Lecture+11+Mendelian_Genetics_2_NOTES

Lecture+11+Mendelian_Genetics_2_NOTES - Lecture 16:...

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Unformatted text preview: Lecture 16: Mendelian Genetics 2 Mendelian II. Mendelian Genetics B. What Mendel did, 5. Law of Assortment Mendel’s next experiment: Mendel’s • Crossing peas that differed in two characters—seed shape and seed color. and • This is a _DIHYBRID__cross (two traits that differ) This _DIHYBRID__ True-breeding parents: • SSYY: spherical yellow seeds • ssyy: wrinkled green seeds ssyy: wrinkled F1 generation: • SsYy: all spherical yellow SsYy • Mendel asked whether, in the gametes produced by SsYy, the Mendel SsYy the traits would be linked, or segregate independently traits II. Mendelian Genetics B. What Mendel did, 5. Law of Assortment Predictions for independent vs. non-independent assortment SY F1 Gametes Gametes ? sy F1 Gametes Gametes sy sy SsY y ssyy SsYy With Independent Assortment Assortment Without Independent Assortment II. Mendelian Genetics B. What Mendel did, 5. Law of Assortment Mendel’s 2nd Law: The Law of Independent Assortment 1. alleles of different genes assort independently 1. alleles during gamete formation, if they are on different chromosomes, • That is, chromosomes assort independently 1. Genes on the same chromosome are physically Genes linked and generally do no assort independently linked • As you’ll see, when genes on the same are widely As separated they may recombine so frequently that it appears as if they assort independently appears II. Mendelian Genetics II. D. Genes and chromosomes II. Mendelian Genetics D. Genes and chromosomes Meiosis & Mendel’s Law of Independent Assortment Assortment This is where the assortment This occurs occurs II. Mendelian Genetics II. D. Genes and chromosomes Deviations from Mendel’s 2nd Law: Linkage Deviations • Morgan discovered that some crosses performed with Drosophila did not Morgan Drosophila yield expected ratios according to the law of independent assortment. yield • Some genes were inherited together; the two loci were on the same Some chromosome, or LINKED. LINKED • All the loci on a chromosome form a linkage group. All linkage II. Mendelian Genetics D. Genes and chromosomes Deviations from Mendel’s 2nd Deviations Law: Crossing Over & Recombination Recombination • Absolute linkage is rare • Genes may recombine during Genes prophase I of meiosis by crossing over over • Chromosomes exchange Chromosomes corresponding segments. The exchange involves two chromatids in the tetrad; both chromatids become recombinant become II. Mendelian Genetics C. From Punnett squares to probabilities Rules about Probabilities: Rules • If an event is certain to happen, probability = _1_____ If _1_____ • If an event cannot possibly happen, probability = ___0______ If ___0______ • All other events have a probability between _0___ and _1__ All _0___ _1__ So in genetics: • For SS homozygotes, the probability that a gamete will carry S = _1 For SS _1 • And for ss homozygotes, the probability that a gamete will carry the And ss s allele = _1_____ _1_____ • For an Ss heterozygote, the probability that a gamete will carry an... For Ss • S allele = __0.5_________ __0.5_________ • s allele = ______0.5_____ ______0.5_____ II. Mendelian Genetics C. From Punnett squares to probabilities The Multiplication Rule: The To calculate the probability of two To independent events happening together, multiply the probabilities of each individual event: event: • Tossing two coins: probability that both Tossing will come up heads is _0.5*0.5=____0.25________ _0.5*0.5=____0.25________ • In this cross, there is 1 way to get an In SS offspring, you flip “heads” on both SS coins = __0.5*0.5=0.25___ coins • And 1 way to get an ss offspring, you And ss flip “tails” on both coins = 0.5*0.5=0.25 flip II. Mendelian Genetics C. From Punnett squares to probabilities The Addition Rule: The The probability of an event that can occur The in two (or more) different ways is the sum in or of the individual probabilities. of • Tossing two coins: there are 2 ways to Tossing get a heads/tail combination: toss heads 1st & tails 2nd OR toss tails 1st & heads heads 2nd heads • So, _(0.5*0.5) +(0.5*0.5) = 0.5____ • In this cross, there are two ways In gametes can combine to get a heterozygote [Ss (¼) or sS (¼)]; heterozygote sS thus, __(0.5*0.5) +(0.5*0.5) = 0.5 _ thus, II. Mendelian Genetics C. From Punnett squares to probabilities What is the probability of getting each gamete type? type? 1/4 (the four above the punnet square__ above What is the probability of ending What up in ANY of the cells on the Punnett square? Punnett ___1/16__ ___1/16__ What is the probability of having What a SSYy genotype? 