Understanding Mendel's Principles of Inheritance

1. Mendel and the Gene 0 14 Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge

2. Roadmap 14 In this chapter you will learn how Mendel’s principles can predict patterns of inheritance starting with expanding to The chromosome theory of inheritance Mendel’s experimental system 14.4 14.1 exploring then examining Extensions to Mendel’sprinciples Experimentswith a singletrait Experimentswith twotraits 14.5 applied to 14.2 14.3 Human inheritance 14.6 explained by explained by The principleof independentassortment The principleof segregation

3. Introduction In 1865, Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas Early in the 20th century, Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance Proposes that meiosis causes the patterns of inheritance that Mendel observed Genetics is the branch of biology that focuses on inheritance

4. Mendel’s Experimental System Gregor Mendel was A 19th-century monk Active member of his city’s agricultural society Mendel was interested in heredity Heredity Is the transmission of traits from parents to their offspring A trait Is any characteristic of an individual

5. What Question Was Mendel Trying to Answer? • Mendel was addressing the basic questions • Why offspring resemble their parents • How transmission of traits occurs • In his time, two hypotheses had been formulated to try to answer this question • Blending inheritance • Parental traits blend • Such that their offspring have intermediate traits • Inheritance of acquired characteristics • Parental traits are modified • Then passed on to their offspring

6. Garden Peas: The First Model Organism in Genetics Genetics The branch of biology that focuses on the inheritance of traits Uses model organisms Because the conclusions drawn from them can be applied to other species

7. Garden Peas: The First Model Organism in Genetics Mendel chose the common garden pea (Pisum sativum) as his model organism because It is easy to grow Its reproductive cycle is short It produces large numbers of seeds Its matings are easy to control Its traits are easily recognizable

8. How Did Mendel Arrange Matings? Peas normally pollinate themselves This process is called self-fertilization Mendel could prevent self-pollination by Removing the male reproductive organs containing pollen from each flower He then used this pollen To fertilize the female reproductive organs of flowers on different plants Thus performing cross-pollination

9. Figure 14.1 (a) Self-pollination Female organ (receives pollen) Male organs (produce pollengrains, which produce sperm cells) Eggs (b) Cross-pollination Collect pollen fromone individual andtransfer it … … to the female organs ofan individual whose maleorgans have beenremoved.

10. What Traits Did Mendel Study? Mendel worked with pea varieties that differed in seven easily recognizable traits: Seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length An individual’s observable features comprise its phenotype Mendel’s pea population had two distinct phenotypes for each of the seven traits

11. What Traits Did Mendel Study? Mendel worked with pure lines They produced identical offspring when self-pollinated He used these plants to create hybrids He mated two different pure lines that differed in one or more traits

12. Inheritance of a Single Trait Mendel’s first experiments involved crossing pure lines that differed in just one trait The adults in the cross were the parental generation The offspring are the F1 generation The F indicates first filial

13. The Monohybrid Cross Mendel first experimented with crossing plants that differed in only one trait When Mendel crossed plants with round seeds and plants with wrinkled seeds: All of the F1 offspring had round seeds This contradicted the hypothesis of blending inheritance The genetic determinant for wrinkled seeds seemed to have disappeared Mendel allowed the F1 progeny to self-pollinate The wrinkled seed trait reappeared in the next F2 generation

14. Figure 14.2 (a) Results of Mendel’s single-trait (monohybrid) cross Male parents(produce round seeds) Female parents(produce wrinkled seeds) F1 generation All produce round seeds Plant, grow, andallow to self-fertilize F2 generation 5474 : 1850 3 : 1 (b) Prediction of blending-inheritance hypothesis Male parents(produce round seeds) Female parents(produce wrinkled seeds) F1 generation All produce slightly wrinkled seeds Plant, grow, and allow to self-fertilize F2 generation All produce slightlywrinkled seeds

15. Dominant and Recessive Traits Mendel called The genetic determinant for wrinkled seeds recessive The determinant for round seeds dominant The terms dominant and recessive Identify only which phenotype is observed in individuals carrying two different genetic determinants Mendel repeated these experiments with each of the other traits In each case, the dominant trait was present in a 3:1 ratio over the recessive trait in the F2 generation

