Mendel and the Gene Idea

1. Mendel and the Gene Idea

2. Two possible explanations existed for how traits are passed from one generation to the next. • Blending hypothesis • Genes from parents blend to make a new set of characteristics

3. Two possible explanations existed for how traits are passed from one generation to the next. • Particulate hypothesis • Genes from parents are discrete units with fixed values for the trait

4. Mendel used the scientific approach to determine how traits are inherited and to identify two laws of inheritance.

5. What did Mendel find? • Mendel determined that inheritance from the parental generation to the filial generation is particulate. • Mendel used peas to test his ideas. Not PEAS again! Can’t you study something else for a while?!

6. Mendel used 7 traits in his initial testing.

7. Peas usually self-pollinate, but Mendel controlled all of the reproduction. By crossing (mating) two true breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers.

8. Naming the Generations P refers to the parent (true-breeding) organisms F1 refers to the first generation of offspring from the P generation F2 refers to the generation of offspring produced by breeding two F1 organisms

9. Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

10. Mendel’s Laws are based on the evidence from his experiments. • Mendel reasoned that • In the F1 plants, only the purple flower factor was affecting flower color in these hybrids • Purple flower color was dominant, and white flower color was recessive

11. Mendel related four concepts in creating his model that explained the 3:1 ratio he found in the F2 generations.

12. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers First, alternative values of genes(alleles) account for variations in inherited characters

13. Second, for each characteran organism inherits two alleles, one from each parent

14. Third, if the two alleles at a locus differthen one, the dominant allele, determines the organism’s appearance • The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

15. Fourth the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes (Meiosis Happens!)

16. Mendel’s Laws • Law of Dominance • If present, a dominant allele will be represented. • Law of Segregation • Alleles segregate during gamete formation. • Law of Independent Assortment • Chromosomes line up randomly and independently in metaphase I • This law came later!

18. A phenotype is a description of the appearance of a gene while a genotype is the genetic code for atrait.

19. Terminology • Homozygous- having identical alleles for a trait • Aka: pure or true breeding • Heterozygous- having different alleles for a trait • Aka: hybrid

20. Using a test cross will allow you to figure out if an organism that displays the dominant trait is heterozygous or homozygous. • By mating the unknown organism with an individual that has the recessive trait you can determine the genotype of the original organism.

21. If even one of the offspring displays the recessive trait, the original organism can be determined to be heterozygous.

22. A dihybrid cross allows us to determine that genes are inherited independently.

23. The laws of probability govern Mendelian inheritance.

24. The multiplication and addition rules can be applied to monohybrid crosses • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities • P(A and B) = P(A)∙P(B)

25. P(A and B) = P(A)∙P(B) • Probability of getting a heart = 1 in 4 • Probability of getting a king = 1 in 13 • Probability of getting a king of hearts = 1 in 52

26. Probability in a monohybrid cross can be determined using this rule

27. The multiplication and addition rules can be applied to monohybrid crosses • The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities • P(A or B)= P(A) + P(B)

28. P(A or B)= P(A) + P(B) • Probability of drawing a heart or a spade from a deck of cards. • Probability of drawing a heart = 1 in 4 • Probability of drawing a spade = ¼

29. Solving Complex Genetics Problems with the Rules of Probability • We can apply the rules of probability • To predict the outcome of crosses involving multiple characters

30. Combining the rules works as well. • What is the probability of drawing a king of spades or a king of hearts? • P(K♠ or K♥) = P(K)∙P(♠) + P(K)∙P(♥)

31. What is the probability of drawing a king of hearts AND a king of spades. • To make the problem easier, lets say we are using separate decks! • P(K♥ and K♠)=

32. Challenge Problem: What is the probability of getting a royal flush from a single deck of cards?

33. You can look at the outcomes of multiple genes at the same time using a larger Punnett Square • A dihybrid or other multicharacter cross • Is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes from such crosses • Each character first is considered separately and then the individual probabilities are multiplied together

34. The Math: x=2n • Possible number of different gametes (x) • n is the number of heterozygous traits in the parent • Possible number of different genotypes (x) • n is the number of dominant traits in the individual

35. How many different gametes can be made from the genotype? • AaBbCCDdEEffGg • There are 4 heterozygous traits, so there are 24 or 16 possible gametes

36. How many possible genotypes are possible for the phenotype? • Purple flowered, round, green peas with yellow inflated pods and tall axial flowers. • There are 5 dominant traits so n=5 • 25=32 different possible genotypes for the phenotype

37. Frequency of Dominant Alleles • Dominant alleles • Are not necessarily more common in populations than recessive alleles

38. Inheritance patterns are often more complex than predicted by simple Mendelian genetics. The relationship between genotype and phenotype is rarely simple.

39. The inheritance of characters by a single gene may deviate from simple Mendelian patterns • Multiple types of dominance • Linkage • Multiple alleles • Pleitropy • Sex linkage • Epistasis • Polygenism

40. The Spectrum of Dominance • Complete dominance • Occurs when the phenotypes of the heterozygote and dominant homozygote are identical • In codominance • Two dominant alleles affect the phenotype in separate, distinguishable ways • In incomplete dominance • The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

41. Complete dominance • Occurs when the phenotypes of the heterozygote and dominant homozygote are identical

42. Codominance occurs • When two dominant alleles affect the phenotype in separate, distinguishable ways

43. Incomplete dominance occurs • When the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

44. Mutation

45. P Generation White CWCW Red CRCR x CR CW Gametes Pink CRCW F1 Generation Gametes CR CR Sperm CR CR Eggs F2 Generation CR CR CR CR CW Cw CW CW CR CW

46. Multiple Alleles • Most genes exist in populations in more than two allelic forms

47. The ABO blood group in humans • Is determined by multiple alleles

48. Pleiotropy • In pleiotropy • A gene has multiple phenotypic effects

50. Some traits may be determined by two or more genes