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Biology 201 Chapter 12 PowerPoint
- 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. 1 Chapter 12 Lecture Outline
- 2. Patterns of Inheritance Chapter 12 2
- 3. 3 Mystery of heredity • Before the 20th century, 2 concepts were the basis for ideas about heredity – Heredity occurs within species – Traits are transmitted directly from parent to offspring • Thought traits were borne through fluid and blended in offspring • Paradox – if blending occurs why don’t all individuals look alike?
- 4. 4 Early work • Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents – Additional variation observed in 2nd generation offspring contradicts direct transmission • T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa – Crossed 2 true-breeding strains – 1st generation resembled only 1 parent strain – 2nd generation resembled both
- 5. 5 Gregor Mendel Chose to study pea plants because: 1. Other research showed that pea hybrids could be produced 2. Many pea varieties were available 3. Peas are small plants and easy to grow 4. Peas can self-fertilize or be cross- fertilized
- 6. 6
- 7. 7 Mendel’s experimental method • Usually 3 stages 1. Produce true-breeding strains for each trait he was studying 2. Cross-fertilize true-breeding strains having alternate forms of a trait – Also perform reciprocal crosses 3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
- 8. 8 Stigma Style Anthers (male) 1. The anthers are cut away on the purple flower. Petals Carpel (female) 4. All progeny result in purple lowers. 3. Pollen is transferred to the purple flower. 2. Pollen is obtained from the white flower. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 9. 9 Monohybrid crosses • Cross to study only 2 variations of a single trait • Mendel produced true-breeding pea strains for 7 different traits – Each trait had 2 variants
- 10. 10 F1 generation • First filial generation • Offspring produced by crossing 2 true- breeding strains • For every trait Mendel studied, all F1 plants resembled only 1 parent – Referred to this trait as dominant – Alternative trait was recessive • No plants with characteristics intermediate between the 2 parents were produced
- 11. 11 F2 generation • Second filial generation • Offspring resulting from the self- fertilization of F1 plants • Although hidden in the F1 generation, the recessive trait had reappeared among some F2 individuals • Counted proportions of traits – Always found about 3:1 ratio
- 12. 12 Purple White Yellow Green Round Wrinkled Green Yellow 1. Flower Color 2. Seed Color 4. Pod Color Dominant Recessive 3.15:1 X X X X 3.01:1 2.96:1 2.82:1 F2 Generation 705 Purple: 224 White 6022 Yellow: 2001 Green 5474 Round: 1850 Wrinkled 428 Green: 152 Yellow 3. Seed Texture Inflated Constricted X 2.95:1 Axial Terminal Tall Short 6. Flower Position 7. Plant Height X X 3.14:1 2.84:1 882 Inflated: 299 Constricted 651 Axial: 207 Terminal 787 T all: 277 Short 5. Pod Shape Dominant Recessive F2 Generation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 13. 13 3:1 is actually 1:2:1 • F2 plants – ¾ plants with the dominant form – ¼ plants with the recessive form – The dominant to recessive ratio was 3:1 • Mendel discovered the ratio is actually: – 1 true-breeding dominant plant – 2 not-true-breeding dominant plants – 1 true-breeding recessive plant
- 14. 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parent generation Self-cross Self-cross Self-cross Self-cross Cross-fertilize Self-cross True- breeding Purple Parent True- breeding White Parent Purple Offspring F1 generation F2 generation (3:1 phenotypic ratio) F3 generation (1:2:1 genotypic ratio) Purple Dominant Purple Dominant Purple Dominant White Recessive True- breeding Non-true- breeding Non-true- breeding True- breeding
- 15. 15 Conclusions • His plants did not show intermediate traits – Each trait is intact, discrete • For each pair, one trait was dominant, the other recessive • Pairs of alternative traits examined were segregated among the progeny of a particular cross • Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive
- 16. 16 Five-element model 1. Parents transmit discrete factors (genes) 2. Each individual receives one copy of a gene from each parent 3. Not all copies of a gene are identical – Allele – alternative form of a gene – Homozygous – 2 of the same allele – Heterozygous – different alleles
- 17. 4. Alleles remain discrete – no blending 5. Presence of allele does not guarantee expression – Dominant allele – expressed – Recessive allele – hidden by dominant allele • Genotype – total set of alleles an individual contains • Phenotype – physical appearance 17
- 18. 18 Principle of Segregation • Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization • Physical basis for allele segregation is the behavior of chromosomes during meiosis • Mendel had no knowledge of chromosomes or meiosis – had not yet been described
- 19. Punnett square • Cross purple-flowered plant with white-flowered plant • P is dominant allele – purple flowers • p is recessive allele – white flowers • True-breeding white-flowered plant is pp – Homozygous recessive • True-breeding purple-flowered plant is PP – Homozygous dominant • Pp is heterozygote purple-flowered plant 19
- 20. 20 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P P p p pp P P p p pp Pp P P p p pppP P P p p pp Pp pP PpPP a. 1. p + p = pp. 2. P + p = Pp. 3. p + P = pP. 4. P + P = PP.