16. Summary Table 14.1

17. A Reciprocal Cross Mendel wanted to determine if gender influenced inheritance He performed a reciprocal cross The mother’s phenotype in the first cross is the father’s phenotype in the second cross The father’s phenotype in the first cross is the mother’s phenotype in the second cross The results of the two crosses were identical Established that it does not matter whether the genetic determinants are located in the male or female parent

18. Figure 14.3 Is the inheritance of seed shape in peas affected by whether the geneticdeterminant is in a male or female gamete? The type of gamete does affect the inheritance of seed shape. The type of gamete does not affect the inheritance of seed shape. The reciprocal cross A cross Round-seeded parentreceives pollen … … from wrinkled-seeded parent Pollen from round-seeded parent … … to female organ ofwrinkled-seeded parent. Male parent Female parent Female parent Male parent Offspring phenotypes will be different in the two crosses. Offspring phenotypes will be identical in the two crosses. Results areidentical First cross: All progeny have round seeds. Reciprocal cross: All progeny have round seeds. It makes no difference whether the genetic determinant for seed shape comes from the male gamete or from the female gamete.

19. Table 14.2

20. Particulate Inheritance Mendel proposed a hypothesis called particulate inheritance Suggests that hereditary determinants maintain their integrity From generation to generation Directly contradicts The blending of inheritance The inheritance of acquired characteristics hypotheses

21. Genes, Alleles, and Genotypes Hereditary determinants for a trait are now called genes Mendel also proposed that Each individual has two versions of each gene These different versions of a gene are called alleles Different alleles are responsible for the variation in the traits that Mendel studied

22. Genes, Alleles, and Genotypes The alleles found in an individual are called its genotype An individual’s genotype has a profound effect on its phenotype

23. The Principle of Segregation Mendel developed the principle of segregation The two members of each gene pair must segregate They separate into different gamete cells During the formation of eggs and sperm in the parents

24. Genetic Notational Convention Mendel used a letter to indicate the gene for a particular trait For example, R represented the gene for seed shape Uppercase (R) indicated a dominant allele Lowercase (r) was for a recessive allele

25. Genetic Notational Convention Individuals have two alleles of each gene Two copies of the same allele Are homozygous RR or rr Two different alleles Are heterozygous Rr

26. Crossing Pure Lines Pure-line individuals always produce Offspring with the same phenotype Because they are homozygous No other allele is present A mating between two pure lines that differ in one trait (RR and rr) results in Offspring that all have a heterozygous genotype Rr A dominant phenotype

27. The Monohybrid Cross A mating of two heterozygous parents Results in offspring that are ¼ RR, ½ Rr, and ¼ rr Produces a 3:1 ratio of phenotypes

28. Figure 14.4 3 1 1 1 1 Offspring genotypes: RR : Rr : rr 4 4 2 4 4 Offspring phenotypes: round : wrinkled (a) A cross between two homozygotes (b) A cross between two heterozygotes Heterozygous mother Homozygous mother R Dominant allele forseed shape (round) R Dominant allele forseed shape (round) r Recessive allele forseed shape (wrinkled) r Recessive allele forseed shape (wrinkled) Meiosis Female gametes Female gametes Homozygous father Heterozygous father Male gametes Meiosis Male gametes Offspring genotypes: All Rr (heterozygous) Offspring phenotypes: All round seeds

29. Summary Table 14.3

30. Testing the Model Mendel’s genetic model Is a set of hypotheses that explain how a particular trait is inherited Explains the results of these crosses A Punnett square Is now used to predict the genotypes and phenotypes Of the offspring from a cross

31. Producing a Punnett Square For one parent, write the gamete genotypes Along the top of the diagram For the other parent, write the gamete genotypes Down the left side of the diagram Draw empty boxes Under the row and to the right of the column of gametes

32. Producing a Punnett Square Fill in each box with the genotypes Written at the top of the corresponding column and at the left of the corresponding row Predict the ratios of each possible offspring genotype and phenotype By tallying the resulting genotypes in all the boxes

33. Single-Trait Cross BLAST Animation: Single-Trait Cross

34. Mendel’s Experiments with Two Traits Mendel used dihybrid crosses Matings between parents that are both heterozygous for two traits To determine whether the principle of segregation holds true if parents differ in more than one trait

