Chapter 14

1. Chapter 14 Mendel and the Gene Idea

2. Preformationism – an pre-Mendel hypothesis of heredity • Claimed either the egg or the sperm (exactly which was a contentious issue) contained a complete preformed individual called a homunculus. • Development was therefore a matter of enlarging the homunculus into a fully formed being.

3. Preformationism – Genetic diseases • Was variously interpreted, examples: • A manifestation of the wrath of God • The mischief of demons and devils • As evidence of either an excess of or a deficit of the father's “seed” • As the result of “wicked thoughts” on the part of the mother during pregnancy

4. Overview: Drawing from the Deck of Genes • What genetic principles account for the transmission of traits from parents to offspring? • Understanding the mathematics of probability was important in Mendel’s discoveries

5. Hypotheses for heredity • “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 • “Particulate” hypothesis of inheritance: the gene idea • Parents pass on discrete heritable units, genes

6. Figure 14.1 Gregor Mendel • Grew up in a part of Austria which is now the Czech Republic • Documented a particulate mechanism of inheritance through his experiments with garden peas • Failed his teacher exams • But was trained in math & botany & research

7. Concept 14.1: 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 • Used quantitative methods • Mendel chose to work with peas • Because they are available in many varieties • Because he could strictly control which plants mated

8. Crossing pea plants TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4

9. First generationAll purple RESULTS First filial gener- ation offspring (F1) 5

10. Some genetic vocabulary • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers* • True-breeding: produce offspring of the same variety when they self-pollinate • Hybridization: the mating or crossing of two true-breeding plants

11. Research decisions • Mendel chose to research • 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”

12. Research design • In a typical breeding experiment • Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents • Are called a P generation • The hybrid offspring of the P generation • Are called an F1 generation • When F1 individuals self-pollinate • An F2 generation is produced

13. The Law of Segregation • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants • All of the offspring were purple • When Mendel crossed the F1 plants • Many of the plants had purple flowers, but some had white flowers

14. P Generation (true-breeding parents)  Purple flowers White flowers EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. F1 Generation (hybrids) All plants had purple flowers F2 Generation RESULTS Both purple-flowered plants and white- flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white. Mendel discovered • A ratio of about three to one, purple to white flowers, in the F2 generation Figure 14.3

15. Mendel reasoned that • In the F1 plants, only the purple flower factor was affecting flower color in these hybrids • Purple flower color was dominant, and white flower color was recessive • Mendel used the term “heritable factor” which has been replaced with “gene”

16. Table 14.1 Mendel observed the same pattern • In many other pea plant characters

17. Mendel’s Model • Mendel developed a hypothesis • To explain the 3:1 inheritance pattern that he observed among his F2 offspring • Four related concepts make up this model

18. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Figure 14.4 1. Alternative versions of genes (alleles) • Account for variations in inherited characters

19. Mendel’s Model Cont. • 2. For each character an organism inherits two alleles, one from each parent • 3. If the two alleles at a locus differ… • Then one, the dominant allele, determines the organism’s appearance • The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

20. Mendel’s Model Cont. • 4. The law of segregation • The two alleles for a heritable character separate (segregate) during gamete formation & end up in different gametes

21. Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? • We can answer this question using a Punnett square • Or probability (mathematics)

22. Punnet Square P Generation Purple flowers Appearance: White flowers Genetic makeup: PP pp p Gametes: P F1 Generation Appearance: Purple flowers Genetic makeup: Pp p Gametes: 1/2 1/2 P Sperm F2 Generation p P P PP Pp Eggs p pp Pp 3 1

23. Useful Genetic Vocabulary • An organism that is homozygous for a particular gene • Has a pair of identical alleles (code) for that gene • Exhibits true-breeding • An organism that is heterozygous for a particular gene • Has a pair of alleles that are different for that gene

24. More Useful Genetic Vocabulary • An organism’s phenotype • Is its physical appearance • An organism’s genotype • Is its genetic makeup (code)

25. Fig. 14-6 Phenotype Genotype PP Purple 1 (homozygous) Pp 3 Purple (heterozygous) 2 Pp Purple (heterozygous) pp White 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1

26. The testcross (works only with true dominant/recessive alleles) • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding themystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

27. The testcross TECHNIQUE  Recessive phenotype, known genotype: pp Dominant phenotype, unknown genotype: PP or Pp? Predictions If PP If Pp or Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p pp Pp pp Pp

