Mendel and the Gene Idea

1. Mendel and the Gene Idea

2. Modern genetics began in an abbey garden, where a monk named Gregor Mendel documented the particulate mechanism of inheritance

3. Pea plants have several advantages for genetics • pea plants are available in many varieties with distinct heritable features (characters) with different variants (traits)

4. another advantage of peas is that Mendel had strict control over which plants mated with which Each pea plant has male (stamens) and female (carpal) sexual organs

6. in nature, pea plants typically self-fertilize, fertilizing ova with their own sperm • however, Mendel could also move pollen from one plant to another to cross-pollinate plants

7. In a typical breeding experiment, Mendel would cross-pollinate (hybridize) two contrasting, true-breeding pea varieties

8. The true-breeding parents are the P generation and their hybrid offspring are the F1 generation

9. Mendel would then allow the F1 hybrids to self-pollinate to produce an F2 generation

10. it was mainly Mendel’s quantitative analysis of F2 plants that revealed the two fundamental principles of heredity: the law of segregation and the law of independent assortment

11. In the law of segregation, the two alleles for a character are packaged into separate gametes For each character, an organism inherits two alleles, one from each parent

12. If two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance The other, the recessive allele, has no noticeable effect on the organism’s appearance

13. A Punnett square predicts the results of a genetic cross between individuals of known genotype

14. An organism with two identical alleles for a character is homozygous for that character (pure) TT or tt

15. Organisms with two different alleles for a character is heterozygous for that character (hybrid) Tt

16. A description of an organism’s traits is its phenotype (see) A description of its genetic makeup is its genotype (letters) • two organisms can have the same phenotype but have different genotypes if one is homozygous dominant and the other is heterozygous

17. It is not possible to predict the genotype of an organism with a dominant phenotype • the organism must have one dominant allele, but it could be homozygous dominant or heterozygous

18. A testcross, breeding a homozygous recessive with dominant phenotype, but unknown genotype, can determine the identity of the unknown allele

21. In the law of independent assortment, each pair of alleles segregates into gametes independently

22. Mendelian inheritance reflects rules of probability • Mendel’s laws of segregation and independent assortment reflect the same laws of probability that apply to tossing coins or rolling dice

23. the probability scale ranged from zero (an event with no chance of occurring) to one (an event that is certain to occur) the probability of tossing heads with a normal coin is ½

24. the probability of rolling a 3 with a six-sided die is 1/6, and the probability of rolling any other number is 1 - 1/6 = 5/6 • when tossing a coin, the outcome of one toss has no impact on the outcome of the next toss

25. each toss is an independent event, just like the distribution of alleles into gametes

27. We can use the rule of multiplication to determine the chance that two or more independent events will occur together in some specific combination • compute the probability of each independent event

28. then, multiply the individual probabilities to obtain the overall probability of these events occurring together • the probability that two coins tossed at the same time will land heads up is 1/2 x 1/2 = 1/4

29. The rule of multiplication also applies to dihybrid crosses • for a heterozygous parent (BbRr) the probability of producing a BR gamete is 1/2 x 1/2 = 1/4

30. We can use this to predict the probability of a particular F2 genotype without constructing a 16-part Punnett square • the probability that an F2 plant will have a BBRR genotype from a heterozygous parent is 1/16 (1/4 chance for a BR ovum and 1/4 chance for a BR sperm)

31. The rule of addition also applies to genetic problems • under the rule of addition, the probability of an event that can occur two or more different ways is the sum of the separate probabilities of those ways

32. For example, there are two ways that F1 gametes can combine to form a heterozygote • the dominant allele could come from the sperm and the recessive from the ovum (probability = 1/4)

33. or, the dominant allele could come from the ovum and the recessive from the sperm (probability = 1/4) • the probability of a heterozygote is 1/4 + 1/4 = 1/2

34. We can combine the rules of multiplication and addition to solve complex problems in Mendelian genetics

35. Let’s determine the probability of finding two recessive phenotypes for at least two of three traits resulting from a trihybrid cross between pea plants that are AaBbRr and Aabbrr

36. there are five possible genotypes that fulfill this condition: aabbRr, aaBbrr, Aabbrr, AAbbrr, and aabbrr

37. we would use the rule of multiplication to calculate the probability for each of these genotypes and then use the rule of addition to pool the probabilities for fulfilling the condition of at least two recessive traits

38. The probability of producing a aabbRr offspring: The probability of producing aa = 1/2 x 1/2 = 1/4 The probability of producing bb = 1/2 x 1 = 1/2 The probability of producing Rr = 1/2 x 1 = 1/2

39. Therefore, the probability of all three being present (aabbRr) in one offspring is 1/4 x 1/2 x 1/2 = 1/16 For aaBbrr: 1/4 x 1/2 x 1/2 = 1/16 For Aabbrr: 1/2 x 1/2 x 1/2 = 2/16 For AAbbrr: 1/4 x 1/2 x 1/2 = 1/16 For aabbrr: 1/4 x 1/2 x 1/2 = 1/16

40. Therefore, the chance of at least two recessive traits is 6/16 • while we cannot predict with certainty the genotype or phenotype of any particular seed from the F2 generation of a dihybrid cross, we can predict the probabilities that it will fit a specific genotype of phenotype

41. Extending Mendelian Genetics Some alleles show incomplete dominance where heterozygotes show a distinct intermediate phenotype, not seen in homozygotes

42. this is not blended inheritance because the traits are separable (particulate) as seen in further crosses • offspring of a cross between heterozygotes will show three phenotypes: both parentals and the heterozygote

43. the phenotypic and genotypic ratios are identical, 1:2:1

44. A clear example of incomplete dominance is seen in flower color of snapdragons A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring

45. self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring

46. Incomplete and complete dominance are part of a spectrum of relationships among alleles • at the other extreme from complete dominance is codominance in which two alleles affect the phenotype in separate, distinguishable ways

47. Because an allele is dominant does not necessarily mean that it is more common in a population than the recessive allele

48. For example, polydactyly, in which individuals are born with extra fingers or toes, is due to an allele dominant to the recessive allele for five digits per appendage

49. however, the recessive allele is far more prevalent than the dominant allele in the population • 399 individuals out of 400 have five digits per appendage