Lecture 3

1. Lecture 3 MendelianGenetics

2. Phenotype and Genotype

3. Genotype and Phenotype • Genotype – genetic constitution of an organism: Total set of alleles of an individual • Describe the following : - AA: Homozygous Dominant (Wild Type) • Aa: Heterozygous • aa: Homozygous Recessive (double mutant) • Phenotype – observable characteristic • Genotype and environment • Contribution of environment varies between genes • Can be controlled by many genes • Random developmental events

5. Mendel’s Experimental Design

6. Stamen Carpel Mendelian Genetics • Modern genetics began with Gregor Mendel’s quantitative genetic experiments • Austrian monk • Mathematician • Numerical and observational data • Several generations

7. Mendelian Genetics • Heritable, obvious traits • Simple crosses at first • Used peas because: • Easy to grow and available • Many distinguishable characteristics • Self-fertilization • True breeding peas

8. Pea Traits

9. Monohybrid Crosses and Mendel’s Principle of Segregation

10. Dominant and Recessive Alleles • Pairs—the factors that control each trait exist in pairs. • Female parent—contributes one factor (♀) • Male parent—contributes one factor (♂) • Together these make a pair • Genes—are the factors that control traits. • Genes consist of pairs of alleles. One that comes from the mother parent and one that comes from the father parent.

11. Dominant and Recessive Alleles • Alleles—the different forms of a gene (such as tall or short, wrinkled or smooth). • Dominant allele—one whose trait always shows up when the allele is present. • Recessive allele—is masked (or covered up) when the dominant allele is present. Recessive alleles only show up if a dominant allele is not present.

12. dominant dominant recessive dominant recessive recessive EXAMPLES recessive dominant Recessive is the green box and dominant is the black box. Each of your parents has a pair of alleles that they can share. If they only give one… answer the following questions. ? dominant ? dominant ? recessive

13. Breeding Crosses • Initial cross is the P generation • Parents • Progeny of parents is first filial generation • F1 generation • Inbreeding of first generation creates second filial generation • F2 generation

14. Monohybrid Crosses aa AA • Cross between true-breeding individuals with one different trait • Mendel’s first crosses • Resembled only one of the parents • Planted progeny and allowed self-fertilization • Revealed both phenotypes Smooth seeds (allele A) are completely dominant to wrinkled seeds(allele a) Aa Aa Aa [S] [W]

17. GENETIC MAKEUP (ALLELES) P PLANTS AA aa Gametes All A Alla F1 PLANTS(hybrids) AllAa Gametes 1/2A 1/2a A A Eggs Sperm AA F2 PLANTS a a Aa Aa Phenotypic ratio3 purple : 1 white aa Genotypic ratio1 AA : 2 Aa : 1 aa Monohybrid Cross • Mendel determined that • Particulate factors for genes, each contains a set of two • Transmitted by both parents • Alternate forms called alleles • True breeding forms contains identical set

18. Monohybrid Cross • F1 generation had both alleles • Only one expresses • One allele masks • Dominant • Recessive • Identical alleles – homozygous • Different alleles - heterozygous

20. Monohybrid Cross

21. Principle of Segregation • Recessive characteristics are masked • Reappear in F2 • Members of a gene pair (alleles) segregated during gamete formation

22. Aa male Formation of sperm F1 genotypes Aa female Formation of eggs 1 2 1 2 a A a A A A 1 2 A 1 4 1 4 F2 genotypes 1 2 A a a a a 1 4 1 4 How cells carry characteristics ?

23. Branch Diagrams • Punnett squares can become messy with more than one gene • Use branch diagram to figure out genotype and phenotype expected frequency

24. TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING All black 1 black : 1 chocolate Test Cross • Mendel did several crosses • Followed over several generations • Selfing also very important • Allowed plants to reveal their genotype and not just their phenotye

25. Test Cross

26. Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment

27. The Principle of Independent Assortment • Factors for different traits assort independently of one another • Genes are inherited independently of each other • Segregate randomly in gametes • Dihybrid Cross

29. Branch Diagram of Dihybrid Cross Phenotype

30. Genotype vs. Phenotype

31. Test Cross With Dihybrid

32. Trihybrid Cross

33. Tribble Traits Activity

34. Statistical Analysis of Genetic Data: The Chi-Square Test

35. Probability & Genetics

36. Statistical Analysis • Data from genetics is quantitative • Use statistics to show deviation of observed results from predicted results • Chance factors cause deviations • Null-hypothesis – no difference between the predicted and observed • If not accepted then have to come up with a new hypothesis for deviation

37. Genetics & Probability • Mendel’s laws: • segregation • independent assortment reflect same laws of probability that apply to tossing coins or rolling dice

38. P P P p PP Pp Probability & genetics • Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin • probability of tossing heads? 50% • probability making a P gamete… 50% 100%

39. P p Pp Probability & genetics • Outcome of 1 toss has no impact on the outcome of the next toss • probability of tossing heads each time? • probability making a P gamete each time? 50% 50%

40. male / sperm P P PP P p P p p P p p Pp pp P female / eggs p Calculating probability sperm egg offspring Pp x Pp 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4 PP Pp 1/2 x 1/2 = 1/4 1/2 Pp pp 1/2 x 1/2 = 1/4

41. Rule of multiplication • Chance that 2 or more independent events will occur together • probability that 2 coins tossed at the same time will land heads up • probability of Pp x Pp  pp 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4

42. Calculating dihybrid probability • Rule of multiplication also applies to dihybrid crosses • heterozygous parents — YyRr • probability of producing yyrr? • probability of producing y gamete = 1/2 • probability of producing r gamete = 1/2 • probability of producing yr gamete = 1/2 x 1/2 = 1/4 • probability of producing a yyrr offspring =1/4 x 1/4 = 1/16

43. P p Pp sperm egg offspring p P Pp 1/2 x 1/2 = 1/4 1/4 1/4 + 1/2 x 1/2 = 1/4 1/2 Rule of addition • Chance that an event can occur 2 or more different ways • sum of the separate probabilities • probability of Pp x Pp Pp

44. Chi-square test • Test to see if your data supports your hypothesis • Compare “observed” vs. “expected” data • is variance from expected due to “random chance”? • is there another factor influencing data? • null hypothesis • degrees of freedom • statistical significance

45. Chi-Square Test • Goodness of fit test • How much observed number deviates from the expected number

46. Chi - Square Test

47. Mendelian Genetics in Humans

48. Pedigree Analysis • Inheritance patterns are studied using family trees • Pedigree analysis • Phenotypic records • Proband is where gene was discovered

49. Examples of Human Genetic Traits • Most genetic disorders are recessive • Due to lack of function • Homozygous recessive expression • Dominant usually selected out • Albinism

50. Characteristics of Recessive Inheritance Traits • Most have normal heterozygous parents • Heterozygotes have 3:1 ratio • When both parents have the trait then all progeny have the trait • Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs