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CHAPTER 14

CHAPTER 14. MENDEL AND THE GENE IDEA. I. OVERVIEW. What genetic principles account for the passing of traits from parents to offspring?

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CHAPTER 14

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  1. CHAPTER 14 MENDEL AND THE GENE IDEA

  2. I. OVERVIEW • What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) offspring • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Mendel documented a particulate mechanism through his experiments with garden peas which began in 1860’s

  3. I. INTRODUCTION A. Blending Theory of Heredity • Pre-Mendel • Proposed that hereditary material from each parent mixes or blends in the offspring B. Particulate Theory of Heredity • Gregor Mendel’s theory • Parents transmit to their offspring inheritable factors (genes) • Began in 1860’s

  4. II. GREGOR MENDEL A. Used quantitative approach B. Austrian monk C. Father of Genetics D. Two reasons he used peas in his experiments: 1. Many easily recognized characteristics (traits) 2. Easy to cross-pollinate

  5. III. VOCABULARY 1. gene—segment of DNA --responsible for production of 1 protein --Mendel’s factor 2. locus—specific position of a gene on a chromosome

  6. 3. characteristic—inherited feature (Ex: flower color) 4. trait—variant of a characteristic (Ex: white or red flowers) 5. diploid—refers to cells with 2 sets of chromosomes (2n) • haploid—refers to cells with 1 set of chromosomes (n) • allele—alternative forms of a gene 8. homozygous—two alleles for a particular characteristic are identical --TT or tt • heterozygous—two alleles for a particular characteristics are different --Tt

  7. 10. genotype—actual genetic makeup --symbols --Tt, TT, or tt 11. phenotype—effects of the genes --description of appearance --tall or short 12. dominant—refers to trait that always appear in offspring of parents with contrasting traits tall x short | tall • recessive—refers to trait that does not appear in offspring of parents with contrasting traits

  8. 14. monohybrid cross—cross in which only one character is considered TT x tt | Tt (monohybrid) 15. dihybrid cross—cross in which two characters are considered TTyy x ttYY | TtYy (dihybrid) 16. P—parental generation --true breeding 17. F1—first filial generation --offspring of P

  9. 18. F2—second filial generation --offspring of F1 19. intercross—cross in which both parents are heterozygous for a characteristic --1 Factor Intercross (1FIC) • Tt x Tt • Phenotypic ratio 3:1 • Genotypic ratio 1:2:1 --2 Factor Intercross (2FIC) • TtYy x TtYy • Phenotypic ratio 9:3:3:1

  10. 20. backcross—cross with a parent or parent-type --Tt x TT or tt 21. testcross—cross unknown genotype with a homozygous recessive --used to attempt to determine if a dominant trait is homozygous or heterozygous --T_ x tt 22. true breeding—offspring have same traits as parents when parents self-fertilize --pure breeding --homozygous 23. hybrid—offspring from a cross of parents differing in one or more characteristics --heterozygous

  11. IV. MENDEL’S EXPERIMENTS P purple x white F1 purple (self-pollinate) F2 705 purple 224 white Ratio 3.15 :1 ~ 3:1

  12. Mendel’s Characters

  13. A. Mendel’s Conclusions 1. For each character, an organism inherits two genes (factors), one from each parent. 2. Alternative forms of genes are responsible for variations in inherited characters. • If the two alleles (factors) differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele). • Symbol for dominant allele—capital letter • Symbol for recessive allele—lowercase letter

  14. Law of Segregation 3. Law of Segregation • The two genes (factors) for each character segregate during gamete formation • Occurs during meiosis

  15. 4. Mendel’s Law of Independent Assortment • States: Each allele pair segregates independently of other gene pairs during gamete formation. • Developed from dihybrid crosses • Refers to behavior of genes during gamete formation • Refers to genes on different chromosomes. • Ex: RrTt x RrTt

  16. Law of Independent Assortment

  17. B. Punnett Square • Named for R. C. Punnett • Special chart used to predict outcome of genetic crosses

  18. C. Inheritance as a Game of Chance 1. Laws of Probability • Probability of 1 means event certain to occur • Probability of 0 means event certain not to occur • Probability of all possible outcomes for an event add up to 1 • Two basic rules of probability:

  19. a. Rule of Multiplication • the probability that independent events will occur simultaneously is the product of their individual probabilities • What is the probability that offspring will be pp if the parents are Pp x Pp? -First determine the probability that an egg will receive a “p” allele (½) -Then determine the probability that a sperm will receive a “p” allele (½) -Solution: ½ x ½ = ¼

  20. b. Rule of Addition • The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. • In the cross Pp x Pp, what is the probability of the offspring being heterozygous (Pp)? -2 ways: egg sperm probability P (½) p (½) = Pp ¼ p (½) P (½) = Pp ¼ -Solution ¼ + ¼ = ½

  21. IV. Concept 14.3: Extending Mendelian Genetics • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

  22. Extending Mendelian Genetics for a Single Gene 1. Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive • When a gene has more than two alleles • When a gene produces multiple phenotypes

  23. V. EXTENDING MENDELIAN GENETICS Complete Dominance • The phenotypes of the heterozygote and the dominant homozygote are indistinguishable.

