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Chapter 11

Chapter 11. Introduction to Genetics The study of heredity Where does an organism get its unique characteristics?. Gregor Mendel – the Father of Genetics. 11.1 The Work of Gregor Mendel Lived mid-1800s, Austria, abbey gardener bred pea plants that had different physical traits

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Chapter 11

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  1. Chapter 11 • Introduction to Genetics • The study of heredity • Where does an organism get its unique characteristics?

  2. Gregor Mendel – the Father of Genetics • 11.1 The Work of Gregor Mendel • Lived mid-1800s, Austria, abbey gardener • bred pea plants that had different physical traits • height, seed color, flower color… • Developed true-breeding varieties • Controlled matings • Recorded traits seen in offspring

  3. Hypothesized that different forms of “factors” determine inherited traits“Factors” = genes • Mendel found that traits appeared in predictable patterns

  4. LE 9-2b Female – carpel (pistil) - stigma - style - ovary, with ovules Petal Male – stamens - anther - pollen - filament Stamen Carpel Parts of a flower

  5. Removed stamens from purple flower White Stamens Carpel Transferred pollen from stamens of white flower to carpel of purple flower Parents (P) Purple Pollinated carpel matured into pod Planted seeds from pod Offspring (F1) LE 9-2c MENDEL’S WORK Manually cross-pollinate Let seeds mature Plant seeds; observe offspring

  6. LE 9-2d Flower color Purple White Mendel’s contrasting traits in peas Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Dwarf Stem length Tall

  7. Terminology of Mendelian genetics • Pure, or True-breeding: offspring are same as parent every generation • Parent self-fertilizes • Hybrid: offspring are a mix of two different parents • Self-fertilize: sperm in pollen fertilize ovules in the same flower • Cross-fertilize (cross): sperm in pollen fertilize ovules in a different plant

  8. Contrasting traits: different varieties of the same trait, easy to tell apart (ex. tall and short) • Parent generation: Traits and genes of parents in a cross (P) • Filial (offspring) generations: F1- offspring of P F2 – offspring of F1

  9. Different forms of genes (alleles) cause variation in inherited traits • For each trait, an organism inherits two alleles, one from each parent • Homozygous: alleles are the same • Heterozygous: alleles are different • Genotype : alleles that make a trait • Phenotype: physical trait

  10. Mendel’s Law of Dominance The Law of Dominance: If the two alleles are different: (heterozygous) • only the dominant trait appears • The recessive allele does not affect appearance (recessive must be homozygous to show up)

  11. 11.2 Applying Mendel’s Principles • Alleles in gametes combine in fertilization • Zygote has a new gene combination • Cannot always tell genes by appearance Only dominant trait appears when different alleles are present

  12. Law of Segregation Allele pairs separate into different gametes in meiosis Egg or sperm has only one allele from a pair Alleles form new pairs in the zygote

  13. Punnett square helps predict outcome for a cross A Punnett Square is a Handy Way of Analyzing Crosses Genotype ratio: 1 PP : 2 Pp : 1 pp Phenotype ratio 3 purple: 1 white New allele pairs Result from Segregation and Recombination

  14. Mendel’s One-Factor Cross (tall x short plants) Short trait disappears in F1 - Hidden by tall Reappears in F2

  15. Green and Yellow Seeds Recessive trait in F2 shows that genes are in pairs What was learned? Law of Segregation - Allele pairs separate in meiosis and go into different gametes, but make new pairs when they come together in a zygote The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells.

  16. Mendel’s white and purple flowers • Purple flower X white flower • F1 – all purple • F2 – 3/4 red : 1/4 white Allele pairs in F1 segregate Form new pairs in F2

  17. Test cross- can find unknown genotype • Dominant phenotype • - can be homozygous or heterozygous • - ex. Purple flower- can bePP or Pp • How to find genotype of purple parent? • Cross with a homozygous recessive parent

  18. TEST CROSS IF unknown parent is homozygous • NO recessive traits in F1 IF unknown parent is heterozygous • SOME recessive traits in F1

  19. Law of Independent Assortment Are different traits inherited together or separately? If true-breeding parents have two traits that differ F1 – all have dominant phenotypes F2 – recombination yields four different phenotypes Mendel performed dihybrid crosses to find out. Mendel’s conclusion: Different characters are inherited independently.

