Segregation

1. Segregation • If we wanted to cross the F1 generation, what are the possible alleles each parent could have?

2. Segregation • Use a Punnett-square to predict the probable offspring from a monohybrid cross of the F1 generation.

3. F1 Cross: Tt x Tt Phenotypic ratio 3 purple : 1 white P Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp p

4. Segregation F2Generation F1Generation PGeneration Tall Tall Tall Tall Short Tall Short Tall

5. Segregation • Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation. • The trait controlled by the recessive allele showed up in some of the F2 plants. • This indicated that at the recessive allele had been separated, or segregated, from the dominant allele.

6. Segregation • Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the sex cells, or gametes. What process did we learn about where chromosomes and DNA separate to form gametes?

7. Segregation • When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. • Therefore, each F1 plant produces two types of gametes—those with the allele for tallness, and those with the allele for shortness.

8. Segregation • Alleles separate during gamete formation.

9. Genetic makeup (alleles) 0 P plants pp PP Gametes All All p P F1 plants (hybrids) All Pp p P 1 – 2 1 – 2 Gametes Sperm p P P PP Pp F2 plants Phenotypic ratio 3 purple : 1 white Eggs Genotypic ratio 1 PP : 2 Pp : 1 pp Pp pp p

10. 11 Review • Gametes are also known as • genes. • sex cells. • alleles. • hybrids.

11. 11.1 Review • The offspring of crosses between parents with different traits are called • alleles. • hybrids. • gametes. • dominant.

12. 11.1 Review • In Mendel’s pea experiments, the male gametes are the • eggs. • seeds. • pollen. • sperm.

13. 11.1 Review • In a cross of a true-breeding tall pea plant with a true-breeding short pea plant, the F1 generation consists of • all short plants. • all tall plants. • half tall plants and half short plants. • all plants of intermediate height.

14. 11.1 Review • If a particular form of a trait is always present when the allele controlling it is present, then the allele must be • dominant.

15. 11.1 Review • A form of a gene that is not expressed when paired with a dominant allele is called a ______________________ recessive allele

16. 11.1 Review • Sections of chromosomes that code for a trait are called ______________________ genes

17. 11.1 Review • Organisms usually get one copy of each gene from each parent. The different forms of the same gene are called ______________________ alleles

18. 11.1 Review • A pea plant has two alleles for color. Allele Y = yellow and y = green. If you have a genotype of Yy, or the plant has both alleles, and the plant is yellow, what does that tell you about each allele? Y must be dominant & y must be recessive.

19. 11.1 Review • What genotype (allele combination) would result in a green plant. What other allele combination could result in a yellow plant. y y is the only combination that could result in a green pea plant. Y Y & Y y could both result in a yellow plant.

20. Chance • Mendel realized that fertilization is a chance event. • Therefore, the principles of probability could be used to explain the results of his genetic crosses. • Probability is the likelihood that a particular event will occur.

21. Flip a coin • What are the chance of a coin landing on heads? On tails? • The chance, or probability, of either outcome is equal. Therefore, the probability that a single coin flip will land heads up is 1 chance in 2. This amounts to 1/2, or 50 percent.

22. Flip a coin • Each coin flip is an independent event, with a one chance in two probability of landing heads up. • With that in mind, if you flip a coin three times in a row, what is the probability that it will land heads up every time? 1/2 × 1/2 × 1/2 = 1/8

23. Using Segregation to Predict Outcomes • Mendel’s cross produced a mixture of tall and short plants. • If each F1 plant had one tall allele and one short allele (Tt), then 1/2 of the gametes they produced would carry the short allele (t).

24. Using Segregation to Predict Outcomes • Because it is recessive, the only way to produce a short (tt) plant is for two gametes carrying the t allele to combine. • Since each gamete produced by the F1 plants has a ½ chance of carrying the t allele, what are the chances of producing a short plant?

25. Roughly one fourth of the F2 offspring should be short, and the remaining three fourths should be tall.

26. Heterozygous cross: Tt x Tt • The probable phenotypic ratio of the F2 generation is 3 tall to 1 short, or 3:1. F2Generation F1Generation PGeneration Tall Tall Tall Tall Short Tall Short Tall

27. Testcross If Purple (P) is dominant to white (p) • If you have a white pea plant, what is it’s genotype? • If you have a purple pea plant, what is it’s genotype? • How could you find out?

