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Inheritance & Mendelian Genetics

Inheritance & Mendelian Genetics

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Inheritance & Mendelian Genetics

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  1. Inheritance&Mendelian Genetics

  2. Gregor Mendel • Modern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas • used experimental method • usedquantitative analysis • collected data & counted them • Most traits in most species do not follow the simple Mendelian pattern, but it was a starting point

  3. Mendel’s work Pollen transferred from white flower to stigma of purple flower • Bred pea plants • cross-pollinate true breeding parents (P) • P = parental • raised seed & then observed traits (F1) • F = filial • allowed offspring to self-pollinate& observed next generation (F2) P anthers removed all purple flowers result F1 self-pollinate F2

  4. What did Mendel’s findings mean? • Traits come in alternative versions • purple vs. white flower color • alleles • different alleles vary in the sequence of nucleotides at the specific locus (locus = location on a chromosome) of a gene • some difference in sequence of A, T, C, G purple-flower allele & white-flower allele are two DNA variations at flower-color locus different versions of gene at same location on homologous chromosomes

  5. Traits are inherited as discrete units • For each characteristic, an organism inherits 2 alleles, 1 from each parent • diploid organism • inherits 2 sets of chromosomes, 1 from each parent • homologous chromosomes - same genetic loci (i.e. same genes), different alleles at those loci

  6. What did Mendel’s findings mean? • Some traits “mask” others • purple & white flower colors are separate traits that do not blend • purple x white ≠ light purple • purplemaskedwhite • dominant allele • functional protein • masks other alleles • recessive allele • allele typically makes a malfunctioning protein mutantallele producingmalfunctioningprotein wild typeallele producingfunctional protein homologouschromosomes

  7. X P purple white F1 all purple Genotype vs. phenotype • Difference between how an organism “looks” & its genetics • phenotype • description of an organism’s trait • the “physical,” the result of gene expression • genotype • description of an organism’s genetic makeup • Its combo of alleles, like “Pp”

  8. Dominant ≠ most common allele • Because an allele is dominant does not mean… • it is more common, healthier, stronger, better, more likely, etc. Polydactyly dominant allele, yet rare!

  9. PP pp x X P purple white F1 all purple Making crosses • Can represent alleles as letters • flower color alleles  P or p • true-breeding purple-flower peas  genotype PP • true-breeding white-flower peas  genotype pp • In research, alleles are usually letter/number/symbol combinations (like ser83psE) Pp

  10. Discussion Which of these are phenotypes and which are genotypes? 1. Curly hair 2. Jj 3. PE1PE2 4. Arthritic knees 5. Type B blood 6. Spotted fur and a pink nose 7. HHGg 8. Purple leaves and spiny stem

  11. Punnett Square reminders • The side and top boxes = parents’ potential gametes, each equally likely. • Inner boxes = potential zygotes. • Punnett Squares predict the odds of each offspring being born with a given genotype/phenotype. • Does not ensure that, say, 50% of the children will definitely be freckled.

  12. Genotypes • Homozygous = same alleles = PP, pp • “True-breeding” = homozygous • Heterozygous = different alleles = Pp • “Carrier” homozygousdominant heterozygous homozygousrecessive

  13. x Test cross • Method to determine genotype in case of dominant phenotype • Breed the dominant phenotype with a homozygous recessive (pp) to determine the identity of the unknown allele How does that work? is itPP or Pp? pp

  14. Discussion Suppose that the Y allele codes for orange fins and the y allele codes for yellow fins. The heterozygous genotype: __ The homozygous dominant genotype: __ The homozygous recessive genotype: __ A fish with yellow fins must have a _____________ genotype. A fish with orange fins could be either _____________ or ___________________. If a fish has orange fins, test-crossing it with a ______-finned fish will produce either 100% _____ or 50% orange/50% yellow. If the former result, the orange fish was _________. If the latter result, the orange fish was _________.