  20. The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character Mendel’s round and wrinkled seeds Mendel's Law of Independent Assortment: each pair of alleles separates independently of other pairs when gametes form in meiosis

  21. Mendel: Round, yellow X wrinkled, green seeds

  22. 11.3 Other Patterns of Inheritance • But many traits have inheritance patterns more complex than Mendel’s principles • Incomplete dominance • - Alleles are not dominant or recessive - Heterozygote shows phenotype somewhere between both homozygous conditions Example: red flower X white flower  pink Mendel’s patterns of inheritance from his crosses are the basis for modern genetics

  23. LE 9-12a P generation White rr Red RR  R r Gametes F1 generation Pink Rr Allele for red and allele for white – neither is dominant over the other - Blended phenotype 1 2 1 2 R r Gametes Sperm 1 2 1 2 R r Red RR Pink rR 1 2 R F2 generation Eggs 1 2 Pink Rr White rr r

  24. Blue Andalusian Chicken Black (BB) X White (WW)  Blue (BW)

  25. Codominance – Both Alleles Are Dominant • Neither allele is recessive • - no blending; both phenotypes show up Roan horse: P = red X white RW has red hairs and white hairs

  26. Calico Cat • Calico (tortoise shell) cat has orange patches and black patches, along with white

  27. Multiple Alleles • Some genes have more than two alleles in the population • Example: human ABO blood group • Three alleles of a single gene • Each person only has two • A and B alleles are codominant to each other (both alleles are expressed in heterozygotes) • O allele is recessive to A and B

  28. The ABO blood group • A and B alleles are codominant (IA IB or A B) • the allele for type O blood (i or O) is recessive • Type A blood – genotypes AA or Ai • Type B blood – genotypes BB or Bi • Type AB blood - genotype AB • Type O blood – genotype ii

  29. Multiple Alleles What does “blood type” mean? Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. ABO blood groups: Identification proteins on red blood cell membrane Rh + or Rh – is a different protein (and different gene) The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene.

  30. LE 9-13 Why does blood type matter? ABO Blood Type Incompatability Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Blood Group (Phenotype) Antibodies Present in Blood Genotypes O A B AB Anti-A Anti-B O ii IAIA or IAi Anti-B A IBIB or IBi B Anti-A AB IAIB

  31. Genes and the Environment • Environment can influence expression of a gene • examples: • a) Skin darkens when exposed to sun • b) plants lacking soil nutrients or water will not grow well

  32. Environment can affect a genetic trait Colder body parts have darker fur

  33. Thomas Hunt Morgan Thomas Hunt Morgan performed some of the most important studies of genetics and crossing over in the early 1900s Morgan in the fly room

  34. T. H. Morgan and fruit fly genetics Used the fruit fly Drosophila Found that genes could be located on specific chromosomes Eye color was on the X chromosome

  35. SEX CHROMOSOMES AND SEX-LINKED GENES • Chromosomes determine sex in many species • In humans • - Male is XY; the Y chromosome has genes for the development of testes • - Female is XX

  36. Male determines gender of baby • Mom always gives X • - if sperm has Y •  boy • - if sperm has X •  girl (male) (female) 44 + XY 44 + XX Parents’ diploid cells 22 + X 22 + X 22 + Y Egg Sperm 44 + XX 44 + XY Offspring (diploid)

  37. Sex-linked traits have unique inheritance pattern • In humans, X chromosome is bigger than Y and has many more genes. • Males only get one X, so recessive trait shows in them more often • Females need two recessive alleles for trait to show • Example: color-blindness

  38. Eye color in fruit flies is sex-linked • “Wild” type (normal) is red; “Mutant” is white

  39. Sex-linked disorders in humans • Color- blindness – cannot tell some colors • - red/green most common • 2) Hemophilia – blood does not clot • Duchenne muscular dystrophy – muscles waste away • ALD -Lorenzo

  40. Can you see a number?

  41. Test Yourself With The Table Below Numbers That You Should See If You Are In One Of The FollowingFour Categories: [Some Letter Choices Show No Visible Numbers]

  42. LE 9-24b Hemophilia in the Russian royal family Queen Victoria Albert Alice Louis Czar Nicholas  of Russia Alexandra Alexis

  43. Gene Linkage • Genes on the same chromosome tend to be inherited together • Punnett and Bateson • “Linked genes” “linkage groups” • Do not follow Mendel's law of independent assortment • Except when crossed-over

  44. Mapping genes on a chromosome • Crossover data can map genes on a chromosome • Rate of crossing over shows relative position of genes on chromosomes • Gene map

  45. LE 9-20a The farther apart are two genes on a chromosome, the more often they cross over. A B a b A B a b A b a B Tetrad Crossing over Gametes

  46. LE 9-21c Mutant phenotypes Short antenna Black body (g) Cinnebar eyes (c) Vestigial wings (l) Brown eyes Gray body (G) Red eyes (C) Normal wings (L) Red eyes Long antenna Wild-type phenotypes

  47. Autosomal Gene Disorders • Cystic Fibrosis - most common inherited disorder in Americans of European descent • - thick mucus, clogs lungs and other organs • - one in 2000 are carriers • - treatment – physically clear mucus from lung, drugs to thin mucus, no cure

  48. Cystic Fibrosis affects many body systems

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