Fundamentals of Genetics

1. Chapter 9 Fundamentals of Genetics

2. Mendel observed seven characteristics of pea plants • Ex: flower color • Each characteristic occurred in two contrasting traits • Trait: genetically determined variant of a characteristic • Ex: yellow flower color Mendel’s Legacy

3. The seven characteristics: • Plant height (long and short) • Flower position along stem (axial and terminal) • Pod color (green and yellow) • Pod appearance (inflated and constricted) • Seed texture (round and wrinkled) • Seed color (yellow and green0 • Flower color (purple and white) Mendel’s Legacy

4. He initially studied each characteristic and its contrasting traits individually • Began growing true-breeding plants • True-breeding: pure; always produced offspring with that trait when they self-pollinate • Produced true-breeding plants by self-pollinating pea plants until he had 14 Mendel’s Experiments

5. He then cross-pollinated pairs of plants that were true-breeding for contrasting traits ofa single characteristics • Ex: he crossed a plant with purple flowers and a plant with white flowers • This was called the P (parent) generation Mendel’s Experiments

6. When the plants matured, he recorded the number of each type of offspring produced by each cross • Called the offspring the F1 generation Mendel’s Experiments

7. He then allowed the F1 generation to self-pollinate, and the next offspring generation was called the F2 generation • He performed hundreds of crosses and documented every result Mendel’s Experiments

10. In one of his experiments, Mendel crossed a plant true-breeding for green pods with a plant that was true-breeding for yellow pods • The F1 generation had all green pods • He then let the F1 generation self-pollinate • The F2 generation had ¾ green pods and ¼ yellow pods Mendel’s Results and Conclusions

11. These results made Mendel believe that each characteristic is controlled by factors • A pair of factors must control each trait Mendel’s Results and Conclusions

12. Mendel got these results through the thousand of crosses • F1 generation: one trait disappeared • F2 generation: trait reappeared in a 3:1 ratio Mendel’s Results and Conclusions

13. Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked the other trait • He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor Mendel’s Results and Conclusions

14. Most of Mendel’s findings agree with what biologists now know about molecular genetics • Molecular genetics: the study of the structure and function of chromosomes and genes Support for Mendel’s Conclusions

15. Allele: each of two or more alternative forms of a gene • Mendel’s factors are now called alleles • Letters are used to represent alleles • Dominant alleles: represented by a capital letter • Recessive alleles: represented by a lowercase letter Support for Mendel’s Conclusions

16. Genotype: an organism’s genetic makeup • Consists of the alleles that the organism inherits from its parents • Ex: flower color • Purple flowers: either PP or Pp • White flowers: pp • P is the dominant allele • P is the recessive allele Genotype and Phenotype

17. Phenotype: an organism’s appearance • Since PP and Pp are dominant genotypes, they will have purple flowers • Since pp is a recessive genotype, they will have the recessive phenotype, which is white flowers Genotype and Phenotype

18. Homozygous: when both alleles of a pair are alike • An organism may be homozygous dominant or homozygous recessive • Ex: PP or pp • Heterozygous: when the two alleles in the pair are different • Ex: Pp Genotype and Phenotype

19. Probability: the likelihood that a specific event will occur • May be expressed as a decimal, a percentage, or a fraction • Determined by the following equation: P= # of times an event is expected to happen # of times an event could happen Probability

20. For example, the dominant trait of yellow seed color appeared in the F2 generation 6,022 times • The recessive trait of green seed color appeared 2,001 times • The total number of individuals was 8,023 Probability

21. Probability that the dominant trait will appear: 6,022 ------------ = 0.75 or 75% or ¾ or 3:1 8,023 Probability

22. The results predicted by probability are more likely to occur when there are many traits Probability

23. Probability that the recessive trait will appear: 2,001 ----------- = 0.25 or 25% or ¼ or 1:3 8,023 Probability

24. Monohybrid cross: a cross in which only one characteristic is tracked • The offspring are called monohybrids • Biologists use a Punnett square to aid them in predicting the probable distribution of inherited traits in the offspring Predicting Results of Monohybrid Crosses

