Mendel's Laws of Inheritance: Exploring Genetic Principles in Pea Plants

1. Genetics • Mendel and the Gene Idea

2. Heredity • What genetic principles account for the transmission of traits from parents to offspring? • 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. • An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units, genes.

3. Gregor Mendel • Documented a particulate mechanism of inheritance through his experiments with garden peas. Gregor Mendel’s monastery garden. Fig. 2.2

4. Themes of Mendel’s work • Variation is widespread in nature • Observable variation is essential for following genes • Variation is inherited according to genetic laws and not solely by chance • Mendel’s laws apply to all sexually reproducing organisms

5. Mendel’s Experimental, Quantitative Approach • 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 • Mendel chose to work with the garden pea (Pisum sativum) • Because they are available in many varieties, • easy to grow, • easy to get large numbers • Because he could strictly control which plants mated with which

6. Removed stamens from purple flower 1 Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower 2 Parental generation (P) Stamens (male) Carpel (female) Pollinated carpel matured into pod 3 Planted seeds from pod 4 Examined offspring: all purple flowers 5 First generation offspring (F1) Crossing Pea Plants

7. Mendel’s experimental design • Statistical analyses: • Worked with large numbers of plants • counted all offspring • made predictions and tested them • Excellent experimentalist • controlled growth conditions • focused on traits that were easy to score • chose to track only those characters that varied in an “either-or” manner

8. Mendel’s Studied Discrete Traits

9. Genetic Vocabulary • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers • Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father • Alternative forms of traits are called alleles.

10. Antagonistic traits Dominant Recessive

11. X X X X X X Mendel’s experimental design • Mendel also made sure that he started his experiments with varieties that were “true-breeding”.

12. Genetic Vocabulary • Phenotype – observable characteristic of an organism • Genotype – pair of alleles present in and individual • Homozygous– two alleles of trait are the same (YY or yy) • Heterozygous – two alleles of trait are different (Yy) • Capitalized traits = dominant phenotypes • Lowercase traits= recessive phenotypes

13. Genetic Vocabulary • Generations: • P = parental generation • F1 = 1st filial generation, progeny of the P generation • F2 = 2nd filial generation, progeny of the F1 generation (F3 and so on) • Crosses: • Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait. • Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.

14. Genotype Phenotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple White 1 pp (homozygous) 1 Ratio 3:1 Ratio 1:2:1 Figure 14.6 Phenotype vs Genotype

15. Phenotype vs. Genotype Dominant & recessive alleles (Fig. 10.7):

16. Mendel’s Experimental Design • 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

17. Mendel’s Observations • 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 • A ratio of about three to one, purple to white flowers, in the F2 generation

18. P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation

19. Mendel’s Rationale • In the F1 plants, only the purple trait was affecting flower color in these hybrids • Purple flower color was dominant, and white flower color was recessive • Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring • There are four related concepts that are integral to this hypothesis

20. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Heredity Concepts 1. Alternative versions of genes account for variations in inherited characters, which are now called alleles 2. For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice 3. If the two alleles at a locus differ, the dominant allele determines the organism’s appearance 4. The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

21. Law of Segregation • Mechanism of gene transmission Fertilization: alleles unite Gametogenesis: alleles segregate

22. Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P0 cross F1 cross Determine the genotype and phenotype

23. Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P0 cross F1 cross Determine the genotype and phenotype

24. Mendelian Genetics • Classical Punett's Square is a way to determine ways traits can segregate Parental P0 cross F1 cross Determine the genotype and phenotype

25. Mendel’s Law Of Segregation, Probability And The Punnett Square P Generation  Appearance:Genetic makeup: Purple flowersPP White flowerspp Gametes: p P F1 Generation Appearance:Genetic makeup: Purple flowersPp 1/2 Gametes: 1/2 p P p P F2 Generation P Pp PP F1 eggs • p pp Pp 3 : 1

26. Mendel’s Monohybrid Cross White (pp) Purple (Pp) Gametes Gametes p p p P Purple (PP) Purple (Pp) P P Pp Pp PP Pp Gametes Gametes P p Pp Pp Pp pp F1 generation All purple F2 generation ¾ purple, ¼ white

27. Smooth and wrinkled parental seed strains crossed. • Punnett square • F1 genotypes: 4/4 Ss • F1 phenotypes: 4/4 smooth

28. Mendel Observed The Same Pattern In Characters

29. The Testcross • In pea plants with purple flowers the genotype is not immediately obvious • A testcross 1. Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype 2. Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

30. Test Cross • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: ? Alternative 2 – Plant with dominant phenotype is heterozygous Alternative 1 – Plant with dominant phenotype is homozygous ? Dominant Phenotype Dominant Phenotype PP Pp Gametes Gametes Recessive phenotype P P P p Recessive phenotype p pp p Gametes Gametes p pp p

