene*cs & Evolu*on Originally gene*cists didn t see the use of evolu*on and evolu*onists didn t see the use of gene*cs. Why not? What did Mendel study? Individual crosses Discrete phenotypes What is the focus of evolu*on? Popula*ons Con*nuous varia*on

63 The Modern Synthesis The solu*on was to apply gene*cs principles to the study of popula*ons. First off, what is evolu*on? A new defini*on came out of this MODERN SYNTHESIS. Became explicit about what evoludon is and what populadon forces can cause it. 70 years afer Darwin & Mendel

64 Focus switched From individual crosses and individual traits to Gene*c varia*on within popula*ons From discrete characters to Quan*ta*ve characters Essen*ally merged Mendelianism and Darwinism

65 Some terminology PopulaDon: a group of individuals of the same species that live in the same area and interbreed, producing fer*le offspring. Is this a nice, concise defini*on? Is this somewhat subjec*ve?

66 Some terminology Gene pool: all copies of every type of allele at every locus in all members of the popula*on Can characterize a popula*on s gene*c make- up this way.

67 Some terminology Allele frequency: The propor*on of all of the alleles of a given gene that are accounted for by one par*cular allele. What is the frequency of a fixed allele?

68 Some terminology EvoluDon: The change in allele frequency in a popula*on over *me. Strictly speaking microevoludon. The following are (for the moment) rhetorical: What causes microevolu*on? How do we know if allele frequencies are changing? What reference can we use?

69 Hardy- Weinberg Equilibrium The frequency of alleles in a popula*on s gene pool will remain constant over genera*ons unless acted upon by agents other than random sexual recombina*on