Mendel and the Gene Idea

1. Mendel and the Gene Idea

2. Inheritance • The passing of traits from parents to offspring. • Humans have known about inheritance for thousands of years.

3. Genetics • The scientific study of the inheritance. • Genetics is a relatively “new” science (about 150 years).

4. Genetic Theories 1. Blending Theory - traits were like paints and mixed evenly from both parents. 2. Incubation Theory - only one parent controlled the traits of the children. Ex: Spermists and Ovists

5. 3. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring.

6. Gregor Mendel • Father of Modern Genetics.

7. Mendel’s paper published in 1866, but was not recognized by Science until the early 1900’s.

8. Reasons for Mendel's Success • Used an experimental approach. • Applied mathematics to the study of natural phenomena. • Kept good records.

9. Mendel was a pea picker. He used peas as his study organism.

10. Why Use Peas? • Short life span. • Bisexual. • Many traits known. • Cross- and self-pollinating. • (You can eat the failures).

11. Cross-pollination • Two parents. • Results in hybrid offspring where the offspring may be different than the parents.

13. Self-pollination • One flower as both parents. • Natural event in peas. • Results in pure-bred offspring where the offspring are identical to the parents.

14. Mendel's Work • Used seven characters, each with two expressions or traits. • Example: • Character - height • Traits - tall or short.

15. Monohybrid or Mendelian Crosses • Crosses that work with a single character at a time. Example - Tall X short

16. P Generation • The Parental generation or the first two individuals used in a cross. Example - Tall X short • Mendel used reciprocal crosses, where the parents alternated for the trait.

17. Offspring • F1 - first filial generation. • F2 - second filial generation, bred by crossing two F1 plants together or allowing a F1 to self-pollinate.

21. Another Sample Cross P1 Tall X short (TT x tt) F1 all Tall (Tt) F2 3 tall to 1 short (1 TT: 2 Tt: 1 tt)

22. Results - Summary • In all crosses, the F1 generation showed only one of the traits regardless of which wasmaleorfemale. • The other trait reappeared in the F2 at ~25% (3:1 ratio).

23. Mendel's Hypothesis 1. Genes can have alternate versions called alleles. 2. Each offspring inherits two alleles, one from each parent.

24. Mendel's Hypothesis 3. If the two alleles differ, the dominant allele is expressed. The recessive allele remains hidden unless the dominant allele is absent. Comment - do not use the terms “strongest” to describe the dominant allele.

25. Mendel's Hypothesis 4. The two alleles for each trait separate during gamete formation. This now called: Mendel's Law of Segregation

26. Law of Segregation

27. Mendel’s Experiments • Showed that the Particulate Model best fit the results.

28. Vocabulary • Phenotype - the physical appearance of the organism. • Genotype - the genetic makeup of the organism, usually shown in a code. • T = tall • t = short

30. Helpful Vocabulary • Homozygous - When the two alleles are the same (TT/tt). • Heterozygous- When the two alleles are different (Tt).

31. 6 Mendelian Crosses are Possible CrossGenotypePhenotype TT X tt all Tt all Dom Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res TT X TT all TT all Dom tt X tt all tt all Res TT X Tt 1TT:1Tt all Dom Tt X tt 1Tt:1tt 1 Dom: 1 Res

32. Test Cross • Cross of a suspected heterozygote with a homozygous recessive. • Ex: T_ X tt If TT - all dominant If Tt - 1 Dominant: 1 Recessive

34. Dihybrid Cross • Cross with two genetic traits. • Need 4 letters to code for the cross. • Ex: TtRr • Each Gamete - Must get 1 letter for each trait. • Ex. TR, Tr, etc.

35. Number of Kinds of Gametes • Critical to calculating the results of higher level crosses. • Look for the number of heterozygous traits.

36. Equation The formula 2n can be used, where “n” = the number of heterozygous traits. Ex: TtRr, n=2 22 or 4 different kinds of gametes are possible. TR, tR, Tr, tr

37. Dihybrid Cross TtRr X TtRr Each parent can produce 4 types of gametes. TR, Tr, tR, tr Cross is a 4 X 4 with 16 possible offspring.

38. Results • 9 Tall, Red flowered • 3 Tall, white flowered • 3 short, Red flowered • 1 short, white flowered Or: 9:3:3:1

39. Law of Independent Assortment • The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait. • Inheritance of height is independent of the inheritance of flower color.

41. Comment • Ratio of Tall to short is 3:1 • Ratio of Red to white is 3:1 • The cross is really a product of the ratio of each trait multiplied together. (3:1) X (3:1)

42. Probability • Genetics is a specific application of the rules of probability. • Probability - the chance that an event will occur out of the total number of possible events.

44. Genetic Ratios • The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization. Ex: 3:1 75% chance of the dominant 25% chance of the recessive

45. Rule of Multiplication • The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities.

46. Example: TtRr X TtRr • The probability of getting a tall offspring is ¾. • The probability of getting a red offspring is ¾. • The probability of getting a tall red offspring is ¾ x ¾ = 9/16

47. Comment • Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares. • Ex: TtrrGG X TtRrgg

48. Solution “T’s” = Tt X Tt = 3:1 “R’s” = rr X Rr = 1:1 “G’s” = GG x gg = 1:0 Product is: (3:1) X (1:1) X (1:0 ) = 3:3:1:1

49. Tips for Dihybrid Problems • Identify all of the alleles that can be identified from the phenotypes of the parents or kids. • Work from the monohybrid ratios to solve for the missing alleles.

50. Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance