Mendelian Genetics and the Inheritance of Genetic Traits

1. Mendelian Genetics and the Inheritance of Genetic Traits Gregor Mendel: Father of Modern Genetics IB Topic 4.3- Theoretical Genetics Campbell: Ch. 14 Allott: Ch. 12

2. Theoretical Genetics Defined: • Theoretical Genetics-concerned with the probabilities associated with producing offspring of a particular genotype or phenotype.

3. Experimental Genetics Began in an abbey Garden • Modern theoretical genetics began with Gregor Mendel’s quantitative experiments with pea plants Stamen Carpel Figure 9.2A, B

4. Gregor Mendel (Father of Genetics) • Discovered the fundamentals of Genetics in the 1860’s • Lived in Austria and studied in Vienna • Worked with Garden Peas (Pisum sativum) • Gathered a huge amount of numerical data • Discovered the frequency of how traits are inherited • Established basic principles of Genetics

5. MENDEL’S PRINCIPLES • The science of heredity dates back to ancient attempts at selective breeding • Until the 20th century, however, many biologists erroneously believed that • characteristics acquired during lifetime could be passed on • characteristics of both parents blended irreversibly in their offspring

6. Reason Mendel worked with Garden Peas • Easy to grow • Many variations were available • Easy to control pollination (self vs cross) • Flower is protected from other pollen sources (reproductive structures are completely enclosed by petals) • Plastic bags can be used for extra protection

7. White 1 Removed stamensfrom purple flower • Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower PARENTS(P) Purple 3 Pollinated carpel matured into pod • This illustration shows his technique for cross-fertilization 4 Planted seeds from pod OFF-SPRING(F1) Figure 9.2C

8. FLOWER COLOR Purple White • Mendel studied seven pea characteristics FLOWER POSITION Axial Terminal • He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity SEED COLOR Yellow Green SEED SHAPE Round Wrinkled POD SHAPE Inflated Constricted POD COLOR Green Yellow STEM LENGTH Figure 9.2D Tall Dwarf

9. Mendel’s Experiment • He set up true-breeding plants (bred for many generations) by allowing them to self-fertilize. • He controlled pollination, looking at 1 or 2 characteristics at a time.

10. 2. He crossed a true breedingplant with a plant of the opposite trait (purple x white). He called this the Parental (P1) generation.

11. 3. He recorded data on the offspring of this cross, calling it the First Filial, or F1 Generation.

12. 4. He self pollinated the F1 offspring5. He recorded data on the offspring of the second generation, calling it the Second Filial generation (F2)

13. Mendel’s Results

14. Analysis • The F1 generation always displayed one trait (he later called this the dominant trait) • The F1 generation must have within it the trait from the original parents - the white trait • The F2 generation displayed the hidden trait, 1/4 of the F2 generation had it (he later called this hidden trait the recessive trait)- 3:1 ratio. • Each individual has two "factors" that determine what external appearance the offspring will have. (We now call these factors genes or alleles)

15. Mendel established three principles (or Laws) from his research: 1. The Principle of Dominance and Recessiveness - one trait is masked or covered up by another trait 2. Law of Segregation - the two factors (alleles) for a trait separate during gamete formation 3. Law of Independent Assortment - factors of a trait separate independently of one another during gamete formation; another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited

16. Genetic Crosses 1. Mendel's factors are now called ALLELES. For every trait a person has, two alleles determine how that trait is expressed. 2. We use letters to denote alleles, since every gene has two alleles, all genes can be represented by a pair of letters. PP = purple, Pp = purple, pp = white

17. Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes GENE LOCI DOMINANT allele P a B P a b RECESSIVE allele GENOTYPE: PP aa Bb HOMOZYGOUSfor thedominant allele HOMOZYGOUSfor therecessive allele HETEROZYGOUS Figure 9.4

18. Let’s do some definitions • Genotype- the alleles possessed by an organism. Ex: BB, or Bb • Phenotype- the characteristics of an organism. Ex: Brown hair

19. More Definitions • Homozygous- having two identical alleles of a gene. Ex: BB or bb • Heterozygous- having two different alleles of a gene. Ex: Bb

20. When we cross-breed 2 things, looking at one factor, we have a: • Monohybrid cross = a cross involving one pair of contrasting traits. Ex. Pp x Pp We can figure the possibilities of offspring using a: • Punnet Square: used to determine the PROBABILITY of having a certain type of offspring given the alleles of the parents

21. How to Solve a Punnett Square 1. Determine the genotypes (letters) of the parents. Bb x Bb2. Set up the punnett square with one parent on each side.3. Fill out the punnett square middle4. Analyze the number of offspring of each type.

22. An Example • In pea plants, round seeds are dominant to wrinkled. The genotypes and phenotypes are: • RR = roundRr = roundrr = wrinkled • If a heteroyzous round seed is crossed with itself (Rr x Rr) a punnett square can help you figure out the ratios of the offspring.

23. Set up your squareRemember, it’s Rr x Rr • Note that the letters get separated on the top and the side. It DOES NOT MATTER which parent goes on top or on the side.

24. Results So,The Phenotypic Ratio is 3:1, Round to Wrinkled The Genotypic Ratio is 1:2:1, and refers to the letters. It is 1 RR, 2 Rr, 1 rr.

25. Monohybrid cross: be able to Predict Genotypes and Phenotypes Try this: what are the genotypic and phenotypic ratios of offspring from a cross between two heterozygous brown-haired people? (Brown is dominant to blond) Now try some more from the worksheets provided.

26. Independent Assortment in Budgie Birds

27. Geneticists use the testcross to determine unknown genotypes • testing a suspected heterozygote by crossing it with a known homozygous recessive.

28. Dihybrid Crosses: Crosses that involve 2 traits.For these crosses your punnett square needs to be 4x4(Note the 9:3:3:1 ratio)

29. Non-single Gene Genetics Incomplete dominance: -neither pair of alleles are completely expressed when both are present. -Typically, a third phenotype is produced, which is a blend of the traits Ex: snapdragons, roses, carnations (pink flowers) Codominance: Two alleles are expressed in a heterozygote condition. Ex: Human Blood types

30. Incomplete dominance results in intermediate phenotypes P GENERATION Whiterr • When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance Red RR Gametes R r PinkRr F1 GENERATION 1/2 R 1/2 r 1/2 R 1/2 R Eggs Sperm RedRR 1/2 r 1/2 r PinkRr PinkrR F2 GENERATION Whiterr Figure 9.12A

31. Many genes have more than two alleles in the population • In a population, multiple alleles often exist for a characteristic • This is called Codominance-When there are multiple alleles, but both express themselves equally in phenotypic expression. Ex- White + Chestnut horse= Roan (white and red hairs mixed together). +

32. Both A and B are dominant. Type O is recessive Four phenotypes Six genotypes Codominance-Also Observed in Blood Types- p. 140 (Allott)

33. Blood types are caused by the presence of a protein cell-surface marker. If an antigen on the surface of the RBC plasma membrane is mixed with the wrong blood type, antigens are bound by antibodies= clumping.

34. 4 Types of Blood • Type A with A antigens on the red cells and anti B antibodies in the plasma. • Type B with B antigens on the red cells and anti A antibodies in the plasma. • Type AB with both A and B antigens on the red cells and no blood type antibodies in the plasma. • Type O with no antigens on the red cells and both anti A and anti B antibodies in the plasma • ** Group O blood cannot be clumped by any human blood, and therefore people with Group O are called universal donors.

35. Blood Donor Chart

37. What is the + and - ? • The Rh blood group (named for the rhesus monkey in which it was discovered) is made up of those Rh positive (Rh+) individuals who can make the Rh antigen and those Rh negative (Rh-) who cannot.

38. Rh factor, cont. • Hemolytic disease of the newborn (HDN) results from Rh incompatibility between an Rh- mother and Rh+ fetus. • Rh+ blood from the fetus enters the mother's system during birth, causing her to produce Rh antibodies. The first child is usually not affected, however subsequent Rh+ fetuses will cause a massive secondary reaction of the maternal immune system. To prevent HDN, Rh- mothers are given an Rh antibody during the first pregnancy with an Rh+ fetus and all subsequent Rh+ fetuses.

39. Blood Type Frequencies of different Ethnic Groups

40. Non-single Gene Genetics Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia combs in roosters coat color in rabbits Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color & Labrador coat color Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

41. A single gene may affect many phenotypic characteristics • A single gene may affect phenotype in many ways • This is called pleiotropy • The allele for sickle-cell disease is an example

42. Pleiotropy – Sickle Cell anemia

43. Effects of Sickle Cell Anemia

44. Explain that polygenic inheritance can contribute to continuous variation using two examples. 1) Human skin color- is thought to be controlled by at least 3 independent genes. AABBCC x aabbcc F1 = AaBbCc , then perform a dihybird cross (AaBbCc), and there are many possible outcomes, such as: AABBCc, AABBcc, AABbcc, AAbbcc, etc. 2) Human hair color- is also thought to be controlled but multiple genes, accounting for the large variety in shade.

45. Polygenic Inheritance

46. P GENERATION aabbcc(very light) AABBCC(very dark) F1 GENERATION AaBbCc AaBbCc Eggs Sperm Fraction of population Skin pigmentation F2 GENERATION Figure 9.16

47. Epistasis • Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice and Labrador coat color

48. Epistasis • Examples: Labrador’s coat color Albino Koala Two Genes Involved: Allele Symbol -Pigment- Black (Dominant) B Chocolate (recessive) b -Expression or deposition of the Pigment E/e BlackYellowChocolate BBEE BBee bbEE BbEE Bbee bbEe BBEe BbEe Which genotype is missing and what group should it be listed under?

49. Epistasis

50. Statistical Tools to Analyze results • Chi-Square: Will tell you how much your data is different from expected (calculated) results. It is Non-Parametric and deals with different categories. Formula:2 = ∑ (o – e)2 e 2: what we are solving: o: observed value e: expected (calculated value)