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 Su on & 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. Fer liza on restores paired 3. Fer liza on restores condi on for chromosomes. paired condi on 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 quan ta ve traits. • Some of the (in hindsight) greatest gene cists of the day didn’t see the connec on. • 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 gene cs. • Why??? What traits make this a good organism? 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”. Finally…
• A er 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. 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 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 determina on. • The traits were SEX- LINKED. 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 What chromosomes will each sex pack into the gametes? Female Gametes X X Male X Gametes Y 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? 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. • Muta on rates increased using mutagens – Chemicals – X-rays – Ultraviolet radia on – 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 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. 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? Mapping Chromosomes
• Let’s take a 3rd 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. 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 condi on. 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. 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? A li le 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? DNA
• Explained a fundamental mystery about living organisms. • Showed that a simple molecule could carry complex instruc ons. • 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-inac va on & Barr bodies. 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? 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 o en? Gene c disorders
• Why would they Male therefore be much more common as XA Y recessive alleles? XA XAXA XAY • No ce that they MUST a A a a
be inherited from the Female X X X X Y mother. Why? • You therefore have differen al expression between the sexes. Sex-linked disorders 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? 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 Non-disjunc on
Aneuploidy Monosomy Trisomy 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? Aneuploidy of sex chromosomes Male • What are the possible XY 0
combina ons? X XXY X
• Y = lethal (of course) Female X XX X • XXY Male 0 YY
• XYY X X XYY • XXX
Female X X XYY • X Male X Y
XX XXX XXY
Female 0 X Y 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? Normal aneuploids
• XYY • XXX • No obvious effects • No syndrome, nearly no • Some increase in height effects. • No evidence of behavioral issues, • Both 1:1000 births despite junk science to the contrary X
• The only viable monosomy known. • Why? • Turner syndrome • 1:2500 female births • Sterile sex organs • Estrogen therapy effec ve Failure of meiosis
• 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 Muta on
• Most gene c diseases are caused by muta ons. • Imperfect duplica on of genome. • But turn this around, what are alleles? • Where must they have come from? • Ul mately, the source of gene c varia on is muta on. • Why is this important? Gene cs review
• 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. Gene cs review
• Deoxyribonucleic acid • Complex macromolecule. • 2 strings of bases arrayed in a double helix ladder. • The pairing of these bases permits copying. Gene cs review
• Some stretches of DNA code for proteins. • Some are regulatory regions. • A lot is “junk”* DNA. • Humans have 3.2 X 109 (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. Gene cs review
• 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) Gene cs review
• Transla on: • 20 amino acids, tRNAs specific to each amino acid. • mRNA read in mul ples of 3. • Each 3-le er code codes for an amino acid. • tRNA reads these and uses rRNA to add each amino acid to the protein chain. Muta on • Again, change in DNA. • 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-le ers available, only 20 amino acids. Types of muta ons • Simplest: point muta on or subs tu on. • One base pair inserted in the place of another. • O en silent (synonymous). • But many gene c diseases are point muta ons. Other types of muta ons
• These o en have more severe effects on fitness. • Can throw a protein- coding region out of frame (frameshi muta ons). • When in mul ples of 3 do not cause frameshi : e.g. cys c fibrosis. 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 permi ed us to understand the gene c basis of evolu on. 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 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 evolu on is and what popula on forces can cause it. • 70 years a er Darwin & Mendel… 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 Some terminology
• Popula on: 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? 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. 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? Some terminology • Evolu on: The change in allele frequency in a popula on over me.
• Strictly speaking microevolu on.
• 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? 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