Chapter 9: Fundamentals of Genetics

1. Chapter 9: Fundamentals of Genetics

2. Figure 14.0 Painting of Mendel

3. 9-1: Gregor Mendel • Austrian Monk • 1860’s • Used garden peas to study specific traits • Used mathematics to quantify findings

4. The Reasons for Peas: • Structure of flower • Easily self pollinate • Easy to hand fertilize • Anther – pollen (male) • Pistil – egg (female) • Presence of distinctive traits • Easily viewed traits in peas include: flower color, flower position, pea color, pod color, pod shape, plant height • No blending of traits (intermediate height or color) • Rapid reproduction cycle • About 90 days from flower to flower • About 4 generations in a year!

5. Figure 14.x1 Sweet pea flowers

6. Mendel’s Garden Peas • Observed seven characteristics of pea plants- heritable trait • Trait- genetically determined variant of a characteristic- such as yellow flower color • Pea characteristics that Mendel observed:

8. Mendel’s Methods • Observed how traits were passed by controlling how the pea plants were pollinated • Pollination- occurs when pollen grains produced in the male reproductive parts of a flower (anthers) are transferred to the female reproductive part of a flower, called the stigma • Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant • Can be prevented by removing all of the anthers from the flowers of a plant • Cross-pollination occurs between flowers of two plants • Pea plants normally reproduce by this method • Can be performed by manually transferring pollen from the flower of a second plant to the stigma of the anther-less plant

9. Figure 14.1 A genetic cross

10. Mendel’s experiments • True breeding (purebred)- for a trait; always produce offspring with that trait when they self-pollinate • Eventually Mendel obtained true-breeding plant types one for each of the 14 traits observed • Mendel cross-pollinated pairs of plants that were true breeding for contrasting traits of a single characteristic • P generation- true breeding parents • F1 generation- offspring • Allowed these to self pollinate and collected seeds • Second filial generation- F2 generation

11. Figure 14.2 Mendel tracked heritable characters for three generations

12. Mendel’s Results and Conclusions • His observations and careful records led him to hypothesize that something within the pea plants controlled the characteristics observed- called factors • Hypothesized that each trait was inherited by a separate factor

13. Recessive and Dominant traits • Whenever Mendel crossed strains, one of the P traits failed to appear in the F1 generation • In every case, that trait appeared in a ratio of 3:1 in the F2 generation • Trait appearing in F1 generation was dominant because it masked the factor for the other trait in the pair • A recessive factor allowed this trait to reappear in the F2 generation

14. Law of Segregation • Paired factors (now called alleles) separate during the formation of reproductive cells • Each reproductive cell, or gamete, receives one factor of each pair • When two gametes combine during fertilization, the offspring have two factors for each characteristic • Law of segregation- states that a pair of factors is separated during the formation of gametes

15. Figure 14.4 Mendel’s law of segregation (Layer 1)

16. Figure 14.4 Mendel’s law of segregation (Layer 2)

18. Law of Independent Assortment • Mendel crossed plants that differed in two characteristics (flower color and seed color) • Found that traits produced by dominant factors do not necessarily appear together • Concluded that factors for individual characteristics are not connected- law of independent assortment- factors separate independently of one another during the formation of gametes

19. Support for Mendel’s conclusions • Most of his findings agree with what biologists now know about molecular genetics- the study of the structure and function of chromosomes and genes • Chromosome- thread- like structure made up of DNA and proteins • Gene- segment of DNA on a chromosome that controls a particular hereditary trait

20. Because chromosomes occur in pairs, genes also occur in pairs • Each of two or more alternative forms of a gene is called an allele • Mendel’s “factors” are now called alleles • Letters are used to represent alleles • Capital letters- dominant • Lowercase letters- recessive • During meiosis, gametes receive one chromosome from each homologous pair of chromosomes • When gametes combine in fertilization, the offspring receives one allele for a given trait from each parent

21. Figure 14.3 Alleles, alternative versions of a gene

22. http://www.youtube.com/watch?v=dOs6tLAYcUQ

23. 9-2: Genetic Crosses • Genotype and Phenotype • Genotype- organism’s genetic makeup • Consists of alleles that the organism inherits from its parents • Phenotype- organism’s appearance • In addition to recessive alleles, certain environmental factors can affect phenotype • Homozygous- when both alleles of a pair are alike • Can be homozygous dominant or recessive • Heterozygous- when two alleles are different

