Introduction to Mendelian Genetics

1. Introduction to Mendelian Genetics

2. The Work of Gregor Mendel A. Genetics is the scientific study of heredity. Every living thing has a set of characteristics inherited from its parent or parents!

3. B. Gregor Mendel • Who & Where: Austrian monk, “Father of Genetics”, born 1822 • What: His work was important to the understanding of heredity. In charge of monastery’s garden; studied traits of pea plants

4. C. So Why Peas? • Pea plant flowers are closed • Self-fertilizing • True-breeding • Have 7 easily visible traits called phenotypes

6. Every time Mendel crossed 2 different traits, only ONE was seen in the offspring!

7. C. Mendel’s Principles Principle of Dominance Principle of Segregation Principle of Independent Assortment

8. Back to Mendel’s Experiments…

9. The Next Generation If these hybrids self-pollinate.. The hidden trait returns! F1 generation: Tt x Tt Result?

10. What happened to Recessive Allele? • Plants with different genotypes • (TTand Tt) • can have the same phenotype (“tall”).

11. Genotype: genetic makeup of organism • Phenotype: physical characteristics of an organism Genotype Phenotype C c T T P p

12. D. Principle of Dominance Definition: some alleles are dominant and others are recessive. The dominant gene shows up in the phenotype when present. Example: Smooth peas S wrinkled peas s

13. E. Principle of Segregation • During sex cell formation, alleles separates from each other • Each gamete has one allele for each trait

14. Each trait is controlled by a gene that is in two contrasting forms • The different forms of a gene are called alleles.

15. Homozygous: two identical alleles Example: TT or tt or SS or ss Heterozygous: two different alleles for the same trait Example: Tt or Ss

16. II. Probability and Punnett Squares Why used? Punnett squares used to predict and compare genetic variations that will result from a cross • The Rules: • Dominant traits are • the 1st letter and • CAPITAL • Recessive are the 2nd • letter and lowercase

17. B. Practice 1) If a homozygous tall person was crossed with a homozygous short person, what are probably offspring? T T Tall= T short= t t T t T t t T t T t

18. Punnett Square 2) Cross two heterozygous tall parents. Tall= T short= t T t What is the genotype ratio? T T T T t 1 TT: 2 Tt: 1 tt What is the phenotype ratio? T t t t t 3 Tall: 1 short

19. Punnett Square 3) The long-eared allele (L) is dominant to the short-eared allele (l). Cross a homozygous long ear with a homozygous short-ear. Cross the F1 generation and give the F2 results.

20. III. Independent Assortment • Mendel discovered that genes for different traits segregate independently during gamete formation Ex. Wrinkled/Smooth and Yellow/Green peas

21. B. Dihybrid Cross: two traitsbeing crossed at the same time Here is a heterozygous tall, heterozygous purple plant: T t P p

22. How can we figure out the alleles present in the gametes? F O I L First Outer Inner Last Using F.O.I.L., list the possible gametes: TPTptPtp

23. Tall and Purple are dominant to short and white. Example: Cross two heterozygous tall, heterozygous purplepea plants. T t P p x T t P p There are four possible gametes each parent can make..

24. MOTHER t T P T p P t p TTPp TtPP T P TTPP TtPp TtPp TTPp TTpp Ttpp T p FATHER t P TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp t p

25. Results in fractions? Phenotypes: Tall, Purple? Tall, White? Short, Purple? Short, White? Ratio from heterozygous dihybrid cross is ALWAYS 9: 3: 3: 1 Alleles assort independently. 9/16 3/16 3/16 1/16

26. Use FOIL to set up these examples: FfPp: SSTt: DdRR: FP Fp fP fp ST ST St St DR DR dR dR

27. IV. Beyond Pure Dominance…. Some alleles are not simply dominant or recessive.. A. Incomplete dominance: Alleles are expressed as a blend. Each allele has a capital letter. Red= R Yellow= Y

28. 1. Cross a red flower with a white flower, showing incomplete dominance. Red=R White=W R R R W W R W R W R W W Genotype: 100% RW Phenotype: PINK!

