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 (F 1 ) filial  allowed offspring to cross-pollinate & observed next generation (F 2 )

4. Mendel’s Data:

5. Mendel’s Experiments true-breeding purple-flower peas true-breeding white-flower peas X 100% purple-flower peas F 1 generation (hybrids) 25% white-flower peas F 2 generation 75% purple-flower peas 3:1 P self-pollinate

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. Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X 100% purple-flower peas 25% white-flower peas 75% purple-flower peas 3:1 PPpp PpPpPpPpPpPpPpPp self-pollinate ???? Phenotype: 100% purple-flower peas F 1 generation (hybrids) F 2 generation 75% purple-flower peas 3:1 P

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

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

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 pp is it PP or Pp?

16. x PPpp Test cross pp P P PpPp PpPpPpPp PpPp pp P p PpPp pp PpPp x PpPp 100% 50%:50% 1:1

17. Mendel’s laws of heredity (#1) PP P P pp p p PpPp P p 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. Law of Segregation What meiotic event creates the law of segregation? Meiosis 1

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 and seed shape Dihybrid crosses

21. Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas x YYRRyyrr P YyRr 100% F 1 generation (hybrids) yellow, round peas self-pollinate F 2 generation 9/16 yellow round peas 9:3:3:1 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas Y = yellow R = round y = green r = wrinkled

22. Dihybrid Cross How are the alleles on different chromosomes handed out?  together or separately? YyRr YRyr YyRr YryRYRyr

23. Dihybrid cross YyRr YRYryR yr YR Yr yR yr YYRRYYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr x 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

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

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

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

31. Incomplete dominance C R C W x C R C W CRCR CWCW male / sperm CRCR CWCW female / eggs CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW 25% 1:2:1 25% 50% 25% 1:2:1 % genotype % phenotype CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 25% 50%

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: WWBB WB +

34. Multiple Alleles More than two possible alleles for one trait Example: in ABO blood group, 3 possible alleles:  I A, I B, 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 30 o C

40. Environmental influence

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