25. Example 1: Homozygous x homozygous • PP and pp • All offspring are Pp • 100% probability that the offspring will have the genotype Pp and thus the phenotype purple flower color

26. Example 2: Homozygous and Heterozygous • BB and Bb • Offspring are BB and Bb • The probability of an offspring having BB genotype is 2/4 or 50% • The probability of an offspring having Bb genotype is 2/4 or 50% • The probability of an offspring have the dominant black coat is 4/4 or 100%

27. Example 3: Heterozygous x Heterozygous • Bb and Bb • The probability of an offspring having a BB genotype is ¼ or 25% • The probability of an offspring having a Bb genotype is 2/4 or 50% • The probability of an offspring having a bb genotype is ¼ or 25%

28. ¾ or 75% of the offspring resulting from this cross are predicted to have a black coat • ¼ or 25% of the offspring are predicted to have a brown coat (recessive phenotype) Example 3: Heterozygous x Heterozygous

29. Genotypic ratio: the ratio of the genotypes that appear in offspring • The probable genotypic ratio of the monohybrid cross represented is 1BB: 2 Bb: 1 bb • Phenotypic ratio: the ratio of the offspring’s phenotypes • The probable phenotypic ratio of the cross is 3 black : 1 brown Example 3: Heterozygous x Heterozygous

30. In guinea pigs, both BB and Bb result in a black coat • How would you determine whether a black guinea pig is homozygous (BB) or heterozygous (Bb)? • Perform a testcross • Testcross: 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 Example 4: Testcross

31. Example 4: Testcross • If the black guinea pig of unknown genotype is homozygous black, all offspring will be black • If the individual with the unknown genotype is heterozygous black, about half will be black

32. In Mendel’s pea-plant crosses, one allele was completely dominant over another • Called complete dominance • In complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype • Ex: PP and Pp produce purple flowered plants Example 5: Incomplete Dominance

33. Sometimes, the F1 offspring will have a phenotype in between that of the parents • Called incomplete dominance • Incomplete dominance occurs when the phenotype of a heterozygote is between the phenotypes determined by the dominant and recessive traits Example 5: Incomplete Dominance

34. Example 5: Incomplete Dominance • Both the allele for red flowers (R) and the allele for white flowers (r) influence the phenotype • Neither allele is completely dominant • When red flowers are crossed with white flowers, all of the F1 offspring have pink flowers

35. 100% of the offspring have the Rr genotype • The probable genotypic ratio is 1RR: 2 Rr: 1 rr • Since neither allele is completely dominant, the phenotypic ratio is 1 red: 2 pink: 1 white Example 5: Incomplete Dominance

36. 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 the phenotype • Example: blood types • Determined by two alleles Example 6: Codominance

37. Dihybrid cross: a cross in which two characteristics are tracked • The offspring are called dihybrids Predicting Results of Dihybrid Crosses

38. Ex: Predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds • Round seeds (R) is dominant over wrinkled seeds (r) • Yellow seeds (Y) is dominant over green seeds (y) Homozygous X Homozygous

39. Homozygous X Homozygous • The Punnett Square used to predict the results of a cross between a parent of the genotype RRYY and a parent of the genotype rryy will contain 16 boxes • Alleles are carried by the male and female gametes

40. The genotype of all of the offspring of this cross will be heterozygous for both traits: RrYy • All have round, yellow seed phenotypes Homozygous X Homozygous

41. Heterozygous X Heterozygous • Cross two pea plants heterozygous for round, yellow seeds • Offspring are likely to have nine different genotypes

42. These 9 genotypes will result in pea plants that have the following four phenotypes: • 9/16 that have round, yellow seeds (genotypes RRYY, RRYy, RrYY, and RrYy) • 3/16 that have round, green seeds (genotypes Rryy and Rryy) • 3/16 that have wrinkled, yellow seeds (genotypes rrYY and rrYy) • 1/16 that have wrinkled, green seeds (genotype rryy) Heterozygous X Heterozygous