Fundamentals of Genetics: Mendel's Laws and Genetic Crosses

1. Chapter 12: Fundamentals of Genetics

2. Gregor Johann Mendel • Austrian monk • Studied the inheritance of traits in peaplants • Developed the laws of inheritance

3. Gregor Johann Mendel • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants • He found that the plants' offspring retained traits of the parents • Called the “Father of Genetics"

4. Genetic Terminology • Trait - any characteristic that can be passed from parent to offspring • Heredity - passing of traits from parent to offspring • Genetics - study of heredity

5. Types of Genetic Crosses • Monohybrid cross - cross involving a single traite.g. flower color • Dihybrid cross - cross involving two traits e.g. flower color & plant height

6. Punnett Square Used to help solve genetics problems

7. Alleles • Alleles - two forms of a gene(dominant & recessive) • Dominant - strongerof two genes expressed in the hybrid; represented byacapital letter (R) • Recessive - gene that shows up less often in a cross; represented by alowercase letter (r)

8. More Terminology • Genotype - gene combination for a trait(e.g. RR, Rr, rr) • Phenotype - the physical feature resulting from a genotype(e.g. red, white)

9. Genotype & Phenotype in Flowers Genotype of alleles:R= red flowerr= yellow flower All genes occur in pairs, so 2alleles affect a characteristic Possible combinations are: GenotypesRRRr rr PhenotypesREDREDYELLOW

10. Genotypes • Homozygous(pure)genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr);SAMEVERSIONof the gene • Heterozygous (hybrid)genotype - gene combination of one dominant & one recessive allele (e.g. Rr);DIFFERENTVERSIONSof the gene

11. Mendel’s Pea Plant Experiments

12. How Mendel Began Mendel allowed the plants to self-pollinate for several generations

13. Eight Pea Plant Traits • Seed shape--- Round (R) or Wrinkled (r) • Seed Color---- Yellow (Y) or  Green (y) • Pod Shape--- Smooth (S) or wrinkled (s) • Pod Color---  Green (G) or Yellow (g) • Seed Coat Color---Gray (G) or White (g) • Flower position---Axial (A) or Terminal (a) • Plant Height--- Tall (T) or Short (t) • Flower color --- Purple (P) or white (p)

16. Generation “Gap” • Parental (P1) Generation= the parental generation in a breeding experiment. • F1Generation= the first-generation offspring in a breeding experiment. (1st filial generation) • From breeding individuals from the P1 generation • F2Generation= the second-generation offspring in a breeding experiment. (2nd filial generation) • From breeding individuals from the F1 generation

17. Following the Generations Cross 2 Pure PlantsTT x tt Results in all HybridsTt Cross 2 Hybridsget3 Tall & 1 ShortTT, Tt, tt

18. Monohybrid Crosses

19. P1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: RoundseedsxWrinkledseeds • RRxrr Genotype(s):Rr Phenotype(s):Round GenotypicRatio = 4 Rr: 0 rr: 0 Rr PhenotypicRatio:4 Round: 0 Wrinkled r r Rr Rr R R Rr Rr

20. P1 Monohybrid Cross Review • Homozygous dominantx Homozygous recessive • Offspringallheterozygous(hybrids) • Offspring calledF1 generation • Offspring all have the Rr genotype • Offspring all have the round phenotype

21. F1 Monohybrid Cross • Trait: Seed Shape • Alleles: R – Round r – Wrinkled • Cross: Roundseeds xRoundseeds • RrxRr Genotype:RR, Rr, rr Phenotype:Round &wrinkled Geno. Ratio:1 RR: 2 Rr: 1 rr Pheno. Ratio:3 round: 1 wrinkled R r RR Rr R r Rr rr

22. F1 Monohybrid Cross Review • Heterozygous x heterozygous • Offspring:25% Homozygous dominantRR50% HeterozygousRr25% Homozygous Recessiverr • Offspring calledF2 generation • Genotypic ratio is1:2:1 • Phenotypic Ratio is 3:1

23. Mendel’s Laws

24. Law of Dominance The dominant trait will always be expressed if present.

25. Law of Segregation • Alleles separate during gamete (egg & sperm) formation and then recombine during fertilization.

26. Applying the Law of Segregation

27. Law of Independent Assortment • Alleles for differenttraits are sorted independently of one another. • This law can be illustrated using dihybrid crosses.

28. Dihybrid Cross • Traits: Seed shape & Seed color • Alleles:R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations

29. Dihybrid Cross RY Ry rY ry RY Ry rY ry

30. Dihybrid Cross RY Ry rY ry Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 9:3:3:1 phenotypic ratio

31. Incomplete DominanceandCodominance

32. Incomplete Dominance r r R R • F1 hybrids (heterozygous) have an appearance somewhat in-between the phenotypesof the two parental varieties. • Example:snapdragons (flower) • red (RR) x white (rr) • RR = red flower • rr = white flower

33. Incomplete Dominance r produces the F1 generation Rr Rr R R Rr Rr All Rr = pink (heterozygous pink) r

34. Incomplete Dominance

35. Codominance • Both alleles are expressed in heterozygous individuals. • Example: Human blood types • 1. Type A = IAIA or IAi • 2. Type B = IBIB or IBi • 3. Type AB = IAIB • 4. Type O = ii

36. Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XX genotype for females • XY genotype for males • Many sex-linked traits carried on X chromosome (i.e., color-blindness)

37. Genetic Practice Problems

38. Breed the P1 generation t t T T • tall (TT) x dwarf (tt) pea plants

39. Solution: t t produces the F1 generation Tt Tt T Tt Tt T All Tt = tall (heterozygous tall) tall (TT) vs. dwarf (tt) pea plants

40. Breed the F1 generation T t T t • tall (Tt) vs. tall (Tt) pea plants

41. Solution: T t produces the F2 generation Tt TT T 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt Tt tt t 1:2:1 genotype 3:1 phenotype tall (Tt) x tall (Tt) pea plants