24. Probability • Likelihood that a specific event will occur • Expressed as a decimal, percentage or a fraction • Probability = Number of times an event is expected to happen Number of times an event could happen • The results of probability are more likely to occur when there are many trials

25. Predicting results of monohybrid crosses • Monohybrid cross- a cross in which only one characteristic is tracked • Offspring are monohybrids • Punnett squares are used to predict the probably distribution of inherited traits in the offspring • Genotypic vs. phenotypic ratio

26. Figure 14.5 Genotype versus phenotype

27. Monohybrid Cross Probability • Genotype • 25% will be homozygous dominant • 50% will be heterozygous • 25% will be homozygous recessive • Phenotype • 75% will be dominant • 25% will be recessive

28. Cross between a purebred (homozygous) green and yellow pea plants Yellow Pea (YY) Y Y y Green Pea (yy) y All plants from this cross will have yellow peas!

29. Therefore: When crossing a homozygous dominant organism with a homozygous recessive organism all offspring will be heterozygous and express the dominant phenotype.

30. Cross between 2 hybrid (heterozygous) yellow pea plants“Monohybrid cross” Yellow Pea Y y Y Yellow Pea y ¾ of the plants from this cross will have yellow peas : ¼ will have green!

31. Therefore: When crossing two heterozygotes, the offspring will end up in a 3:1 phenotypic ratio or a 1:2:1 genotypic ratio.

32. What happens when . . . You cross one individual that is heterozygous with another that is homozygous recessive? (Round = R; wrinkled = r)

33. Therefore. . . You will always get 1:1 (50%) phenotypic and genotypic ratios

34. Test Cross • An individual of unknown genotype is crossed with a homozygous recessive individual • Can determine the genotype of any individual whose phenotype expresses the dominant trait

35. Figure 14.6 A testcross

36. In a test cross, if recessive traits show in the offspring then the unknown parent must be heterozygous!

37. Incomplete vs. Complete Dominance • Complete dominance- when one allele is completely dominant over another • Heterozygous plant and homozygous plants are indistinguishable in phenotype (both have PP and Pp for the color purple) • Incomplete dominance- sometimes F1 generation has a phenotype in-between that of the parents • Example: Producing pink flowers from white and red parents

38. Common in flowers and the coat color of some animals

39. Figure 14.9 Incomplete dominance in snapdragon color

40. Figure 14.9x Incomplete dominance in carnations

41. Codominance • Occurs when both alleles for a gene are expressed in a heterozygous offspring • Neither allele is dominant or recessive • Nor do the alleles blend in phenotype • Blood type • Determined by the carbohydrates that coats the cell surface • A • AB • B • O (neither)

42. Blood alleles are represented with I

43. Figure 14.10 Multiple alleles for the ABO blood groups

44. Predicting Results of Dihybrid Crosses • Dihybrid cross- cross in which two characteristics are tracked • Must consider how four alleles from each parent can combine • Measures 2 traits at a time • You can do this mathematically (multiplying probabilities) • You can do this with a Punnett Square

45. In peas, Yellow is dominant over green and Round is dominant over wrinkled. • If we crossed two dihybrid plants (RrYy) we would need a 4X4 square Punnett. • To find the gamete possibilities use “FOIL”

46. Round Yellow Peas (RrYy) dihybrids will produce a variety of gametes

48. When crossing two individuals who are heterozygous for both traits, the probable phenotype outcome of a dihybrid cross will be 9:3:3:1 • 9/16 will be round & yellow (both dominant) • 3/16 will be round and green (one dominant, one recessive) • 3/16 will be wrinkled & yellow (one recessive, one dominant) • 1/16 will be wrinkled and green (both recessive)

49. Hidden recessives!

50. Pedigree Charts • Show how a trait is inherited in a family • Used to study traits in humans as well as other organisms • Can reveal the presence of carriers for traits • People who carry the (recessive) gene but do not show the trait