Mendel and Inheritance

1. Mendel and Inheritance MUPGRET Workshop December 4, 2004

2. Genetic variation • In the beginning geneticists studied differences they could see in plants. • These differences are called morphological differences. • Individual variants are referred to as phenotypes, ex. tall vs. short plants or red vs. white flowers.

3. Trait • A broad term encompassing a distribution of phenotypic variation. • Example: • Trait: Disease resistance • Phenotype: resistant vs. susceptible • Morphological differences associated with the trait might include fungal infection, fungal growth, sporulation, etc.

4. Mendel • Monk at the St. Thomas monastery in the Czech Republic. • Performed several experiments between 1856 and 1863 that were the basis for what we know about heredity today. • Used garden peas for his research. • Published his work in 1866.

5. Mendel • Results are remarkably accurate and some have said they were too good to be unbiased. • His papers were largely ignored for more than 30 years until other researchers appreciated its significance.

6. Garden Pea • Pisum sativum • Diploid • Differed in seed shape, seed color, flower color, pod shape, plant height, etc. • Each phenotype Mendel studied was controlled by a single gene.

7. Terms • Wild-type is the phenotype that would normally be expected. • Mutant is the phenotype that deviates from the norm, is unexpected but heritable. • This definition does not imply that all mutants are bad; in fact, many beneficial mutations have been selected by plant breeders.

8. Advantages of plants • Can make controlled hybrids. • Less costly and time consuming to maintain than animals. • Can store their seed for long periods of time. • One plant can produce tens to hundreds of progeny.

9. Advantages of plants • Can make inbreds in many plant species without severe effects that are typically seen in animals. • Generation time is often much less than for animals. • Fast plants (Brassica sp.) • Arabidopsis

10. Allele • One of two to many alternative forms of the same gene (eg., round allele vs. wrinkled allele; yellow vs. green). • Alleles have different DNA sequences that cause the different appearances we see.

11. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Principle of Segregation(Mendel’s First Law)

12. Important Observations • F1 progeny are heterozygous but express only one phenotype, the dominant one. • In the F2 generation plants with both phenotypes are observedsome plants have recovered the recessive phenotype. • In the F2 generation there are approximately three times as many of one phenotype as the other.

13. Mendel’s Results

14. 3 : 1 Ratio • The 3 : 1 ratio is the key to interpreting Mendel’s data and the foundation for the the principle of segregation.

15. The Principle of Segregation • Genes come in pairs and each cell has two copies. • Each pair of genes can be identical (homozygous) or different (heterozygous). • Each reproductive cell (gamete) contains only one copy of the gene.

16. Mendel’s Principle of Segregation • In the formation of gametes, the paired hereditary determinants separate (segregate) in such a way that each gamete is equally likely to contain either member of the pair. • One male and one female gamete combine to generate a new individual with two copies of the gene.

17. Genotypes versus phenotypes Yy  Yy 1:2:1 YY:Yy:yy 3:1 yellow: green

18. The Punnett Square

19. Test cross reveals unknown genotype

20. X Parental Lines Round Wrinkled All round F1 progeny Self-pollinate 3 Round : 1 Wrinkled Round 5474 Wrinkled 1850 Round vs. Wrinkled

21. Round vs. wrinkled • The SBEI causes the round vs. wrinkled phenotype. • SBEI = starch-branching enzyme • Wrinkled peas result from absence of the branched form of starch called amylopectin. • When dried round peas shrink uniformly and wrinkled do not.

22. Round vs. wrinkled • The non-mutant or wild-type round allele is designated W. • The mutant, wrinkled allele is designated w. • Seeds that are Ww have half the SBEI of wild-type WW seeds but this is enough to make the seeds shrink uniformly. • W is dominant over w.

23. Round vs. wrinkled • An extra DNA sequence is present in the wrinkled allele that produces a non-functional SBEI and blocks the starch synthesis pathway at this step resulting in a lack of amylopectin.

24. A Molecular View Parents F1 F2 Progeny WW ww Ww ¼WW ¼Ww ¼wW ¼ww 1: 2 : 1 Genotype = 3: 1 Phenotype

25. Dihybrid crosses reveal Mendel’s law of independent assortment • A dihybrid is an individual that is heterozygous at two genes • Mendel designed experiments to determine if two genes segregate independently of one another in dihybrids • First constructed true-breeding lines for both traits, crossed them to produce dihybrid offspring, and examined the F2 for parental or recombinant types (new combinations not present in the parents).