28. 0 Geneticists use the testcross to determine unknown genotypes • Testcross • Mating between an unknown genotype and a homozygous recessive individual • Will show whether the unknown genotype includes a recessive allele • Used by Mendel to confirm true-breeding genotypes

29. 0 Testcross: B_ Genotypes bb Two possibilities for the black dog: BB Bb or B b B Gametes b Bb b Bb bb Offspring 1 black : 1 chocolate All black

30. Independent Assortment • How do alleles segregate when more than one gene is involved? • The principle of independent assortmentstates that genes for different traits can segregateindependently during the formation of gametes.

31. Independent Assortment • Mendel wondered if the segregation of one pair of alleles affects another pair. • Mendel performed an experiment that followed two different genes as they passed from one generation to the next. • Because it involves two different genes, Mendel’s experiment is known as a dihybridcross.

32. Independent Assortment • Mendel crossed true-breeding plants that produced only round yellow peas with plants that produced wrinkled green peas.

33. Independent Assortment • The round yellow peas had the genotype RRYY, which is homozygous dominant. • The wrinkled green peas had the genotype rryy, which is homozygous recessive.

34. Independent Assortment • All of the F1 offspring produced round yellow peas. These results showed that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas.

35. Independent Assortment • The Punnett square shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color.

36. Independent Assortment • Mendel then crossed the F1 plants to produce F2 offspring. Mendel observed that: • 315 F2 seeds were round and yellow • 32 seeds were wrinkled and green These represented the two parental phenotypes. • 209 seeds had combinations of phenotypes, and therefore combinations of alleles, that were not found in either parent.

37. Independent Assortment • The alleles for seed shape segregatedindependently of those for seed color • Genes that segregate independently—such as the genes for seed shape and seed color in pea plants—do not influence each other’s inheritance.

38. Create a dihybrid cross for the heterozygous F1 generationRrYy x RrYy

40. Independent assortment rryy RRYY ry • Parental generation: roundyellow seedswrinkled green seeds • F1 generation: all plants with round yellow seeds • F2 generation: • 9/16 of plants with round yellow seeds • 3/16 of plants with round green seeds • 3/16 of plants with wrinkled yellow seeds • 1/16 of plants with wrinkled green seeds Gametes RY RrYy Sperm RY 1 – 4 Ry ry 1 – 4 1 – 4 1 – 4 rY RY 1 – 4 RrYy RRYY RrYY RRYy 1 – 4 rY RrYY rrYY RrYy rrYy Eggs Yellow round 9 –– 16 Ry 1 – 4 RRYy RRyy Rryy RrYy Green round 3 –– 16 1 – 4 ry Yellow wrinkled Rryy rrYy rryy 3 –– 16 RrYy Actual results (support hypothesis) Green wrinkled 1 –– 16

41. 0 The law of independent assortment is revealed by tracking two characters at once • Example of a dihybrid cross • Parental generation: round yellow seeds  wrinkled green seeds • F1 generation: all plants with round yellow seeds • F2 generation: • 9/16 of plants with round yellow seeds • 3/16 of plants with round green seeds • 3/16 of plants with wrinkled yellow seeds • 1/16 of plants with wrinkled green seeds

42. Independent Assortment • Mendel’s experimental results were very close to the 9:3:3:1 ratio that the Punnett square shown predicts. • Mendel had discovered the principle ofindependent assortment. • The principle of independent assortment states that genes for different traits can segregateindependently during gamete formation.

43. Do Now 01Write out the following: • Mendel’s theory of segregation: • Mendel’s theory of Independent Assortment: • For the Smurfs, the allele for blue skin (S) is dominant to the allele for red skin (s). What are the chances for a mating between a heterozygousblueSmurf and a redSmurf resulting in a redSmurfbaby? Diploid cells have pairs of genes, on pairs of homologous chromosomes. The two genes of each homologouspair are separated from each other during meiosis, so they end up in different gametes. By the end of meiosis, genes of pairs of homologous chromosomes have sortedout for distribution into one gamete of another independently of gene pairs of other chromosomes.

44. What happens when traits don’t follow Mendel’s rules?

45. Alleles • An allele may be fullydominant, incompletely dominant, or codominant with its partner on the homologous chromosome.

46. Incomplete Dominance • When one allele is not completely dominant over another it is called incomplete dominance. • In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes.

47. Incomplete Dominance RR • A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW

48. Codominance • In codominance, both alleles contribute to the phenotype. • In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

49. Codominance • Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately.

50. Multiple Alleles • Genes that are controlled by more than two alleles are said to have multiple alleles. • An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. • A rabbit's coat color is determined by a single gene that has at least four different alleles.