31. ? Alternative 1 – Plant with dominant phenotype is homozygous Dominant Phenotype PP Gametes Recessive phenotype P P Alternative 2 – Plant with dominant phenotype is heterozygous p pp Pp Pp ? Gametes Dominant Phenotype Pp Pp p Pp Gametes P p Recessive phenotype If all offspring are purple; unknown plant is homozygous. p pp Pp Gametes pp p Pp pp If half of offspring are white; unknown plant is heterozygous. Test Cross • To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype:

32.  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

33. The Law of Independent Assortment • Mendel derived the law of segregation by following a single trait • 2 alleles at a single gene locus segregate when the gametes are formed • The F1 offspring produced in this cross were monohybrids, heterozygous for one character • Mendel identified his second law of inheritance by following two characters at the same time • Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently • Crossing two, true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

34. Dihybrid Cross • With his monohybrid crosses, Mendel determined that the 2 alleles at a single gene locus segregate when the gametes are formed. • With his dihybrid crosses, Mendel was interested in determining whether alleles at 2 different gene loci segregate dependently or independently.

35. Dihybrid Cross • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y). • Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.

36. Dihybrid Cross • For example, in pea plants seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) while seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y). • Mendel crossed 2 pure-breeding plants: one with round yellow seeds and the other with green wrinkled seeds.

37. Dependent Segregation • If dependent segregation (assortment) occurs: • Alleles at the 2 gene loci segregate together, and are transmitted as a unit. • Therefore, each plant would only produce gametes with the same combinations of alleles present in the gametes inherited from its parents: r ry y R R Y Y Parents R Y r y Parental Gametes Rr Yy F1 Offspring R Y r y F1 Offspring’s Gametes What is the expected phenotypic ratio for the F2?

38. F2 With Dependent Assortment: r y R Y R Y r y Ratio is 3 round, yellow : 1 wrinkled, green

39. Independent Segregation • Alleles at the 2 gene loci segregate (separate) independently, and are NOT transmitted as a unit. Therefore, each plant would produce gametes with allele combinations that were not present in the gametes inherited from its parents: r ry y R R Y Y Parents R Y r y Parental Gametes Rr Yy F1 Offspring R Y R y r Y r y F1 Offspring’s Gametes What is the expected phenotypic ratio for the F2?

40. Mendelian Genetics Dihybrid cross - parental generation differs in two traits example-- cross round/yellow peas with wrinkled/green ones Round/yellow is dominant What are the expected phenotype ratios in the F2 generation? round, yellow = round, green = wrinkled, yellow = wrinkled, green =

41. F2 with independent assortment: RY Ry rY ry RY Ry rY ry Phenotypic ratio is 9 : 3 : 3 : 1

42. A Dihybrid Cross • How are two characters transmitted from parents to offspring? • As a package? • Independently? • A dihybrid cross • Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation

43. P Generation YYRR yyrr 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. Gametes YR yr  F1 Generation YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm Yr 1 ⁄4 1 ⁄4 YR 1 ⁄4 yR yr 1 ⁄4 Sperm Eggs 1⁄2 yr 1⁄2 YR 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 3 ⁄4 1 ⁄4 yr 1 ⁄4 CONCLUSION The results support the hypothesis ofindependent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. Phenotypic ratio 3:1 Yyrr YyRr yyRr yyrr 1 ⁄16 3 ⁄16 3 ⁄16 9 ⁄16 Phenotypic ratio 9:3:3:1 315 108 Phenotypic ratio approximately 9:3:3:1 101 32

44. Dihybrid cross • F2 generation ratio:9:3:3:1

45. Law of Independent Assortment • Mendel’s dihybrid crosses showed a 9:3:3:1 phenotypic ratio for the F2 generation. • Based on these data, he proposed the Law of Independent Assortment, which states that when gametes form, each pair of hereditary factors (alleles) segregates independently of the other pairs.

46. Laws Of Probability Govern Mendelian Inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • The multiplication rule • States that the probability that two or more independent events will occur together is the product of their individual probabilities • 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

47. Laws Of Probability - Multiplication Rule • The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself • Following the self-hybridization of a heterozygous purple pea plants (Pp), what is the probability that a given offspring will be homozygous for the production of white flowers (pp)? • Probability that a pollen seed will carry p: ½ • Probability that an egg will carry p: ½ • Probability that the offspring will be pp: • 1/2 X 1/2 = 1/4

48. Laws Of Probability - Addition Rule • The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities • Following the self-hybridization of a heterozygous purple pea plant (Pp), what is the probability that a given offspring will be purple? • Probability of maternal P uniting with paternal P: 1/4 • Probability of maternal p uniting with paternal P: 1/4 • Probability of maternal P uniting with paternal p: 1/4 • Probability that the offspring will be purple: • 1/4 + 1/4 + 1/4 = 3/4

49. Probability In A Monohybrid Cross • Can be determined using these rules 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

50. Mendel’s conclusions • Genes are distinct entities that remain unchanged during crosses • Each plant has two alleles of a gene • Alleles segregated into gametes in equal proportions, each gamete got only one allele • During gamete fusion, the number of alleles was restored to two