1/16+1/16=1/8_ 1/16+1/16=1/8_ What is the probability of having What a green spherical phenotype? green ___3/16___ What is the probability of having What a spherical phenotype? _12/16=3/4 _12/16=3/4 II. Mendelian Genetics C. From Punnett squares to probabilities Work these examples at home: Work Joint probability that an F2 seed will be spherical and yellow = Joint probability that an F2 seed will be spherical and green= Joint probability that an F2 seed will be wrinkled and yellow = II. Mendelian Genetics D. Interactions between Alleles Mendel was lucky in choosing traits that assorted independently and had only simple dominance/recessive interactions among alleles. That made it easier for him to deduce his laws. But in many cases, alleles interact with each other and the environment in complex ways: interact 1. Interactions between alleles at the same locus Interactions • Dominance vs. recessiveness • Incomplete dominance • Co-dominance These interactions These • Effects of multiple alleles can lead to a • Pleiotropy complex relationship 1. Interactions _between alleles at different loci__ Interactions between the • Epistasis genotype and phenotype phenotype 1. Environmental effects Environmental II. Mendelian Genetics II. D. Interactions, 1. Same Locus Interactions Some alleles are neither dominant nor recessive, so that both are Some partially expressed—heterozygote has an intermediate phenotype: partially _incomplete dominance (aka partial dominance)________ _incomplete Example: snapdragons Here the R allele codes Here for red pigment production and the r allele codes for NO pigment codes So the Rr genotype So Rr makes half as much pigment as the RR and is pigment thus pink II. Mendelian Genetics D. Interactions, 1. Same Locus Interactions Some alleles are BOTH dominant and thus both fully expressed— fully often giving an intermediate phenotype: __codominance_ often __codominance_ Example: blood groups Here both A and B alleles are fully Here expressed, creating antigens on the surface of blood cells surface These antigens determine your blood These type and necessitate a match for blood transfusions transfusions The O allele produces no antigens The II. Mendelian Genetics D. Interactions, 1. Same Locus Interactions A given gene may have more than two alleles— multiple alleles__ given • Example: coat color in rabbits controlled by 4 alleles coat 1. 2. 3. 4. C (gray) c (albino) ch (point restricted) (point cch (chinchilla) Multiple alleles increase the number of possible phenotypes II. Mendelian Genetics D. Interactions, 1. Same Locus Interactions A single allele can have multiple phenotypic effects = _pleiotropy (pleiotropic effects) (pleiotropic • Example: allele for coloration pattern in Siamese cats; the same allele results in crossed eyes—both result from the same protein results _one gene, multiple effects II. Mendelian Genetics D. Interactions, 2. Interactions between loci Interaction between genes where the presence of a particular allele at one gene determines how another gene will be expressed = ___epistasis__ ___epistasis__ Example: 2-locus control of coat color in Labrador retrievers 2-locus At B locus, allele B (black) dominant to b (brown) At At E locus, allele E (pigment deposition) is dominant to e (no pigment deposition—yellow) deposition—yellow) If you’re an ee homozygote at E-locus, then B-locus genotype is If ee E-locus then B-locus irrelevant; thus the E-gene effects the expression of the B-gene E-gene B-gene multiple multiple genes, one effect__ effect__ BBEE, BbEE, BbEe, BBEe bbEE, bbEe BBee, bbee, Bbee ee, bbee, Bb II. Mendelian Genetics II. D. Interactions, 3. Environmental Interactions _environment_ also affects phenotype in many ways. Light, temperature, nutrition, etc., can affect expression of the genotype genotype • Example: Siamese cats and certain rabbit breeds—enzyme that Example: produces dark fur is inactive at higher temperatures. produces • Note where the kitty and bunny are dark: on lower temperatures Note on the body. Darker at extremities. II. Quantitative Genetics • Mendel’s characters were discrete and __qualitative_ Mendel’s __qualitative_ • For more complex characters, phenotypes vary continuously For over a range—_quantitative_ variation, or continuous over _quantitative_ continuous • Quantitative variation is usually due to both genes and Quantitative environment environment • Most quantitative characters are controlled by multiple genes at Most different loci: they are ___polygenetic_______________ ___polygenetic_______________ II. Quantitative Genetics When a large enough number of genes influence a character, it will have a continuous, normal frequency distribution will Discrete phenotypic classes Continuous phenotypic variation Called a Called bell-curve bell-curve or _normal distribution distribution or or Gaussian distribution distribution II. Quantitative Genetics The effects of multiple loci on wheat chaff color (Nilsson-Ehle, 1909) (Nilsson-Ehle, Geneticists have found 3 loci (A, B, & C), Geneticists each with 2 alleles (A,A’; B,B’; C,C’) each Each _contributes_ to the color of the Each chaff in wheat chaff II. Quantitative Genetics The more alleles involved, the more the distribution of traits resembles a ___normal distribution resembles A few lectures from now, we’ll talk a bit more about __quantitative few genetics_- which addresses how you predict the outcome of crosses and study selection on quantitative characters and QG handles traits with multiple loci in a statistical way, and is MUCH QG easier than drawing hideously-complex Punnett squares... easier Discrete phenotypic classes Continuous phenotypic variation ...
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