1. 1 CHAPTER 11 GENETICS

2. 2 Gregor Mendel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ned M. Seidler/Nationa1 Geographic Image Collection

3. 3 Gregor Mendel The garden pea: Organism used in Mendel’s experiments A good choice for several reasons: Easy to cultivate Short generation Normally self-pollinating, but can be cross-pollinated by hand True-breeding varieties were available Simple, objective traits

4. 4 Garden Pea Anatomy stamen anther a. Flower Structure filament stigma style ovules in ovary carpel Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5. 5 All peas are yellow when one parent produces yellow seeds and the other parent produces green seeds. Brushing on pollen from another plant Cutting away anthers Garden Pea Anatomy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6. Garden Pea Anatomy Trait *Dominant Characteristics *Recessive Stem length Pod shape Seed shape Seed color Flower position Flower color Pod color b. Green Purple Axial Yellow Round Inflated TallShort Constricted Wrinkled Green Terminal White Yellow Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

7. Figure 14.5 Genotype versus phenotype

8. Figure 14.6 A testcross

9. Figure 14.2 Mendel tracked heritable characters for three generations

10. 10 11.2 Mendel’s Laws Mendel performed cross-breeding experiments Used “true-breeding” (homozygous) plants Chose varieties that differed in only one trait (monohybrid cross) Performed reciprocal crosses Parental generation = P First filial generation offspring = F 1 Second filial generation offspring = F 2 Formulated the Law of Segregation

11. 11 Mendel’s Laws Law of Segregation: Each individual has a pair of factors (alleles) for each trait The factors (alleles) segregate (separate) during gamete (sperm & egg) formation Each gamete contains only one factor (allele) from each pair of factors Fertilization gives the offspring two factors for each trait

12. 12 Mendel’s Laws Classical Genetics and Mendel’s Cross: Each trait in a pea plant is controlled by two alleles (alternate forms of a gene) Dominant allele (capital letter) masks the expression of the recessive allele (lower-case) Alleles occur on a homologous pair of chromosomes at a particular gene locus Homozygous = identical alleles Heterozygous = different alleles

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

14. Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross

15. 15 Mendel’s Laws Genotype Refers to the two alleles an individual has for a specific trait If identical, genotype is homozygous If different, genotype is heterozygous Phenotype Refers to the physical appearance of the individual

16. 16 Mendel’s Laws A dihybrid cross uses true-breeding plants differing in two traits Mendel tracked each trait through two generations. Started with true-breeding plants differing in two traits The F 1 plants showed both dominant characteristics F 1 plants self-pollinated Observed phenotypes among F 2 plants Mendel formulated the Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traits All possible combinations of factors can occur in the gametes P generation is the parental generation in a breeding experiment. F 1 generation is the first-generation offspring in a breeding experiment. F 2 generation is the second-generation offspring in a breeding experiment

17. 17 Classical View of Homologous Chromosomes Replication alleles at a gene locus sister chromatids b. Sister chromatids of duplicated chromosomes have same alleles for each gene. a. Homologous chromosomes have alleles for same genes at specific loci. G R S t G R S t G R S t g r s T g r s T g r s T Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

18. Figure 15.1 The chomosomal basis of Mendel’s laws

19. 19 Mendel’s Laws Punnett Square Allows us to easily calculate probability, of genotypes and phenotypes among the offspring Punnett square in next slide shows a 50% (or ½) chance The chance of E = ½ The chance of e = ½ An offspring will inherit: The chance of EE =½  ½=¼ The chance of Ee =½  ½=¼ The chance of eE =½  ½=¼ The chance of ee =½  ½=¼

20. Rules of Probability Probability scale ranges from 0 to 1 The possibilities of all outcomes must add up to 1 In every fertilization involving gametes, the ovum has a ½ chance of carrying a dominant allele and ½ chance of carrying a recessive allele Two basic laws of probability can help are the rule of multiplication and the rule of addition

21. Rule of Multiplication How can we determine the chance that two or more independent events will occur together in some combination? Compute the probability for each independent event, then multiply these individual probabilities to obtain the overall probability of these events occurring together

22. Rule of Addition The probability of an event that can occur in two or more different ways is the sum of the separate probabilities of those ways

23. 23 Mendel’s Laws Genetic disorders are medical conditions caused by alleles inherited from parents Autosome - Any chromosome other than a sex chromosome (X or Y) Genetic disorders caused by genes on autosomes are called autosomal disorders Some genetic disorders are autosomal dominant An individual with AA has the disorder An individual with Aa has the disorder An individual with aa does NOT have the disorder Other genetic disorders are autosomal recessive An individual with AA does NOT have the disorder An individual with Aa does NOT have the disorder, but is a carrier An individual with aa DOES have the disorder

24. 24 Autosomal Recessive Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. I II III IV Key Generations Autosomal recessive disorders Most affected children have unaffected parents. Heterozygotes (Aa) have an unaffected phenotype. Two affected parents will always have affected children. Close relatives who reproduce are more likely to have affected children. Both males and females are affected with equal frequency. A? aa A? Aa A? Aa * A? aa aa = affected Aa = carrier (unaffected) AA = unaffected A? = unaffected (one allele unknown)