  15. P P P p PP Pp pp p p Mendel’s 1st law of heredity • Law of segregation • during meiosis, alleles segregate • homologous chromosomes separate • each allele for a trait is packaged into a separate gamete • You only give 1 allele per gene to your child

  16. Law of Segregation Suppose there’s an eye color locus, with the alleles B for brown eyes or b for blue eyes. A man has the genotype Bb, which gives him the phenotype brown eyes. Meiosis produces his gametes… b He can make gametes that are EITHER B or b. Half of his gametes will be one, half will be the other. That’s segregation! b b S Phase b b b b Meiosis I B Meiosis II B B B Normal cell in G1 B B B Four Gametes

  17. Discussion Monohybrid cross practice! Show Punnett Squares to support your answer. • If two black bees (bees A and B) have 676 babies, all black; two red bees (bees C and D) have 983 babies, all red; and a different two black bees (bees E and F) have 524 babies, 220 red and 304 black, what was each bee’s genotype? Use any letter for the alleles that you want. • What generation were bees A,B,C,D,E, and F a part of? What generation were their children a part of?

  18. Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characters • seed color andseed shape • dihybrid crosses Mendelwas working outmany of the genetic rules!

  19. F1 generation (hybrids) yellow, round peas 100% F2 generation Dihybrid cross P true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr self-pollinate 9:3:3:1 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas

  20. 9/16 yellow round YyRr YyRr 3/16 green round YR YR yr YR yR Yr yr Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr or Dihybrid cross YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

  21. yellow green round wrinkled Mendel’s 2nd law of heredity • Law of independent assortment • different loci (genes) separate into gametes independently • non-homologous chromosomes align independently • classes of gametes produced in equal amounts • YR = Yr = yR = yr YyRr Yr Yr yR yR YR YR yr yr 1 : 1 : 1 : 1

  22. Discussion • Complete a Punnett Square for this dihybrid cross problem! • If A = tall and a = short, while B = fuzzy and b = smooth… • What are the odds that a parent heterozygous for both traits and a short smooth parent will have a tall and fuzzy offspring?

  23. Law of Independent Assortment EXCEPTION • If genes are on same chromosome & close together • will usually be inherited together • rarely crossover separately • “linked”

  24. Rules of Probability • Probability scale ranges from 0 to 1 • Rule of Multiplication:determine the chance that two or more independent events will occur together in some specific combination. • Compute the probability of each independent event. • Then, multiply the individual probabilities to obtain the overall probability of these events occurring together. • Rule of Addition: probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways.

  25. Rules of Probability • For instance, if I roll a six-sided die, what are the odds I’ll get a number that is equal to or less than 2? Which law did you use? • If I roll two dice, what are the odds I’ll get a 1 both times? Which law did you use?

  26. Discussion • You have been using both rules all along! • How does the rule of multiplication come into play in a monohybrid cross? • The rule of addition?

  27. Rules of Probability • What are the odds that a homozygous red-haired, heterozygous green-eyed, white-chinned cat (AAEeww) and a dark-haired, heterozygous green-eyed, white-chinned cat (aaEeww) would have a kitten with the genotype AaEeww? • We can solve each gene as a separate monohybrid problem, then multiply!

  28. Discussion • AAEeww x aaEeww = ?% AaEeww

  29. Discussion • Determine the probability of finding two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are PpYyRr and Ppyyrr. • There are five possible genotypes that fulfill this condition: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr. • Hint: Use the rule of multiplication to calculate the probability for each of these genotypes, and then use the rule of addition to pool the five probabilities.

  30. Answer: • The probability of producing a ppyyRr offspring: • The probability of producing pp = 1/2 x 1/2 = 1/4. • The probability of producing yy = 1/2 x 1 = 1/2. • The probability of producing Rr = 1/2 x 1 = 1/2. • Therefore, the probability of all three being present (ppyyRr) in one offspring is 1/4 x 1/2 x 1/2 = 1/16. • For ppYyrr: 1/4 x 1/2 x 1/2 = 1/16. • For Ppyyrr: 1/2 x 1/2 x 1/2 = 2/16 • For PPyyrr: 1/4 x 1/2 x 1/2 = 1/16 • For ppyyrr: 1/4 x 1/2 x 1/2 = 1/16 • Therefore, the chance of at least two recessive traits is 6/16 = 3/8.

  31. Beyond Mendel’s Lawsof Inheritance

  32. Mendel chose peas luckily • Relatively simple genetically • most characters are controlled by a single gene with each gene having only 2 alleles • one completely dominant over the other • All the genes he chose happened to be on different chromosomes - whew!

