CHAPTER 14

1. CHAPTER 14 INTRODUCTION TO GENETICS

2. MODELS OF HEREDITY 1. BLENDING MODEL - genetic material contributed by the two parents mixes - over many generations, a freely mating population will give rise to a uniform population of individuals - everyday observation contradicts this model - does not explain why traits sometimes skip generations

3. 2. Particulate model (the gene idea) - parents pass on discrete heritable units (genes) that retain their separate identities in offspring Modern genetics began with the work of Gregor Mendel, who documented this particulate model of inheritance

4. Figure 14.0x Mendel

5. Figure 14.0 Painting of Mendel

6. GREGOR MENDEL Mendel begin breeding garden peas around 1857 to study inheritance - there was a long tradition of breeding plants at the monastery where he lived - he probably chose to work with peas because there are many varieties CHARACTER- a heritable feature that varies among individuals, such as flower color

7. TRAIT- a variant for a character, such as purple or white flowers By using peas, Mendel was also able to control which plants mated - each pea flower has both male (stamens) and female (carpel) parts - these plants usually self-fertilize

8. Mendel cross-pollinated the plants: - he removed immature stamens from a plant before they produced pollen - he then dusted the carpel of with pollen from another flower

9. Figure 14.1 A genetic cross

10. Mendel began all of his experiments with TRUE-BREEDING - when the plants self-pollinate, all offspring are of the same variety Mendel crossed 2 true-breeding varieties (example: white vs. purple flowers) - the parents are called theP1 generation

11. - their hybrid offspring are called the F1 generation - allowing the F1 generation to self-pollinate produced the F2 generation

12. RESULTS OF THE EXPERIMENTS When Mendel crossed pure purple and pure white flowered plants, he got all purple flowers in the F1 generation - if the blending model had been correct, these flowers should have been pale purple What happened to the white? - when Mendel allowed F1 plants to self-pollinate, white flowers reappeared

13. Figure 14.2 Mendel tracked heritable characters for three generations

14. - his results worked out to about 3 purple to 1 white flower Mendel reasoned that the factor for white flowers did not disappear in the F1 plants, but only the purple-flower factor was affecting flower color in this generation - Mendel called the purple color a DOMINANT TRAIT and the white color a RECESSIVE TRAIT

15. Mendel observed other characters with the same results: Flower position- axial or terminal Seed color- yellow or green Seed shape- round or wrinkled Pod shape- inflated or constricted Pod color- green or yellow Stem length- tall or dwarf

16. Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

17. LAW OF SEGREGATION Mendel developed a hypothesis to explain his results that can be broken down into 4 parts: 1. Alternative versions of genes account for variations in inherited characters - ALLELES- alternative versions of a gene Ex: Gene is flower color, alleles are purple and white

18. - the purple allele and the white allele are 2 DNA variations possible at the flower color locus on one of a pea plant’s chromosomes

19. Figure 14.3 Alleles, alternative versions of a gene

20. 2. For each character, an organism inherits two alleles, one from each parent - RECALL: a diploid organism has homologous pairs of chromosomes, one from each parent

21. 3. If the 2 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 4. The two alleles for each character segregate (separate) during gamete production

22. - an ovum and a sperm each get only one of the two alleles that are present in the somatic cells of the organism - this is where the name of the law, the LAW OF SEGREGATION, comes from PUNNETT SQUARE- a diagram used to predict the results of a genetic cross

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

24. SOME IMPORTANT VOCABULARY HOMOZYGOUS- an organism having a pair of identical alleles for a character Ex: a pea plant that is true-breeding for purple flowers (PP) - can also be for white flowers (pp) HETEROZYGOUS- an organism having 2 different alleles for a gene - for the flowers, would be Pp- produces a purple flower

25. PHENOTYPE- an organism’s traits - this is the PHYSICAL APPEARANCE or ABILITY - for the flowers, the phenotypes are either purple or white GENOTYPE- an organism’s genetic makeup - for the flowers, the phenotypes could be PP, pp, or Pp

26. Figure 14.5 Genotype versus phenotype

27. TESTCROSS A TESTCROSS is a genetic cross performed when the genotype of one of the parents is unknown - the genotype of the parent can be determined by looking at the offspring

28. LAW OF INDEPENDENT ASSORTMENT The Law of Segregation was derived by performing breeding experiments using only a single character - the F1 hybrids produced in these crosses are called MONOHYBRIDS (Monohybrid crosses) Mendel also performed DIHYBRID CROSSES- crosses involving 2 separate characters

29. Mendel studied seed color and seed shape - he knew that the allele for yellow seeds is dominant over green seeds, and the allele for round seeds is dominant over wrinkled seeds - he crossed 2 true-breeding plants that differed in BOTH characters: YYRR x yyrr

30. Mendel wondered if the 2 characters, seed color and seed shape, were transmitted from parents to offspring as a package - in other words, will Y and R alleles always stay together? Or - are they inherited independently of each other?

31. For both possibilities, the F1 plants were heterozygous for both traits: YyRr - Mendel needed to see what would happen when these plants self-pollinated (in the F2 generation)

32. THE RESULTS Experimental results supported the hypothesis that each character is independently inherited - the 2 alleles for seed color segregate independently of the 2 alleles for seed shape - Mendel always ended up with the 9:3:3:1 phenotypic ratio

33. The independent segregation of each pair of alleles during gamete formation is now called the LAW OF INDEPENDENT ASSORTMENT

34. PROBABILITY RECALL: The probability scale ranges from 0 to 1 - an event certain to occur has a probability of 1 - an event certain NOT to occur has a probability of 0 - with a coin, the chance of tossing heads is ½, tails is ½

35. This can be applied in fertilization: - the ovum has ½ chance of carrying a dominant allele, and ½ chance of carrying a recessive allele - the same odds apply to the sperm - like 2 separate coin tosses, allele segregation occurs 2 independent events

36. Rule of Multiplication “What is the chance that 2 coins tossed simultaneously will land heads up?” - we will find the probability for each independent event and then multiply these events together ½ x ½ = ¼

37. “An F1 plant is Pp for purple flower color. What is the probability that an F2 plant will have white flowers?” - both ovum and sperm must carry a p ½ x ½ = ¼ “What is probability of an F2 plant having genotype YYRR?” - probability of YR is ¼ ¼ x ¼ = 1/16

38. Rule of Addition “What is the probability that an F2 plant from a monohybrid cross will be heterozygous?” - there are 2 ways that this can occur: * the dominant allele can come from the ovum and the recessive from the sperm, or vice versa - to find the probability that an event can occur in 2 or more different ways, we add the probabilities

39. ¼ + ¼ = ½

40. We can combine these rules for more complicated crosses: PpYyRr x Ppyyrr Calculate the fraction of offspring predicted to show the recessive phenotypes for at least 2 of the 3 traits : Ppyyrr, ppYyrr, ppyyRr, PPyyrr, ppyyrr - we will combine the rules

41. PpYyRr x Ppyyrr ppyyRr: ¼ (prob. of pp) x ½ x ¼ = 1/16 ppYyrr: ¼ x ½ x ½ = 1/16 Ppyyrr: ½ x ½ x ½ = 2/16 PPyyrr: ¼ x ½ x ½ = 1/16 ppyyrr: ¼ x ½ x ½ = 1/16 ___________________________________ Chance of at least 2 = 6/16 recessive traits or 3/8

42. Try it: PPYyrr x PpyyRr “What is the probability of getting dominant phenotypes for at least 2 of the 3 traits?”