- 21. 21 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. p P p P Pp Pp Pp Pp White parent pp b. P P p p pp Pp Purple parent PP Purple heterozygote Pp Purple heterozygote Pp F1 generation PP pP F2 generation 3 Purple:1 White (1PP: 2Pp :1pp )
- 22. 22 Human traits • Some human traits are controlled by a single gene – Some of these exhibit dominant and recessive inheritance • Pedigree analysis is used to track inheritance patterns in families • Dominant pedigree – juvenile glaucoma – Disease causes degeneration of optic nerve leading to blindness – Dominant trait appears in every generation
- 23. 23
- 24. 24 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 21 2 3 4 51 21 Dominant Pedigree Generation I Generation II Generation III Key affected female affected male unaffected female unaffected male 3
- 25. • Recessive pedigree – albinism – Condition in which the pigment melanin is not produced – Pedigree for form of albinism due to a nonfunctional allele of the enzyme tyrosinase – Males and females affected equally – Most affected individuals have unaffected parents 25
- 26. 26 1 2 1 2 1 2 3 3 1 2 3 4 4 5 5 6 7 Recessive Pedigree Generation I Generation II Generation III Generation IV Heterozygous Homozygous recessive Key male carrier female carrieraffected female affected male unaffected female unaffected male Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. One of these persons is heterozygous Mating between first cousins
- 27. 27 Dihybrid crosses • Examination of 2 separate traits in a single cross • Produced true-breeding lines for 2 traits • RRYY x rryy • The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait • Allow F1 to self-fertilize to produce F2
- 28. 28 F1 self-fertilizes •RrYy x RrYy •The F2 generation shows all four possible phenotypes in a set ratio – 9:3:3:1 – R_Y_:R_yy:rrY_:rryy – Round yellow:round green:wrinkled yellow:wrinkled green
- 29. 29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross-fertilization RY Ry rY ry Meiosis Meiosis rr yy Parent generation RR YY Rr Yy F1 generation Meiosis (chromosomes assort independently into four types of gametes)
- 30. 30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RY Ry rY ry RR yy Rr yy Rr yy rr yy 9/16 3/16 3/16 1/16 round, yellow round, green wrinkled, yellow wrinkled, green RY Ry rY ry F1 X F1 (RrYy X RrYy) F2 generation RR YY RR Yy Rr YY Rr Yy RR Yy Rr Yy rr Yy rr Yy rr YYRr YyRr YY Rr Yy
- 31. 31 Principle of independent assortment • In a dihybrid cross, the alleles of each gene assort independently • The segregation of different allele pairs is independent • Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
- 32. 32 Probability • Rule of addition – Probability of 2 mutually exclusive events occurring simultaneously is the sum of their individual probabilities • When crossing Pp x Pp, the probability of producing Pp offspring is – probability of obtaining Pp (1/4), PLUS probability of obtaining pP (1/4) – ¼ + ¼ = ½
- 33. 33 • Rule of multiplication – Probability of 2 independent events occurring simultaneously is the product of their individual probabilities • When crossing Pp x Pp, the probability of obtaining pp offspring is – Probability of obtaining p from father = ½ – Probability of obtaining p from mother = ½ – Probability of pp = ½ x ½ = ¼
- 34. 34 Testcross • Cross used to determine the genotype of an individual with dominant phenotype • Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) • Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent
- 35. 35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P p P P p p Heterozygous dominant Homozygous recessive Alternative 2: Half of the offspring are white and the unknown flower is heterozygous (Pp) PP or Pp then If Pp Dominant Phenotype (unknown genotype) If PP then Alternative 1: All offspring are purple and the unknown flower is homozygous dominant (PP) Homozygous recessive Homozygous dominant PpPp Pp pp
