1. MCAT Prep Exam PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor Genetics Lecture 5 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents eggs Ee spem Punnett square Offspring Ee E e Ee EE ee TTtt t T Tt eggs sperm Offspring Tt T t TT Tt tt

2. 2 Gregor Mendel Austrian monk Studied science and mathematics at University of Vienna Conducted breeding experiments with the garden pea Pisum sativum Carefully gathered and documented mathematical data from his experiments Formulated fundamental laws of heredity in early 1860s Had no knowledge of cells or chromosomes Did not have a microscope

3. 3 Blending Inheritance Theories of inheritance in Mendel’s time: Based on blending Parents of contrasting appearance produce offspring of intermediate appearance Mendel’s findings were in contrast with this He formulated the particulate theory of inheritance Inheritance involves reshuffling of genes from generation to generation

4. 4 One-Trait Inheritance 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

5. 5 Mendel’s Monohybrid Crosses: An Example Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TTtt t T P generation P gametes F 1 gametes F 2 generation F 1 generation Tt Tt sperm T t TTTt tt Offspring Phenotypic Ratio short 1 tall 3 Allele Key T = tall plant t = short plant

6. 6 Law of Segregation Each individual has a pair of factors (alleles) for each trait (Gene), one inherited from each parent Trait (Genes) exist in alternate forms (alleles) The factors (alleles) segregate (separate) during gamete (sperm & egg) formation i.e. the two alleles separate during meiosis, resulting in gametes that carry only one allele for any inherited trait If two allele in an individual organism are different, only the dominant allele will be fully expressed Fertilization gives the offspring two factors for each trait

7. 7 Modern Genetics View 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

8. 8 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.

9. 9 Genotype versus Phenotype 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

10. 10 Punnett Square Table listing all possible genotypes resulting from a cross All possible sperm genotypes are lined up on one side All possible egg genotypes are lined up on the other side Every possible zygote genotypes are placed within the squares

11. 11 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 x ½=¼ The chance of Ee =½ x ½=¼ The chance of eE =½ x ½=¼ The chance of ee =½ x ½=¼

12. 12 Punnett Square Showing Earlobe Inheritance Patterns Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents Ee eggs spem Punnett square Offspring Ee E e Ee EE ee Allele keyPhenotypic Ratio unattached earlobes 3 1 E = unattached earlobes e = attached earlobes attached earlobes

13. 13 Monohybrid Test cross Individuals with recessive phenotype always have the homozygous recessive genotype However, individuals with dominant phenotype have indeterminate genotype May be homozygous dominant, or Heterozygous Test cross determines genotype of individual having dominant phenotype

14. 14 One-Trait Test Cross Insert figure 11.7a here Phenotypic Ratio eggs a. sperm t T t Offspring Tttt Tt tt Allele Key short1 tall1 T = tall plant t = short plant Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

15. 15 One-Trait Test Cross Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. b. tT Tt Offspring eggs sperm TTtt Allele Key Phenotypic Ratio All tall plants T = tall plant t = short plant

16. 16 Two-Trait Inheritance Dihybrid cross uses true-breeding plants differing in two traits Observed phenotypes among F 2 plants Formulated 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 Mendel tracked each trait through two generations. P generation is the parental generation in a breeding experiment. F1 generation is the first-generation offspring in a breeding experiment. F2 generation is the second-generation offspring in a breeding experiment

17. 17 Two-Trait (Dihybrid) Cross Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TtGg ttgg TTGG Offspring P generation P gametes F 2 generation F 1 F 1 gametes eggs sperm TGTg TG Tg tg tG tg TG TTGgTtGGTtGg TTGgTtgg TtGGttGGttGg TtGgTtggttGg ttggTTGG TtGg TTgg TtGg Allele Key = = = = T t G g tall plant short plant green pod yellow pod Phenotypic Ratio 9 tall plant, green pod 3 tall plant, yellow pod 3 short plant, green pod 1 short plant, yellow pod

18. 18 Independent Assortment and Segregation during Meiosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parent cell has two pairs of homologous chromosomes. All possible combina- tions of chromosomes and alleles occur in the gametes as suggested by Mendel's two laws. All orientations of ho- mologous chromosomes are possible at meta- phase I in keeping with the law of independent assortment. At metaphase II, each daughter cell has only one member of each homologous pair in keeping with the law of segregation. A AA AA A A AB ab Ab aB a a a a a a a A A b A aa a B B BB B B B B B B a B b bb AA bbBB aa b b b b b A b b either or

19. 19 Human Genetic Disorders 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 disorder Other genetic disorders are autosomal recessive An individual with AA does NOT have disorder An individual with Aa does NOT have disorder, but is a carrier An individual with aa DOES have the disorder

20. 20 Autosomal Recessive Pedigree Chart Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. aa Aa A? Aa * aa A? Key aa = affected Aa = carrier (unaffected) AA = unaffected A? = unaffected (one allele unknown) I II III IV

21. 21 Autosomal Dominant Pedigree Chart Aa aa Aa aa Aa A? Aa Autosomal dominant disorders 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. * I II III Key AA = affected Aa = affected A? = affected (one allele unknown) aa = unaffected Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

22. 22 Cystic Fibrosis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cl - H2OH2O H2OH2O H2OH2O thick mucus defective channel nebulizer percussion vest © Pat Pendarvis

