Chapter 14

1. Chapter 14 Mendel and the Gene Idea

2. Figure 14.1 Gregor Mendel • Gregor Mendel discovered the basic principles of heredity • By breeding garden peas in carefully planned experiments

3. Mendel’s Experimental, Quantitative Approach • Mendel chose to work with peas • Because they are available in many varieties • Because he could strictly control which plants mated with which

4. 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) Figure 14.2 Research Method Crossing Pea Plants

5. Artificial Selection • 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”

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

7. The Law of Segregation • When Mendel crossed true-breeding white and true-breeding purple flowered pea plants • All of the offspring had purple flowers • When Mendel crossed the F1 plants • Many of the plants had purple flowers, but some had white flowers Figure 14.3

8. The 3:1 ratio is revealed • Mendel discovered • A ratio of about three to one, purple to white flowers, in the F2 generation • 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

9. Table 14.1 The 3:1 ratio applies to the F2 generation • Mendel observed the same 3:1 ratio in the F2 generation • In many other pea plant characters

10. Useful Genetic Vocabulary • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple flowers or white flowers • Gene: a segment of DNA that codes for a heritable feature • Locus: the specific location on a chromosome where a particular gene is always found • Allele: one alternative version of a gene, such as P or p • Phenotype: the physical appearance or physiological makeup of an organism, such as purple flowers • Genotype: the genetic makeup of an organism, such as PP, Pp, or pp • Homozygous: both homologous chromosomes carry the identical allele for that gene (“true-breeding”), such as PP or pp • Heterozygous: the homologous chromosomes carry two different alleles for that gene (“hybrid”), such as Pp

11. Phenotype Genotype Purple PP (homozygous dominant) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple pp (homozygous recessive) White 1 1 Ratio 3:1 Ratio 1:2:1 Figure 14.6 Phenotype versus genotype

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

13. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Figure 14.4 Mendel’s Model: Part 1 • First, alternative versions of genes • Account for variations in inherited characters, which are now called alleles

14. Mendel’s Model: Part 2 • Second, for each character • An organism inherits two alleles, one from each parent • A genetic locus is actually represented twice

15. Mendel’s Model: Part 3 • Third, 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

16. Mendel’s Model: Part 4 • Fourth, the law of segregation • The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

17. Punnet Squares • 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 Figure 14.5

18. Predicting the Outcome of Crosses Problem 1: A true-breeding purple-flowered pea is crossed with a true-breeding white-flowered plant. Calculate the expected genotypic frequency and phenotypic frequency of the F1 and F2 offspring. P = purple flowers p = white flowers 1. Determine the gametes involved Parent 1 PP Parent 2 pp

19. 2. Set up the Punnett Square PP x PP 3. Calculate the F1 generation Genotypic frequency= Phenotypic frequency=

20. 4. Cross the offspring Aa x Aa 5. Calculate the F2 generation Genotypic frequency= Phenotypic frequency=

21. The Law of Independent Assortment • Mendel derived the law of segregation • By following one character, flower color • The F1 offspring produced in this cross • Were monohybrids, heterozygous for one character • Mendel identified the law of independent assortment • By following two characters at the same time • The F1 offspring produced in this cross • Were dihybrids, heterozygous for both characters

22. The 9:3:3:1 ratio is revealed • A dihybrid cross • Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation in a characteristic 9:3:3:1 ratio • New combinations of characters are created, showing independent assortment for each pair of alleles Figure 14.8

23. Predicting the Outcome of Crosses Problem 2: Pea plants heterozygous for seed color and seed shape (YyRr) are allowed to self–pollinate, and 400 of the resulting seeds are planted. How many offspring would be predicted to have round yellow seeds? The mating of YyRr x YyRr is a dihybrid cross • Produces four phenotypes in the characteristic 9:3:3:1 ratio: • 9/16 Yellow Round • 3/16 Yellow Wrinkled • 3/16 Green Round • 1/16 Green Wrinkled 9/16 x 400 = 225

24. The Testcross • A testcross • 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

25. DD x dd Dd x dd Predicting the Outcome of Crosses Problem 3: In dogs, there is an hereditary deafness caused by a recessive gene (d). A kennel owner has a male dog that she wants to use for breeding purposes if possible. The dog can hear, so the owner knows his genotype is either DD or Dd. Draw the Punnett squares to illustrate the two possible testcrosses. A testcross is the mating of an individual with the dominant phenotype with an individual that is homozygous recessive

26. The Rules of Probability • Mendel’s laws of segregation and independent assortment • Reflect the rules of probability • The multiplication rule (either, or) • States that the probability that two or more independent events will occur together is the product of their individual probabilities • The rule of addition (and, both) • States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

27. Solve Complex 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

28. D B B d D B DD BB Dd BB b Bb D DD Bb Dd Bb x BB DD x Dd Predicting the Outcome of Crosses Problem 4: An organism with the genotype BbDD is mated to one with the genotype BBDd. Assuming independent assortment of these two genes, write the genotypes of all possible offspring from this cross and calculate the chance of each genotype occurring using the rules of probability. 1. Consider the character B 3. Consider the character D 2. Write the possible genotypes and frequencies 4. Write the possible genotypes and frequencies Genotypic frequency= Genotypic frequency= BB = ½ Bb = ½ DD = ½ Dd = ½