31. D. Co-dominance • Both traits dominate, seen separately! Red Horse White Horse

32. Give you ROAN!

33. 1. Example of Codominant Problem Red feathers are codominant to white feathers in chickens. CR= red CW= white Cross a homozygous Red with a homozygous white feathered chicken. CR CR GENOTYPE: 100% CW CR CW CR CW CR CW PHENOTYPE: 100% Red and white mixed feathers CW CR CW CR CW

34. C. Multiple Alleles • One trait, many allele options! • But remember: an individual cannot inherit more than two actual alleles, even if more than two possible alleles exist. Example: Blood type A, B, AB, O!

35. Blood Type Problem I • Cross a homozygous Type A with a heterozygous Type B. What are the possible phenotypes of offspring? IA IA IB IA IB Phenotypes: 50% Type AB 50% Type A IA i i

36. Blood Type Problem II • Cross a heterozygous Type A man with a heterozygous Type B woman. Is it possible for them to have an O child? i IA Phenotypes: 25% Type AB 25% Type A 25% Type B 25% Type O IB IA IB IB i IA i i i i

37. Blood Type Problem III • Cross a heterozygous Rh+ man with a Rh- woman. What are the possible phenotypes of offspring? Rh- Rh+ Phenotypes: 50% Type + 50% Type - Rh- Rh+ Rh- Rh- Rh- Rh+ Rh- Rh- Rh- Rh-

38. Rabbits have 4 basiccolors (alleles!) • brown • chinchilla or grey • It is recessive to brown. • himalayan or white with black tips. • It is recessive to both brown and chinchilla. • albino • It is recessive to all.

39. Chinchilla Full color AIbino Himalayan

40. D. Polygenic Traits • Traits produced by many genes with many alleles • Most human traits are polygenic • Most variety of expression • There are 3 genes that contribute to skin color.. And many alleles for each gene!

42. More examples: • Height • Weight • Intelligence • Eye color

43. V. Sex Determination In humans, the X and Y chromosomes control the sex of offspring. Outcome is always 50% chance of a male, and 50% chance of a female

44. Sex-linked traits • Traits controlled by genes on the sex chromosomes are called sex-linked. • Alleles for sex-linked traits are written assuperscriptson the X chromosomes only. Example: Red eyes in fruit flies found in females Males tend to have white eyes, which is recessive. XR XR Xr y

45. X and Y sex chromosomes are non-homologous • Any allele on the X chromosome will NOT be masked by a matching allele on the Y chromosome.

46. Why are sex-linked disorders more common in males than in females? • Males have just one X chromosome containing an allele. So all X-linked alleles are automatically expressed in males, even if they are recessive.

47. C. Examples of Sex-Linked • Color blindness • Duchenne Muscular Dystrophy • Hemophilia

48. Frank and Awilda at Breakfast • Frank: Are you sure you want to wear that new shirt to work today? A green and red shirt like that would be better for Christmas, not for St. Patrick's Day. • Awilda: Oh no! Not again! I hate being color blind! I really thought this shirt was just different shades of green. Where's the red? At Dinner That Night • Awilda: We should try to find a way to make sure we only have sons, no daughters. I don't want to have any daughters who might be color blind and have so many problems like I do. Color blindness wouldn't matter so much for a boy.

49. Frank: Remember, the doctor said that, since I'm not color blind, none of our daughters would be color blind, only our sons. • Awilda: That doesn't make any sense. Our daughters should be color blind like me and our sons should be normal like you. • Frank: No, the doctor said the gene for color blindness is on the X chromosome, so only our sons will inherit your colorblindness. • Awilda: I don't agree. Girls have more X chromosomes than boys, so girls should be more likely to be color blind.

50. Help Frank to explain to Awilda why the doctor was right by answering the following questions. 1. What are the genotypes of Awilda and Frank? (Since the allele for color blindness is recessive and located on the X chromosome, use the symbol Xc for an X chromosome with the allele for color blindness and XC for an X chromosome with the normal allele.) Awilda: Frank: Xc Xc XC y