26. Mendel and two genes Round Yellow Wrinkled Green x All F1 Round, Yellow Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108

27. Punnett Square with Dihybrid Cross

28. Dihybrid cross produces a predictable ratio of phenotypes

29. Ratio for a cross with 2 genes • Crosses with two genes are called dihybrid. • Dihybrid crosses have genetic ratios of 9:3:3:1.

30. The law of independent assortmentMendel’s second law • During gamete formation different pairs of alleles (different genes) segregate independently of each other

31. Mendel and two genes Wrinkled Yellow 101 Wrinkled Green 32 Round Yellow 315 Round Green 108 Yellow = 416 Green = 140 Round = 423 Wrinkled = 133 Each gene has a 3 : 1 ratio.

32. Summary of Mendel's work • Inheritance is particulate - not blending • There are two copies of each trait in a germ cell • Gametes contain one copy of the trait • Alleles (different forms of the trait) segregate randomly • Alleles are dominant or recessive - thus the difference between genotype and phenotype • Different traits assort independently

33. Rules of Probability Independent events - probability of two events occurring together What is the probability that both A and B will occur? Solution = determine probability of each and multiply them together. Mutually exclusive events - probability of one or another event occurring. What is the probability of A or B occurring? Solution = determine the probability of each and add them together.

34. PRODUCT RULE From James Birchler

35. SUM RULE Mutually exclusive ways! From James Birchler

36. From James Birchler

37. All Dominant Dominant Recessive All Recessive From James Birchler

38. P RRYYTTSS X rryyttss RYTS ryts gametes F1 RrYyTtSs X RrYyTtSs RYTS RYTS RYTs RYTs RYtS RYtS RYts RYts RyTS RyTS RyTs RyTs RytS RytS Ryts Ryts gametes rYTS rYTS rYTs rYTs rYts rYts rYTS rYTS ryTs ryTs rYtS rYtS rYts rYts ryts ryts F2 What is the ratio of different genotypes and phenotypes? Laws of probability for more complex crosses

39. Punnett Square method - 24 = 16 possible gamete combinations for each parent Thus, a 16  16 Punnett Square with 256 genotypes That’s one big Punnett Square! Loci (Genes) Assort Independently - So we can look at each locus independently to get the answer. Branch diagrams are also convenient tools

40. Extensions to Mendel Complexities in relating genotype to phenotype

41. Some Extensions to Mendel’s Analysis • Single-gene inheritance • In which pairs of alleles show deviations from complete dominance and recessiveness • In which different forms of the gene are not limited to two alleles • Where one gene may determine more than one trait • Multifactorial inheritance in which the phenotype arises from the interaction of one or more genes with the environment, chance, and each other

42. Dominance is not always complete • Crosses between true-breeding strains can produce hybrids with phenotypes different from both parents • Incomplete dominance • F1 hybrids that differ from both parents express an intermediate phenotype. Neither allele is dominant or recessive to the other • Phenotypic ratios are same as genotypic ratios • Codominance • F1hybrids express phenotype of both parents equally • Phenotypic ratios are same as genotypic ratios

43. Summary of dominance relationships

44. Incomplete dominance in snapdragons

45. Codominant blood group alleles

46. A gene can have more than two alleles • Genes may have multiple alleles that segregate in populations • Alleles may be unique to every pair of alleles in an individual

47. Mendelian inheritance in humans • Many traits in humans are due to the interaction of multiple genes and do not show a simple Mendelian pattern of inheritance. • A few traits represent single-genes. Examples include sickle-cell anemia, cystic fibrosis, Tay-Sachs disease, and Huntington’s disease • Because we can not do breeding experiments on humans, we must use pedigrees to study inheritance • Pedigrees are an orderly diagram of relevant genetic features extending through multiple generations • Pedigrees help us infer if a trait is from a single gene and if the trait is dominant or recessive

48. Anatomy of a pedigree

49. A vertical pattern of inheritance indicates a rare dominant trait Hunitington’s disease: A rare dominant trait

50. A horizontal pattern of inheritance indicates a rare recessive trait Cystic fibrosis: a recessive condition