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Chapter 12 Lecture Outline
See separate PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes and animations. 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Patterns of Inheritance
Chapter 12 2
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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?
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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
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Gregor Mendel Chose to study pea plants because:
Other research showed that pea hybrids could be produced Many pea varieties were available Peas are small plants and easy to grow Peas can self-fertilize or be cross-fertilized
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Mendel’s experimental method
Usually 3 stages Produce true-breeding strains for each trait he was studying Cross-fertilize true-breeding strains having alternate forms of a trait Also perform reciprocal crosses Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
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Petals Carpel (female) Stigma Style Anthers (male) 1. The anthers are
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Petals Carpel (female) Stigma Style Anthers (male) 1. The anthers are cut away on the purple flower. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2. Pollen is obtained from the white flower. 3. Pollen is transferred to the purple flower. 4. All progeny result in purple lowers.
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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
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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
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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
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Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dominant Recessive F2 Generation Dominant Recessive F2 Generation 1. Flower Color 5. Pod Shape 882 Inflated: 299 Constricted 705 Purple: 224 White X X 2.95:1 3.15:1 Purple White Inflated Constricted 2. Seed Color 6. Flower Position 6022 Yellow: 2001 Green 651 Axial: 207 Terminal X X 3.01:1 Yellow Green 3.14:1 Axial Terminal 3. Seed Texture 7. Plant Height 5474 Round: 1850 Wrinkled 787 T all: 277 Short X 2.96:1 X Round Wrinkled 2.84:1 4. Pod Color Tall Short 428 Green: 152 Yellow X 2.82:1 Green Yellow
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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
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Copyright © The McGraw-Hill Companies, Inc
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. True- breeding Purple Parent True- breeding White Parent Parent generation Cross-fertilize Purple Offspring F1 generation Self-cross Purple Dominant Purple Dominant Purple Dominant White Recessive F2 generation (3:1 phenotypic ratio) True- breeding Non-true- breeding Non-true- breeding True- breeding Self-cross Self-cross Self-cross Self-cross F3 generation (1:2:1 genotypic ratio)
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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
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Five-element model Parents transmit discrete factors (genes)
Each individual receives one copy of a gene from each parent Not all copies of a gene are identical Allele – alternative form of a gene Homozygous – 2 of the same allele Heterozygous – different alleles
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Alleles remain discrete – no blending
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
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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
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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
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P p P p P P Pp p pp p pp P p P p P Pp P PP Pp p pP pp p pP pp a.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P p P p P P Pp p pp p pp 1. p + p = pp. 2. P + p = Pp. P p P p P Pp P PP Pp p pP pp p pP pp 3. p + P = pP. 4. P + P = PP. a.
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F2 generation 3 Purple:1 White
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. White parent pp p p Purple parent PP P Pp Pp P Pp Pp F1 generation Purple heterozygote Pp P p Purple heterozygote Pp P PP Pp p pP pp F2 generation 3 Purple:1 White (1PP: 2Pp :1pp ) b.
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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
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Dominant Pedigree Generation I 1 2 Generation II 1 2 3 4 5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dominant Pedigree Generation I 1 2 Generation II 1 2 3 4 5 Generation III 1 2 3 Key unaffected male affected male unaffected female affected female
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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
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Recessive Pedigree One of these persons is heterozygous Generation I 1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Recessive Pedigree One of these persons is heterozygous Generation I 1 2 Heterozygous Generation II 1 2 3 4 5 Generation III 1 2 3 4 5 6 7 Generation IV Mating between first cousins 1 2 3 Homozygous recessive Key unaffected male affected male male carrier unaffected female affected female female carrier
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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
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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
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(chromosomes assort independently into four types of gametes)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RR YY rr yy Parent generation Meiosis Meiosis Cross-fertilization Rr Yy F1 generation Meiosis (chromosomes assort independently into four types of gametes) RY Ry rY ry
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F1 X F1 (RrYy X RrYy) RY Ry rY ry RY RR YY RR Yy Rr YY Rr Yy Ry
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F1 X F1 (RrYy X RrYy) RY Ry rY ry RY RR YY RR Yy Rr YY Rr Yy Ry F2 generation RR Yy RR yy Rr Yy Rr yy rY Rr YY Rr Yy rr YY rr Yy ry Rr Yy Rr yy rr Yy rr yy 9/16 round, yellow 3/16 round, green 3/16 wrinkled, yellow 1/16 wrinkled, green
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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
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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
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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
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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
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Dominant Phenotype (unknown genotype) Homozygous dominant Heterozygous
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dominant Phenotype (unknown genotype) Homozygous dominant Heterozygous dominant Homozygous recessive Homozygous recessive P P P p If PP If Pp p p Pp Pp then then Pp pp PP or Pp Alternative 1: All offspring are purple and the unknown flower is homozygous dominant (PP) Alternative 2: Half of the offspring are white and the unknown flower is heterozygous (Pp)
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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
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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
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30 20 Number of Individuals 10 5′0″ 5′6″ 6′0″ ' Height
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30 20 Number of Individuals 10 5′0″ 5′6″ 6′0″ ' Height (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
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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
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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
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Incomplete dominance Codominance
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
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CRCR CWCW Parent generation Cross-fertilization CRCW F1 generation CR
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CRCR CWCW Parent generation Cross-fertilization CRCW F1 generation CR CW CR F2 generation CRCR CRCW CW CRCW CWCW 1 : 2 : 1 CRCR: CRCW: CWCW
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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
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Blood Type Sugars Exhibited Donates and Receives Alleles IAIA, IAi
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Blood Type Sugars Exhibited Donates and Receives Alleles IAIA, IAi (IA dominant to i) Receives A and O Donates to A and AB A Galactosamine IBIB, IBi (IB dominant to i) Receives B and O Donates to B and AB B Galactose IAIB (codominant) Both galactose and galactosamine Universal receiver Donates to AB AB ii (i is recessive) Receives O Universal donor O None
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Environmental influence
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Coat color in Himalayan rabbits and Siamese cats Allele produces an enzyme that allows pigment production only at temperatures below 30oC Temperaturebelow 33º C, tyrosinase active, dark pigment Temperature above 33º C, tyrosinase inactive, no pigment © DK Limited/Corbis
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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
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a. b. White (AAbb) White (aaBB) Parental generation
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. White (AAbb) White (aaBB) Parental generation Cross-fertilization All Purple (AaBb) F1 generation AB Ab aB ab AB AABB AABb AaBB AaBb Ab F2 generation AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb 9/16 Purple: 7/16 White a. Enzyme A Enzyme B Precursor (colorless) Intermediate (colorless) Pigment (purple) b.
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