- 36. 36 Extensions to Mendel • Mendel’s model of inheritance assumes that – Each trait is controlled by a single gene – Each gene has only 2 alleles – There is a clear dominant-recessive relationship between the alleles • Most genes do not meet these criteria
- 37. 37 Polygenic inheritance • Occurs when multiple genes are involved in controlling the phenotype of a trait • The phenotype is an accumulation of contributions by multiple genes • These traits show continuous variation and are referred to as quantitative traits – For example – human height – Histogram shows normal distribution
- 38. 38 30 20 10 0 0 5′6″ '6′0″5′0″ Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. NumberofIndividuals (top): From Albert F. Blakeslee, “CORN AND MEN: The Interacting Infl uence of Heredity and Environment—Movements for Betterment of Men, or Corn, or Any Other Living Thing, One-sided Unless Th ey Take Both Factors into Account,” Journal of Heredity, 1914, 5:511-8, by permission of Oxford University Press Height
- 39. 39 Pleiotropy • Refers to an allele which has more than one effect on the phenotype • Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions • This can be seen in human diseases such as cystic fibrosis or sickle cell anemia – Multiple symptoms can be traced back to one defective allele
- 40. 40 Multiple alleles • May be more than 2 alleles for a gene in a population • ABO blood types in humans – 3 alleles • Each individual can only have 2 alleles • Number of alleles possible for any gene is constrained, but usually more than two alleles exist for any gene in an outbreeding population
- 41. 41 • Incomplete dominance – Heterozygote is intermediate in phenotype between the 2 homozygotes – Red flowers x white flowers = pink flowers • Codominance – Heterozygote shows some aspect of the phenotypes of both homozygotes – Type AB blood
- 42. 42 Parent generation 1 : 2 : 1 CR CW Cross-fertilization CW CW CR CR F1 generation CR CW CR CWCR CR CR CW CR CW CW CW CR CR : CR CW : CW CW F2 generation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
- 43. 43 Human ABO blood group • The system demonstrates both – Multiple alleles • 3 alleles of the I gene (IA , IB , and i) – Codominance • IA and IB are dominant to i but codominant to each other
- 44. 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Alleles AB NoneO GalactosamineA GalactoseB Blood Type Sugars Exhibited Donates and Receives Receives A and O Donates to A and AB Receives B and O Donates to B and AB Universal receiver Donates to AB Receives O Universal donor Both galactose and galactosamine IA IA , IA i (IA dominant to i) IB IB , IB i (IB dominant to i) IA IB (codominant) ii (i is recessive)
- 45. Environmental influence • Coat color in Himalayan rabbits and Siamese cats – Allele produces an enzyme that allows pigment production only at temperatures below 30o C 45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © DK Limited/Corbis Temperaturebelow 33º C, tyrosinase active, dark pigment Temperature above 33º C, tyrosinase inactive, no pigment
- 46. 46 Epistasis • Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment • R.A. Emerson crossed 2 white varieties of corn – F1 was all purple – F2 was 9 purple:7 white – not expected
- 47. 47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. AB Ab aB ab AABB AABb AaBB AaBb AABb AAbb AaBb Aabb AaBB AaBb aaBB aaBb AaBb Aabb aaBb aabb 9/16 Purple: 7/16 White AB Ab aB ab Cross-fertilization a. b. Parental generation F1 generation F2 generation Pigment (purple) Enzyme B Enzyme APrecursor (colorless) Intermediate (colorless) White (aaBB) White (AAbb) All Purple (AaBb)