25. 25 Autosomal Dominant Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Affected children will usually have an affected parent. Heterozygotes (Aa) are affected. Two affected parents can produce an unaffected child. Two unaffected parents will not have affected children. Both males and females are affected with equal frequency. AA = affected Aa = affected A? = affected (one allele unknown) aa = unaffected I II III Aa aa Aa * A? aa Aa Key Generations Autosomal dominant disorders

26. Non-single gene genetics Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types

27. Figure 14.9 Incomplete dominance in snapdragon color

28. 28 Blood Types Some traits are controlled by multiple alleles (multiple allelic traits) The gene exists in several allelic forms (but each individual only has two alleles) ABO blood types The alleles: I A = A antigen on red blood cells, anti-B antibody in plasma I B = B antigen on red blood cells, anti-A antibody in plasma i = Neither A nor B antigens on red blood cells, both anti-A and anti-B antibodies in plasma The ABO blood type is also an example of codominance More than one allele is fully expressed Both I A and I B are expressed in the presence of the other

29. 29 ABO Blood Type Genotype I A I A, I A i I B I B, I B i I A I B ii Phenotype A B AB O

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

31. Figure 14.15 Pleiotropic effects of the sickle-cell allele in a homozygote

32. Figure 14.11 An example of epistasis

33. Figure 14.12 A simplified model for polygenic inheritance of skin color

34. 34 Extending the Range of Mendelian Genetics X-Linked Inheritance In mammals The X and Y chromosomes determine gender Females are XX Males are XY

35. 35 Extending the Range of Mendelian Genetics X-Linked Inheritance The term X-linked is used for genes that have nothing to do with gender X-linked genes are carried on the X chromosome. The Y chromosome does not carry these genes Discovered in the early 1900s by a group at Columbia University, headed by Thomas Hunt Morgan. Performed experiments with fruit flies They can be easily and inexpensively raised in simple laboratory glassware Fruit flies have the same sex chromosome pattern as humans

36. 36 X – Linked Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. XrYXrY XRYXRY XRXR XrXr Y XrYXrY XrXr XRXR Y Offspring eggs sperm P generation P gametes F 2 generation F 1 generation F 1 gametes Allele Key = = XRXR XrXr red eyes white eyes Phenotypic Ratio all red-eyed red-eyed white-eyed females: males: 1 1 XRYXRY XRXrXRXr XRXRXRXR XRXR XRXrXRXr XRXRXRXR

37. X – Linked Inheritance

38. 38 Extending the Range of Mendelian Genetics Several X-linked recessive disorders occur in humans: Color blindness The allele for the blue-sensitive protein is autosomal The alleles for the red- and green-sensitive pigments are on the X chromosome. Menkes syndrome Caused by a defective allele on the X chromosome Disrupts movement of the metal copper in and out of cells. Phenotypes include kinky hair, poor muscle tone, seizures, and low body temperature Muscular dystrophy Wasting away of the muscle Caused by the absence of the muscle protein dystrophin Adrenoleukodystrophy X-linked recessive disorder Failure of a carrier protein to move either an enzyme or very long chain fatty acid into peroxisomes. Hemophilia Absence or minimal presence of clotting factor VIII or clotting factor IX Affected person’s blood either does not clot or clots very slowly.

39. 39 X-Linked Recessive Pedigree Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. XBXBXBXB XbYXbY grandfather daughterXBXbXBXb XBYXBYXBYXBYXbXbXbXb XbYXbY XBXbXBXb grandson XBYXBYXBXBXBXB XbYXbY Key X B X B = Unaffected female X B X b = Carrier female X b X b = Color-blind female X b Y = Unaffected male X b Y = Color-blind male X-Linked Recessive Disorders More males than females are affected. An affected son can have parents who have the normal phenotype. For a female to have the characteristic, her father must also have it. Her mother must have it or be a carrier. The characteristic often skips a generation from the grandfather to the grandson. If a woman has the characteristic, all of her sons will have it.

40. 40 Expressivity refers to variations in a phenotype among individuals carrying a particular genotype. Penetrance is the proportion of individuals carrying a particular variant of a gene (allele or genotype) that also express an associated trait (phenotype).

41. Chromosomal errors, I Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II Aneuploidy: chromosome number is abnormal Monosomy ~ missing chromosome Trisomy ~ extra chromosome (Down syndrome) Polyploidy~ extra sets of chromosomes

42. Figure 15.11 Meiotic nondisjunction

43. Chromosomal errors, II Alterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another

44. Figure 15.13 Alterations of chromosome structure

45. Figure 15.14 Down syndrome

46. Figure 15.x2 Klinefelter syndrome

47. Figure 15.x3 XYY karyotype

48. Chromosomal Disorders Down syndrome XXY Klinefelter syndrome XYY XXX XO Turner syndrome

49. Extranuclear Genes Not all genes are located on the nuclear chromosomes. These genes do not exhibit Mendelian genetics. Mitochondrial DNA which is given from the mother only, can in some rare cases cause some disorders. If the Mitochondrial DNA is defected this would reduce the amount of ATP made.