  33. Extending Mendelian genetics • The inheritance of traits can rarely be explained by simple Mendelian genetics • Various patterns of inheritance: incomplete dominance, codominance, pleiotropy, lethality, epistasis, polygenetic traits, multiallelic genes, sex-linked traits… • Not all traits just determined by nuclear DNA: environmental effects, gene regulation, mitochondrial DNA…

  34. Incomplete dominance • Heterozygote shows a NOVEL, intermediate, blended phenotype • example: • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color RR WW RW RR RW WW

  35. 100% pink flowers F1 generation (hybrids) 100% 25% red 50% pink 25% white 1:2:1 F2 generation Incomplete dominance X true-breeding red flowers true-breeding white flowers P self-pollinate

  36. Co-dominance • 2 alleles affect the phenotype equally & separately • Phenotype is not blended, it’s both of the true-breeding phenotypes simultaneously • Speckled chickens, Roan cows, human ABO blood groups • 3 alleles • IA, IB, i • IA & IB alleles are co-dominant • glycoprotein antigens on RBC • IAIB = both antigens are produced • i allele recessive to both

  37. Genetics of Blood type

  38. Pleiotropy • Most genes are pleiotropic • one gene affects more than one trait • dwarfism (achondroplasia)

  39. Lethal pleiotropy Aa x aa Aa x Aa dominantinheritance a a A a Aa Aa AA Aa A A achondroplastic achondroplastic achondroplastic lethal a a aa aa Aa aa typical achondroplastic typical typical 50% affected:50% typical or1:1 67% affected:33%typicalor2:1

  40. Discussion • What if an allele is lethal recessive? • Suppose that in a plant, the recessive allele for yellow seeds is lethal, the dominant allele for green seeds is not. • What phenotypic ratios would you get from a cross of… • GG x Gg? • Gg x Gg? • Gg x gg? (gg produced by genetically engineering gametes while leaving the somatic cells intact)

  41. Epistasis • One gene completely masks another gene • coat color in mice = 2 separate genes • C,c:pigment (C) or no pigment (c) • B,b:more pigment (black=B) or less (brown=b) • cc = albino, no matter B allele • 9:3:3:1 becomes 9:3:4 B_C_ B_C_ bbC_ bbC_ _ _cc _ _cc How would you know thatdifference wasn’t random chance? Chi-square test!

  42. Epistasis in Labrador retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b) eebb eeB– E–bb E–B–

  43. Polygenic inheritance • Some traits determined by additive effects of 2 or more genes • phenotypes on a continuum • human traits • skin color • height • weight • intelligence • behaviors

  44. albinism Skin color: Albinism • However, albinism can be inherited as a single gene trait • aa = albino enzyme melanin tyrosine

  45. Sex linked traits • Genes are on sex chromosomes • as opposed to autosomal chromosomes • first discovered by T.H. Morgan’s “Fly Lab” at Columbia U. • Drosophila breeding • good genetic subject • prolific • 2 week generations • 4 pairs of chromosomes • XX=female, XY=male

  46. Classes of chromosomes autosomalchromosomes sexchromosomes

  47. Discovery of sex linkage true-breeding red-eye female true-breeding white-eye male X P Huh!Sex matters?! 100% red eye offspring F1 generation (hybrids) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

  48. Let’s reconsider Morgan’s flies… P x x F1 XRXR XrY XRXr XRY F2 Xr Y XR Y XR XR XRXr XRY XRXR XRY BINGO! F1 XR Xr XRXr XRY XRXr XrY 100% red females 50% red males; 50% white males 100% red eyes

  49. Genes on sex chromosomes • Y chromosome • few genes other than SRY • sex-determining region • master regulator for maleness • turns on genes for production of male hormones • many effects = pleiotropy! • X chromosome • other genes/traits beyond sex determination • mutations: • hemophilia • Duchenne muscular dystrophy • color-blindness

  50. Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Duchenne muscular dystrophy Becker muscular dystrophy Chronic granulomatous disease Retinitis pigmentosa-3 Adrenal hypoplasia Glycerol kinase deficiency Norrie disease Retinitis pigmentosa-2 Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Androgen insensitivity Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease PRPS-related gout Lowe syndrome Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Lesch-Nyhan syndrome HPRT-related gout Immunodeficiency, X-linked, with hyper IgM Lymphoproliferative syndrome Hunter syndrome Hemophilia B Hemophilia A G6PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked Albinism-deafness syndrome Fragile-X syndrome Human X chromosome • Sex-linked • usually means“X-linked” • more than 60 diseases traced to genes on X chromosome