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CHAPTER 12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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? 2 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 3 4 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 5 6 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 7 F1 generation • First filial generation • Offspring produced by crossing 2 truebreeding 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 8 F2 generation • Second filial generation • Offspring resulting from the selffertilization 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 9 10 3:1 is 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 11 12 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 13 5 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 14 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 15 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 16 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 17 18 19 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 20 21 22 • 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 23 24 Dihybrid crosses • Examination of 2 separate traits in a single cross • Produced true-breeding lines for 2 traits • RR YY 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 25 • 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 26 27 28 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 29 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 30 • 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 31 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 32 33 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 34 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 35 36 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 37 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 38 • 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 39 40 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 41 42 Environmental influence • Coat color in Himalayan rabbits and Siamese cats – Allele produces an enzyme that allows pigment production only at temperatures below 30oC 43 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 44 45