23. 23 Incomplete Dominance Heterozygote has phenotype intermediate between that of either homozygote Homozygous red has red phenotype Homozygous white has white phenotype Heterozygote has pink (intermediate) phenotype Phenotype reveals genotype without test cross

24. 24 Incomplete Dominance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. R 1 R 2 R 1 R 2 R 1 R 2 R 1 R 2 R 1 R 1 R 2 R 2 R 1 R 2 R 1 R 2 eggs sperm Offspring Key 1 R 1 R 1 2 R 1 R 2 1 R 2 R 2 red pink white

25. 25 Multiple Allelic Traits Some traits controlled by multiple alleles The gene exists in several allelic forms (but each individual only has two) ABO blood types The alleles: I A = A antigen on red cells, anti-B antibody in plasma I B = B antigen on red cells, anti-AB antibody in plasma I = Neither A nor B antigens, both antibodies

26. 26 Multiple Allelic Traits

27. 27 Pleioptropic Effects Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits. Marfan syndrome have disproportionately long arms, legs, hands, and feet; a weakened aorta; poor eyesight

28. 28 Polygenic Inheritance Occurs when a trait is governed by two or more genes having different alleles Each dominant allele has a quantitative effect on the phenotype These effects are additive Result in continuous variation of phenotypes

29. 29 X – Linked Inheritance In mammals The X and Y chromosomes determine gender Females are XX Males are XY The term X-linked is used for genes that have nothing to do with gender 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

30. 30 X – Linked Inheritance Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Offspring eggs sperm P generation P gametes F 2 generation F 1 generation F 1 gametes XRXRXRXR XrYXrY XrXr XRXR Y XRYXRYXRXrXRXr XrXr XRXR XRXrXRXr XRXRXRXR XrYXrYXRYXRY XRXR Y Allele Key X R = red eyes X r = white eyes Phenotypic Ratio females: all red-eyed males : 1 red-eyed 1 white-eyed

31. 31 Human X-Linked Disorders 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. Muscular dystrophy Wasting away of the muscle 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 a clotting factor VIII, or clotting factor IX Affected person’s blood either does not clot or clots very slowly.

32. X-Linked Recessive Pedigree 32 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.

33. Genetic Mapping in Eukaryotes: Genes on non-homologous chromosomes assort independently. Genes on the same chromosome (syntenic) are physically linked (linkage group) and may be inherited together. Classical genetics: Analyzes the frequency of allele recombination in genetic crosses. New combinations of parental alleles are produced by recombination (crossing-over during meiosis I). Test-crosses are used to determine which genes are linked and create a linkage map showing the relative distances of markers on each chromosome. Genetic Mapping

34. 34 How does linkage affect Mendelian segregation patterns? Discovery of genetic linkage in Drosophila (Thomas H. Morgan, ~1911) Morgan determined that the gene for white-eyes (w) and a gene for miniature wings (m) occur on the X-chromosome. Cross female white-miniature (wm/wm) & wild type male (w+m+/Y): wm x w+m+  w+m+ & wm wm (Y) F 1  wild type (w+m+/wm) females and white-eyed miniature wing (wm/ Y) males. Genetic Mapping

35. Discovery of genetic linkage in Drosophila (Thomas H. Morgan, ~1911) F 1 x F 1  w+m+ xwm wm (Y) F 2 “parental” genotypes and phenotypes (same allele states as F 1 ): w+m+wild-type female(n = 439) wm w+m+wild-type male(n = 352) (Y) wmwhite-eye/miniature female (n = 359) wm wmwhite-eye/miniature male (n = 391) (Y) Genetic Mapping

36. Morgan also observed F 2 non-parental genotypes and phenotypes: w+mwild-type eye/miniature female(n = 235) wm w+mwild-type eye/miniature male(n = 210) (Y) wm+white-eye/wild-type wing female (n = 218) wm wm+white-eye/wild-type wing male (n = 237) (Y) F 1 x F 1  w+m xwm wm+ (Y) Non-parental combinations of linked genes are called recombinants. 50% recombinant phenotypes are expected if independent assortment occurs. Morgan observed 900/2,441 (36.9%) recombinant phenotypes and concluded that the two genes must be linked. Genetic Mapping

38. Mechanism of crossing-over gives rise to recombinant (non-parental) genotypes and phenotypes for linked genes. The distance between gene loci on a chromosome determines the degree of genetic linkage

39. 39 Terminology Pleiotropy A gene that affects more than one characteristic of an individual Sickle-cell (incomplete dominance) Codominance More than one allele is fully expressed ABO blood type (multiple allelic traits) Epistasis A gene at one locus interferes with the expression of a gene at a different locus Human skin color (polygenic inheritance)

40. 40 Review Blending Inheritance Monohybrid Cross Law of Segregation Modern Genetics Genotype vs. Phenotype Punnett Square Dihybrid Cross Law of Independent Assortment Human Genetic Disorders

41. MCAT Prep Exam PowerPoint® Lecture Slides are prepared by Dr. Isaac Barjis, Biology Instructor Genetics Lecture 5 41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parents eggs Ee spem Punnett square Offspring Ee E e Ee EE ee TTtt t T Tt eggs sperm Offspring Tt T t TT Tt tt