Chapter 14 Mendel and the Gene Idea

1. Chapter 14 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. Gregor Mendel • Father of Modern Genetics.

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

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

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

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

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

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

12. 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.

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

17. 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)

18. 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).

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

20. 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.

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

22. Law of Segregation

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

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

26. 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

27. 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

29. 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.

30. 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.

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

32. 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.

34. 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)

35. 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.

37. 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

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

39. 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

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

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

42. Incomplete Dominance • When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents.(blending) Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white

44. Result • No hidden Recessive. • 3 phenotypes and 3 genotypes • Red = CR CR • Pink = CRCW • White = CWCW

45. Another example

46. Codominance • Both alleles are expressed equally in the phenotype. • Ex. MN blood group • MM • MN • NN

47. Result • No hidden Recessive. • 3 phenotypes and 3 genotypes

48. Multiple Alleles • When there are more than 2 alleles for a trait. • Ex. ABO blood group • IA - A type antigen • IB - B type antigen • i - no antigen

49. Result • Multiple genotypes and phenotypes. • Very common event in many traits.

50. Alleles and Blood Types TypeGenotypes A IA IA or IAi B IB IB or IBi AB IAIB O ii