Mendel, Genes, and Inheritance

1. Mendel, Genes, and Inheritance Chapter 12

2. Gregor Mendel • Austrian Monk with a strong background in plant breeding and mathematics • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

3. Mendel’s Work • Bred pea plants • cross-pollinated true breeding parents (P) • raised seed & then observed traits (F1) • filial • allowed offspring to cross-pollinate & observed next generation (F2)

4. Mendel’s Data:

5. 100% purple-flower peas F1 generation (hybrids) 100% self-pollinate 75% purple-flower peas 25% white-flower peas 3:1 F2 generation Mendel’s Experiments true-breeding purple-flower peas true-breeding white-flower peas X P

6. Genes • Sequences of DNA that contain information about specific traits, by coding for individual proteins • Passed from parents to offspring • Each has a specific location (locus) on a chromosome Alleles • Different versions of the same trait

7. Genes and inheritence • For each characteristic (gene), a diploid organism inherits 2 sets of alleles • 1 from each parent • Homologous chromosomes • Pairs of chromosomes that contain the same traits (genes) • For each trait there are two alleles (one on each homologous chromosome)

8. Genetic Terms purple-flower allele & white-flower allele are 2 DNA variations at flower-color locus different versions of gene on homologous chromosomes

9. Mendel’s Findings: Dominance • Some alleles for a given traits mask others • Dominant allele • Fully expressed in a hybrid • Designated by a capital letter • e.g. P = Purple allele • Recessive allele • no noticeable effect in a hybrid • Designated by a lowercase letter • E.g. p = White allele

10. Genotype vs. phenotype • Phenotype • Description of an organism’s trait • “visible” characteristic • Genotype • Description of an organism’s genetic makeup; i.e. which alleles are present in the organism • Alleles may be the same or they may be different: • Homozygous = same alleles; PP, pp • Heterozygous = different alleles; Pp

11. self-pollinate Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X P PP pp Phenotype: 100% purple-flower peas 100% purple-flower peas F1 generation (hybrids) 100% Pp Pp Pp Pp 75% purple-flower peas 75% purple-flower peas 25% white-flower peas 3:1 3:1 F2 generation ? ? ? ?

12. PP 25% male / sperm P p Pp 50% 75% P Pp female / eggs pp p 25% 25% Punnett squares % genotype % phenotype Pp x Pp PP Pp Pp pp 1:2:1 3:1

13. purple PP homozygous dominant purple Pp heterozygous Phenotype vs. genotype • 2 organisms can have the same phenotype but have different genotypes

14. Dominant phenotypes • It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it. • So how can you figure it out? PP? Pp?

15. Test cross • Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x is itPP or Pp? pp

16. x x Test cross PP pp Pp pp p p p p Pp Pp Pp Pp P P P Pp Pp p pp pp 50%:50%1:1 100%

17. P P P p p PP Pp pp p Mendel’s laws of heredity (#1) • Law of segregation • when gametes are produced during meiosis, homologous chromosomes separate from each other • each allele for a trait is packaged into a separate gamete

18. Meiosis 1 Law of Segregation • What meiotic event creates the law of segregation?

19. Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters • flower color • seed color • monohybrid crosses

20. Dihybrid cross • Other of Mendel’s experiments followed the inheritance of 2 different characters • seed color andseed shape • Dihybrid crosses

21. yellow, round peas F1 generation (hybrids) 100% self-pollinate 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas 9:3:3:1 F2 generation Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas P x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr

22. YyRr YyRr YR YR yR Yr yr yr Dihybrid Cross • How are the alleles on different chromosomes handed out? • together or separately?

23. 9/16 yellow round 3/16 green round YR Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

24. Mendel’s laws of heredity (#2) Law of independent assortment: • each pair of alleles segregates into gametes independently • 4 classes of gametes are produced in equal amounts • YR, Yr, yR, yr • only true for genes on separate chromosomes YyRr Yr Yr yR yR YR YR yr yr

25. Meiosis 1 Law of Independent Assortment • What meiotic event creates the law of independent assortment?

26. The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring

27. Review: Mendel’s laws of heredity • Law of segregation • each allele segregates into separate gametes • Law of independent assortment • Observable in dihybrid (or more) cross • 2 or more traits • each pair of alleles for genes on separate chromosomes segregates into gametes independently • Each gamete carries one allele of each trait

28. Extending Mendelian genetics • Mendel worked with a simple system • peas are genetically simple • most traits are controlled by a single gene • each gene has only 2 alleles, 1 of which is completely dominant to the other • The relationship between genotype & phenotype is rarely that simple

29. Incomplete dominance • Heterozygotes show an intermediate phenotype • RR = red flowers • rr = white flowers • Rr = pink flowers • make 50% less color

30. self-pollinate 25% red 50% pink 25% white Incomplete dominance X true-breeding red flowers true-breeding white flowers P 100% pink flowers F1 generation (hybrids) 100% 1:2:1 F2 generation

31. CRCR 25% 25% male / sperm CR CW CRCW 50% 50% CR CRCW female / eggs CWCW CW 25% 25% Incomplete dominance % genotype % phenotype CRCW x CRCW CRCR CRCW CRCW CWCW 1:2:1 1:2:1

32. Codominance • BOTH alleles express in heterozygotes • Example: In chickens, feather color trait has two alleles: B= Black feathers W = White feathers • What is phenotype of BB, WW? • What is the phenotype of BW?

33. Codominance: + WW BB WB

34. Multiple Alleles • More than two possible alleles for one trait • Example: in ABO blood group, 3 possible alleles: • IA, IB, i • 4 blood types: A, B, O, AB • How?

35. Pleiotropy • Most genes are pleiotropic • one gene affects more than one phenotypic character • wide-ranging effects due to a single gene: • dwarfism (achondroplasia) • gigantism (acromegaly)

36. Epistasis • One gene masks another • coat color in mice = 2 genes • pigment (C) or no pigment (c) • more pigment (black=B) or less (brown=b) • cc = albino, no matter B allele • 9:3:3:1 becomes 9:3:4

37. Epistasis in Labrador retrievers • 2 genes: E & B • pigment (E) or no pigment (e) • how dark pigment will be: black (B) to brown(b)

38. Polygenic inheritance (continuous variation) • Some phenotypes determined by additive effects of 2 or more genes on a single character • phenotypes on a continuum • human traits • skin color • height • weight • eye color • intelligence • behaviors

39. Environmental influence • The expression of some genes can be influenced by the environment • for example: coat color in Himalayan rabbits and Siamese cats • an allele produces an enzyme that allows pigment production only at temperatures below 30oC

40. Environmental influence

41. Acknowledgement • Much of this presentation was modified from the Biology Zone website: http://bio.kimunity.com/