Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHAPTER 12

2 Mystery of heredity Before the 20 th 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?

3 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

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5 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

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7 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

8 F 1 generation First filial generation Offspring produced by crossing 2 true- breeding strains For every trait Mendel studied, all F 1 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

9 F 2 generation Second filial generation Offspring resulting from the self- fertilization of F 1 plants Although hidden in the F 1 generation, the recessive trait had reappeared among some F 2 individuals Counted proportions of traits –Always found about 3:1 ratio

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11 3:1 is 1:2:1 F 2 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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13 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 F 2 generation in the ratio of ¾ dominant to ¼ recessive

14 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

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

16 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

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

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20 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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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

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25 Dihybrid crosses Examination of 2 separate traits in a single cross Produced true-breeding lines for 2 traits RR YY x rryy The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait Allow F 1 to self-fertilize to produce F 2

26 F 1 self-fertilizes RrYy x RrYy The F 2 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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29 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

30 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

31 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

32 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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34 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

35 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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37 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

38 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

39 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

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41 Human ABO blood group The system demonstrates both –Multiple alleles 3 alleles of the I gene (I A, I B, and i) –Codominance I A and I B are dominant to i but codominant to each other

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Environmental influence Coat color in Himalayan rabbits and Siamese cats –Allele produces an enzyme that allows pigment production only at temperatures below 30 o C 43

44 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 –F 1 was all purple –F 2 was 9 purple:7 white – not expected

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