35. Mendel’s Experiments with Two Traits Mendel’s experiments tested two contrasting hypotheses Independent assortment Alleles of different genes are transmitted independently of each other Dependent assortment The transmission of one allele depends on the transmission of another

36. The Principle of Independent Assortment Mendel’s results supported the principle of independent assortment The Punnett square from a dihybrid cross predicts Should be 9 different offspring genotypes and 4 phenotypes Four possible phenotypes should be present in a ratio of 9:3:3:1

37. The Principle of Independent Assortment Mendel accepted the hypothesis that Alleles of different genes are transmitted independently of one another This result became known as the principle of independent assortment

38. Figure 14.5 (a) Hypothesis of independent assortment:Alleles of different genes don’t staytogether when gametes form. (b) Hypothesis of dependent assortment:Alleles of different genes stay together when gametes form. Female parent Female parent r Recessive allelefor seed shape (wrinkled) y Recessive allelefor seed color (green) Female gametes Female gametes Male parent Male parent Male gametes Male gametes F1 offspring all RrYy F1 offspring all RrYy R  Dominant allelefor seed shape (round) F2 female parent F2 female parent Y  Dominant allelefor seed color (yellow) Female gametes Female gametes F2 maleparent Male gametes F2 maleparent Male gametes F2 offspring genotypes: F2 offspring phenotypes: F2 offspring genotypes: Dashes in a genotypemean that either allelecan be present F2 offspring phenotypes: (c) Mendel’s results 556 total F2 phenotypes Data are consistent withthe predictions ofindependent assortment. Number Fraction of offspring

39. Two-Trait Cross BLAST Animation: Two-Trait Cross

40. Mendel’s Experiments Web Activity: Mendel’s Experiments

41. Using a Testcross to Confirm Predictions In a testcross A parent that is homozygous recessive for a particular trait Is mated with a parent that has the dominant phenotype but an unknown genotype Because the genetic contribution of the homozygous recessive parent is known The genotype of the other parent can be inferred from the results

42. Using a Testcross to Confirm Predictions Mendel used the testcross To further confirm the principle of independent assortment

43. Figure 14.6 Parent of known phenotypebut unknown genotype Homozygousrecessive parent Could be Could be All All All Offspring predicted ifunknown parent ishomozygous dominantat both genes Offspring predicted if unknownparent is heterozygousat both genes

44. The Chromosome Theory of Inheritance The chromosome theory of inheritance Arose out of Sutton and Boveri’s careful observations of meiosis It states that chromosomes are Composed of Mendel’s hereditary determinants What we now call genes The physical separation of alleles during anaphase of meiosis I Is responsible for Mendel’s principle of segregation

45. Figure 14.7 Rr parent Dominant allelefor seed shape Recessive allelefor seed shape Chromosomes replicate Meiosis I Allelessegregate Meiosis II Gametes PRINCIPLE OF SEGREGATION: Pairs of alleles are separatedduring meiosis in the formation of gametes.

46. The Chromosome Theory of Inheritance The genes for different traits assort independently of one another at meiosis I because They are located on different nonhomologous chromosomes They assort independently of one another This phenomenon explains Mendel’s principle of independent assortment

47. Figure 14.8 Chromosomes Chromosomes could line upthis way could line upthis way Replicated chromosomesprior to meiosis Alleles for seed shape Alleles for seed color Meiosis I Meiosis I Meiosis II Meiosis II Gametes 1/4 RY 1/4 ry 1/4 Ry 1/4 rY PRINCIPLE OF INDEPENDENT ASSORTMENT: The genes for seed shape and seedcolor assort independently, because they are located on different chromosomes.

48. Principle of Independent Assortment Web Activity: The Principle of Independent Assortment

49. Testing the Chromosome Theory Early in the 20th century, Thomas Hunt Morgan adopted Fruit flies (Drosophila melanogaster) As a model organism for genetic research Morgan’s first goal was to identify different phenotypes Wild type The most common phenotype for each trait

50. Testing the Chromosome Theory Mutation Phenotypes that differed from the wild type Resulted from a change in a gene Mutants Individuals with traits attributable to mutation