Mendel and the Gene Idea

1. Mendel and the Gene Idea CHAPTER 14

2. What genetic principlesaccount for the transmission of traits from parents to offspring?

3. One possible explanation of heredity is a “blending” hypothesis • The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green

4. An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea • Parents pass on discrete heritable units, genes

5. Gregor Mendel • Documented a particulate mechanism of inheritance through his experiments with garden peas • DNA wasn’t known yet

6. Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned experiments

7. Mendel’s Experimental, Quantitative Approach • Mendel chose to work with peas because • They are available in many varieties • He could strictly control which plants mated with which • Self pollinating (able to start with pure plants) • Lots of offspring

8. Removed stamens from purple flower 1 APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower 2 TECHNIQUE TECHNIQUE RESULTS TECHNIQUE Parental generation (P) Stamens (male) Carpel (female) Pollinated carpel matured into pod 3 Planted seeds from pod 4 When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. Examined offspring: all purple flowers 5 First generation offspring (F1) CROSSING PEA PLANTS

9. SOME GENETIC VOCABULARY • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers

10. More Genetic Vocabulary • An organism that is homozygous for a particular gene • Has a pair of identical alleles for that gene (RR or rr) • Exhibits true-breeding (pure) • An organism that is heterozygous for a particular gene • Has a pair of alleles that are different for that gene (Rr) • Hybrid

11. An organism’s phenotype • Is its physical appearance • An organism’s genotype • Is its genetic makeup

12. Phenotype Genotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple pp (homozygous) White 1 1 Ratio 3:1 Ratio 1:2:1 Phenotype Versus Genotype

13. Mendel chose to track • Only those characters that varied in an “either-or” manner • Mendel also made sure that • He started his experiments with varieties that were “true-breeding”

14. In a typical breeding experiment • Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents • Are called the P1 generation • Cross RR x rr yielded all Rr (hybrids)

15. The hybrid offspring of the P1 generation • Are called the F1 generation • When F1 individuals self-pollinate • The F2 generation is produced • Hybrid x Hybrid Rr x Rr • Offspring 25% RR, 50% Rr, 25% rr • 3:1 phenotypic ratio • 1:2:1 genotypic ratio

16. The Law of Segregation • Mendel derived the law of segregation • By following a single trait • The F1 offspring produced in this cross • Were monohybrids, heterozygous for one character

17. The Law of Segregation • The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

18. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers • Alternative Versions Of Genes • Account for variations in inherited characters, which are now called alleles • Capital letter (dominant allele) • Lowercase (recessive)

19. For each character • An organism inherits two alleles, one from each parent • A genetic locus is actually represented twice

20. The Testcross or F2 • Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype • Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait • RR x Rr or rr x Rr • 1:1 ratio

21.  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. If PP, then all offspring purple: If Pp, then 1⁄2 offspring purple and 1⁄2 offspring white: p p p p RESULTS P P Pp Pp Pp Pp P p Pp pp Pp pp THE TESTCROSS

22. The Law of Independent Assortment • Mendel derived the law of independent assortment • By following a two traits • The F1 offspring produced in this cross • Were dihybrids, heterozygous for both characters

23. EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. P Generation YYRR yyrr yr Gametes YR  F1 Generation YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm Yr 1 ⁄4 1 ⁄4 1 ⁄4 yr YR yR 1 ⁄4 Sperm Eggs 1⁄2 yr 1⁄2 YR RESULTS 1 ⁄4 YR Eggs YyRr YYRR YYRr YyRR 1 ⁄2 YR F2 Generation (predicted offspring) YYRR YyRr 1 ⁄4 Yr YYrr YyRr Yyrr YYrr yr 1 ⁄2 yyrr YyRr 1 ⁄4 yR YyRR YyRr yyRr yyRR CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. 3 ⁄4 1 ⁄4 yr 1 ⁄4 Phenotypic ratio 3:1 Yyrr YyRr yyRr yyrr 1 ⁄16 3 ⁄16 3 ⁄16 9 ⁄16 Phenotypic ratio 9:3:3:1 315 108 101 Phenotypic ratio approximately 9:3:3:1 32 • A dihybrid cross • Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation

24. Using the information from a dihybrid cross, Mendel developed the law of independent assortment • Each pair of alleles segregates independently during gamete formation

25. LAWS OF PROBABILITY

26. The Laws Of Probability Govern Mendelian Inheritance • Mendel’s laws of segregation and independent assortment • Reflect the rules of probability

27. The Multiplication and Addition Rules Applied to Monohybrid Crosses • The Multiplication Rule • States that the probability that two or more independent events will occur together is the product of their individual probabilities • Probability of two independent events A,B: • P(A and B) = P(A)*P(B)

28. Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm   Sperm r R 1⁄2 1⁄2 R R r R R 1⁄2 1⁄4 1⁄4 Eggs r r R r r 1⁄2 1⁄4 1⁄4 • Probability in a monohybrid cross • Can be determined using this rule

29. The Rule Of Addition • States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

30. Solving Complex Genetics Problems with the Rules of Probability • We can apply the rules of probability • To predict the outcome of crosses involving multiple characters

31. A dihybrid or other multicharacter cross • Is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes from such crosses • Each character first is considered separately and then the individual probabilities are multiplied together

32. Probability Problem • If H horns is dominant to h hornless, and T fancy tail in dragons is dominant to t plain tail, what is the probability that results from a cross of Dragon #1 HHtt x HhTt Dragon #2: • Dragon #1 always contributes H & Dragon #2 contributes H half the time and h the other half SO the probability of having horns (HH x Hh) is equal to 1 and no horns is equal to 0

33. Probability Problem cont.) Dragon #1 HHtt x HhTt Dragon #2 • Dragon #1 always contributes a t while Dragon #2 contributes a T half the time and a t half the time • (tt x Tt) • SO the probability of having a fancy tail is ½ and the probability of having a plain tail is ½

34. Probability Problem cont.) SO what’s the probability of having a baby dragon with: • Horns and fancy tail? 1 x ½ = ½ • Horns and a plain tail? 1 x ½ = ½ • Hornless and fancy tail? 0 x ½ = 0 • Hornless and plain tail? 0 x ½ = 0

35. OTHER FACTORS AFFECTING INHERITANCE

36. Inheritance patterns are oftenmore complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely simple • The inheritance of characters by a single gene may deviate from simple Mendelian patterns

37. The Spectrum of Dominance • Complete dominance • Occurs when the phenotypes of the heterozygote and dominant homozygote are identical • Codominance • Two dominant alleles affect the phenotype in separate, distinguishable ways • Human ABO blood type is an example of codominance

38. The ABO blood group in humans • Is determined by multiple alleles • A and B are dominant to 0

39. P Generation White CWCW Red CRCR  Gametes CR CW Pink CRCW F1 Generation 1⁄2 1⁄2 Gametes CR CR 1⁄2 Sperm 1⁄2 CR CR Eggs F2 Generation 1⁄2 CR CR CR CR CW 1⁄2 Cw CW CW CR CW • Incomplete Dominance • The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

40. Pleiotropy • In Pleiotropy • A gene has multiple phenotypic effects • Frizzle gene in chickens causes feathers to curl outward, abnormal body temperature, and greater digestive capacity

41. Extending Mendelian Genetics for Two or More Genes • Some traits • May be determined by two or more genes • In Epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus

42. Example of Epistasis - Fruit Color in Squash • Color is recessive to no color at one allelic pair • This recessive allele must be expressed before the specific color allele at a second locus is expressed. • At the first gene, white colored squash is dominant to colored squash, and the gene symbols are W=white and w=colored

43. Fruit Color in Squash cont.) • At the second gene, yellow is dominant to green, and the symbols used are Y=yellow, y=green • The presence of the dominant W allele masks the effect of either the Y or y alleles • W_Y_ & W_yy give white (12) • wwY_ is yellow (3) • wwyy is green (1) • 12:3:1 ratio

44. BbCc BbCc  Sperm 1⁄4 1⁄4 bC Bc 1⁄4 BC bc 1⁄4 Eggs 1⁄4 BBCC BbCC BbCc BC BBCc 1⁄4 bC BbCC bbCC bbCc BbCc 1⁄4 BBcc BBCc BbCc Bbcc Bc bbcc Bbcc BbCc 1⁄4 bbCc bc 4⁄16 9⁄16 3⁄16 • Another Example Of Epistasis

45.  AaBbCc AaBbCc aabbcc Aabbcc AABBCc AABBCC AaBbcc AABbCc AaBbCc 20⁄64 15⁄64 Fraction of progeny 6⁄64 1⁄64 • Quantitative variation usually indicates Polygenic Inheritance • An additive effect of two or more genes on a single phenotype • Skin color • Eye Color • Hair color

47. Nature and Nurture: The Environmental Impact on Phenotype • Another departure from simple Mendelian genetics arises • When the phenotype for a character depends on environment as well as on genotype • May inherit genes for height, but not receive the needed nutrition to grow tall • Heart disease & cancer

48. The Norm Of Reaction • Is the phenotypic range of a particular genotype that is influenced by the environment (iron in soil affects color)

49. GENETIC PATTERNS IN HUMANS

50. Pedigree Analysis • A pedigree • Is a family tree that describes the interrelationships of parents and children across generations • Male • Female