28. The Law of Independent Assortment • Mendel derived the law of segregation • By following a single trait • The F1 offspring produced in this cross • Were monohybrids, heterozygous for one character (like Rr)

29. An organism inherits two alleles, one from each parent • Mendel identified his second law of inheritance • By following two characters at the same time • Crossing two, true-breeding parents differing in two characters • Produces dihybrids in the F1 generation, heterozygous for both characters (PpRr)

30. Using probability • How are two characters transmitted from parents to offspring? • As a package? • Independently? • If they are inherited independently, then one character doesn’t affect the other • Think of rolling dice…

31. EXPERIMENT Dihybrid cross of independent alleles YYRR yyrr P Generation Gametes yr YR  F1 Generation YyRr Hypothesis of dependent assortment Hypothesis of independent assortment Predictions Sperm or Predicted offspring of F2 generation 1/4 1/4 1/4 yr 1/4 YR yR Yr Sperm YR yr 1/2 1/2 1/4 YR YYRr YYRR YyRR YyRr 1/2 YR YyRr YYRR 1/4 Yr Eggs YYRr YYrr Yyrr YyRr Eggs 1/2 yr YyRr yyrr 1/4 yR YyRR YyRr yyRR yyRr 3/4 1/4 1/4 yr Phenotypic ratio 3:1 Yyrr yyRr YyRr yyrr 3/16 1/16 9/16 3/16 Phenotypic ratio 9:3:3:1 RESULTS Phenotypic ratio approximately 9:3:3:1 315 108 101 32

32. Concept 14.2: The laws of probability govern Mendelian inheritance • Using the information from a dihybrid cross, Mendel developed the law of independent assortment • Each pair of alleles segregates independently during gamete formation • Because it follows the mathematics of independent events in probability

33. The multiplication & 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 (A x B) • 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 (A + B)

34. 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 Figure 14.9 Probability in a monohybrid cross • Can be determined using this rule

35. 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 • 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

37. Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • Mendel choose a system that was relatively simple genetically • Each character studied was controlled by a single gene • Each gene had only two alleles, one of which is completely dominant to the other • The relationship between genotype and phenotype is rarely so simple.

38. Extending Mendelian Genetics for a Single Gene • 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

39. The Spectrum of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

40. Incomplete dominance P Generation Red White CRCR CWCW • The phenotype of F1 hybrids is somewhere between the phenotypes of the 2 parental varieties CR CW Gametes Pink F1 Generation 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

41. The Relation Between Dominance and Phenotype • Do not really “interact” • Alleles lead to synthesis of different proteins that interact to produce a phenotype

42. Pea Seed Shape • Dominant allele (round) codes for enzyme that helps convert sugar to starch • Recessive allele (wrinkled) has a defective enzyme • Lots of sugar causes lots of water intake (through osmosis) • When seed dries, it looks wrinkled • (This is different than the explanation in the book, which doesn’t make sense.)

43. Tay-Sachs disease • Brain cells cannot metabolize certain lipids, and baby dies at young age • 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

44. Frequency of Dominant Alleles • Dominant alleles • Are not necessarily more common or better recessive alleles • For example, • Polydactyl (6 digits per hand) is dominant • but not more prevalent

45. Multiple Alleles • Most genes exist in populations • In more than two allelic forms

46. Carbohydrate Allele Phenotype (blood group) Red blood cell appearance Genotype IA A B IB i none (a) The three alleles for the ABO blood groups and their associated carbohydrates IAIA or IA i A B IBIB or IB i AB IAIB ii O (b) Blood group genotypes and phenotypes

47. Pleiotropy • In pleiotropy • A gene has multiple phenotypic effects • Pleiotrophic alleles are responsible for multiple symptoms associated with certain hereditary diseases • Cystic fibrosis • Sickle-cell disease

48. Extending Mendelian Genetics for Two or More Genes • Some traits • May be determined by two or more genes • Epistasis & polygenic inheritance are situations in which two or more genes are involved in determining phenotype

49. Epistasis • In epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus • Some animal colors • B = black • b = brown • C = color • c = no color

50. Epistasis  BbCc BbCc Sperm 1/4 1/4 1/4 1/4 BC bC Bc bc Eggs 1/4 BC BBCc BBCC BbCC BbCc 1/4 bC bbCC bbCc BbCC BbCc 1/4 Bc BBcc Bbcc BBCc BbCc 1/4 bc BbCc bbCc Bbcc bbcc : 4 9 : 3