  24. V. EXTENDING MENDELIAN GENETICS A. Incomplete Dominance • Dominant phenotype is not fully expressed in heterozygote, resulting in a third phenotype that is intermediate between homozygous dominant and homozygous recessive. • Example: P red flowers x white flowers F1pink flowers (self-pollinate) F2¼ red½ pink ¼ white

  25. Degrees of Dominance • Complete Dominance- the phenotypes of the heterozygote and the dominant homozygote are indistinguishable. • Incomplete Dominance- neither allele is completely dominant and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.

  26. Fig. 14-10-3 P Generation Red White CRCR CWCW CR CW Gametes Pink F1Generation CRCW 1/2 1/2 CR CW Gametes Sperm 1/2 1/2 CR CW F2 Generation 1/2 CR CRCW CRCR Eggs 1/2 CW CRCW CWCW

  27. Symbols: R—red W—white RR x WW RW x RW ¼ RR½ RW ¼ WW

  28. B. What is a dominant allele? Complete Incomplete Codominance Dominance Dominance (A is dominant) (A is incompletely (no dominance) dominant) AA and Aa Aa Aa Same Intermediate Both alleles phenotype phenotype expressed between AA and aa

  29. The Relation Between Dominance and Phenotype • A dominant allele does not subdue a recessive allele; alleles don’t interact • Alleles are simply variations in a gene’s nucleotide sequence • For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype • Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain

  30. B. Codominance • Defined as the full expression of both alleles in a heterozygote • Ex: MN blood type • Tay Sachs • At the organismal level, the allele is recessive • At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant • At the molecular level, the alleles are codominant

  31. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United States is born with extra fingers or toes • The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage

  32. D. Multiple Alleles • More than two alternative forms (alleles) of a gene • Ex: ABO blood group • The four phenotypes of the ABO blood group (A, B, AB, O) in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • 3 alleles produce 4 possible phenotypes—A, B, O, AB • IA and IB are dominant to i (recessive). • IA and IB are codominant to each other. • Each person carries only 2 alleles.

  33. ABO Blood Group

  34. Blood Problem • Identification bracelets were accidentally removed from three newborn babies. Blood typings were taken to help in the identification procedures. The blood types for the babies and their parents were: Baby 1- type A, Baby 2- type O, Baby 3- type AB • Mr. Black = type A Mr. Black = type B • Mr. Green = type AB Mrs. Green = type O • Mr. White = type O Mrs. White = type O • Which baby could belong to Mr. and Mrs. Black? • Which baby could belong to Mr. and Mrs. Green? • Which baby could belong to Mr. and Mrs. White?

  35. E. Pleiotrophy • Ability of a single gene to have multiple phenotypic effects • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases • Ex: sickle cell anemia, cystic fibrosis

  36. F. Epistasis • Interaction between two nonallelic genes in which one alters the phenotypic expression of the other (a gene at one locus alters the phenotypic expression of a gene at a second locus) • Dihybrid cross results will deviate from expected ratio of 9:3:3:1 • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

  37. Epistasis • Ex: albinism (no pigmentation) BB, Bb-black coat color bb-brown coat color CC, Cc-can make pigment (melanin) cc-albino (can not make pigment)

  38. BbCc x BbCc • Black B_ C_ • Brown bb C_ • Albino __ cc

  39. Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance

  40. G. Polygenic Inheritance • Additive effect of two or more genes determines a single phenotypic character • Ex: Skin pigmentation in humans is controlled by at least 3 separately inherited genes. --incomplete dominance --AABBCC- very dark --aabbcc-very light --AaBbCc-intermediate shade --phenotype can be affected by environmental factors (sun exposure)

  41. H. Environmental Effects on Phenotype • Coat color can be influenced by temperature. -Ex: Siamese cats • Curly wing mutant of Drosophila loses the ability to fly at less than 16° C • Color of flowers of Hydrangeas because of soil pH I. Phenocopy • Environmentally produced phenotype that simulates the effects of a particular gene -Ex: The drug thalidomide mimics the birth defect phocomelia.

  42. Environmental Genetic Interaction

  43. Environmental Effects on Phenotype (Nature vs. Nuture) • Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype • Coat color can be influenced by temperature. -Ex: Siamese cats • Curly wing mutant of Drosophila loses the ability to fly at less than 16° C • Color of flowers of Hydrangeas because of soil pH

  44. Siamese Cat--Coco

  45. J. Norm of Reaction • Refers to range of phenotypic possibilities due to environmental influences • The norm of reaction is the phenotypic range of a genotype influenced by the environment • Nature vs. Nurture (genetics vs. environment) • Norms of reaction are broadest for polygenic characters such as skin color which are usually referred to as multifactorial (both genetic and environmental factors influence phenotype).

  46. For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

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