29. Calculate the chance of each genotype using the rules of probability Genotypic frequency= Genotypic frequency= BB = ½ Bb = ½ DD = ½ Dd = ½ 5. Combine the two characters and write the possible genotypes ½ x ½ = ¼ ½ x ½ = ¼ ½ x ½ = ¼ ½ x ½ = ¼ BBDD BbDD BBDd BBDd

30. Extending Mendelian Genetics for a Single Gene • The inheritance of characters by a single gene may deviate from simple Mendelian patterns • In the cases Mendel studied involving heterozygotes, the heterozygote was identical to one of the parents • one allele was expressed but the other was not • we call this complete dominance

31. Codominance • In codominance • Two different alleles of a gene are both fully expressed and appear together in the hybrid • The phenotype of F1 hybrids shows the phenotypes of both parental varieties • The human blood type AB • Is an example of codominance

32. Incomplete Dominance • In incomplete dominance • Two different alleles of a gene are both partially expressed and appear together in the hybrid • The phenotype of F1 hybrids is between the phenotypes of the two parental varieties • Flower color in snapdragons • Is an example of incomplete dominance Figure 14.10

33. Table 14.2 Multiple Alleles • Most genes exist in populations • In more than two allelic forms • The ABO blood group in humans • Is determined by multiple alleles • IA, IB, and i

34. Pleiotropy • In pleiotropy • A gene has multiple phenotypic effects • Sickle-cell disease • Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • Symptoms include physical weakness, pain, organ damage, and even paralysis

35. Gg x Gg Predicting the Outcome of Crosses Problem 5: When two gray mice are mated, the phenotypic ratio of fur color in the offspring is 1 black : 2 gray : 1 white. Set up the Punnet square for this mating. Is this an example of complete dominance, codominance, incomplete dominance, multiple alleles, or pleiotropy? Gray color is between black and white. • This is an example of incomplete dominance: The phenotype of the F1 hybrids is between the phenotypes of the two parental varieties. Therefore the gray mice are heterozygous, and the black and white mice are homozygous. Gg = gray GG = black gg = white

36. Predicting the Outcome of Crosses Problem 6: A child of blood type A has a mother who is blood type O. What are the genotypes of the child and the mother? What type or types may the father belong to? A mother of blood type O has the genotype ii • Blood type A may be caused by IAIA or IAi. Since we know the child inherited i from its mother, we know the child must be IAi. • Now we know the child inherited IA from its father, we know the father must be IAi, IAIA or IAIB and so his blood type is either A or AB.

37. Extending Mendelian Genetics for Two or More Genes • Some traits • May be determined by two or more genes

38. Epistasis • In epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus • Coat color in mice • Is an example of epistasis • The C gene for color acts as an “on/off” switch Figure 14.11

39. Polygenic Inheritance • In polygenetic inheritance • Many genes influence a character showing a range of variation (height, weight, beak size, crop yield, etc) • Each gene has a small effect, and the effects are additive • Human skin color • Is an example of polygenic inheritance Figure 14.12

40. Nature and Nurture • An organism’s phenotype • Includes its physical appearance, internal anatomy, physiology, and behavior • Reflects its overall genotype and unique environmental history • Multifactorial characters • Are those that are influenced by both genetic and environmental factors

41. Multifactorial Disorders • Many human diseases • Have both genetic and environment components • Examples include • Heart disease and cancer

42. Recessively Inherited Disorders • Recessively inherited disorders • Show up only in individuals homozygous for the allele • Examples: cystic fibrosis, sickle-cell anemia • Carriers • Are heterozygous individuals who carry the recessive allele but are phenotypically normal

43. Dominantly Inherited Disorders • Dominantly inherited disorders • Show up in any individual who has a copy of the allele • Examples: achondroplasia, Huntington’s Disease • Achondroplasia • Is a form of dwarfism that is lethal when homozygous for the dominant allele • Huntington’s disease • Is a degenerative disease of the nervous system that has no obvious phenotypic effects until about 35 to 40 years of age

44. Predicting the Outcome of Crosses Problem 6: Achondroplasia (dwarfism) is governed by an allele which is dominant and which is lethal in the homozygous dominant condition. a. What is the genotype of a person with achondroplasia? b. Show the genotypes and give the ratio of phenotypes in offspring from parents who both have achondroplasia. A = achondroplasia a = normal • a. The genotype of a person with achondroplasia is Aa, because aa is normal and AA is lethal

45. A a A AA Aa Aa x Aa a Aa aa b. Set up the Punnett Square Aa = achondroplasia aa = normal AA = lethal Show the genotypes and give the ratio of phenotypes in offspring Aa, aa Genotypes = Ratio of Phenotypes = 2 achondroplasia : 1 normal