Mendelian Inheritance Chapter 11

Mendelian Inheritance 2Outline Blending Inheritance Monohybrid Cross  Law of Segregation Modern Genetics  Genotype vs. Phenotype  Punnett Square Dihybrid Cross  Law of Independent Assortment Human Genetic Disorders

3 Gregor Mendel

Mendelian Inheritance 4 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

5 Fruit and Flower of the Garden Pea

6 Mendel’s Monohybrid Crosses: An Example

7 Garden Pea Traits Studied by Mendel

Mendelian Inheritance 8 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

9 Mendel’s Monohybrid Crosses: An Example

Mendelian Inheritance 10 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

Mendelian Inheritance 11 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 Fertilization gives the offspring two factors for each trait

Mendelian Inheritance 12 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

13 Homologous Chromosomes

Mendelian Inheritance 14 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

Mendelian Inheritance 15 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

16 Punnett Square Showing Earlobe Inheritance Patterns

17 Try this one MONOHYBRID CROSS Cross a heterozygous tall plant with a heterozygous tall plant (use T = tall and t = short) Determine expected genotype and phenotype ratios.

Mendelian Inheritance 18 Monohybrid Testcross 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

Mendelian Inheritance 19 One-Trait Test Cross Unknown is Heterozygous

Mendelian Inheritance 20 One-Trait Test Cross Unknown is Homozygous Dominant

Mendelian Inheritance 21 IS MY DOG A PURE BREED? Do a test cross: G = golden retriever g = other g = other

Mendelian Inheritance 22 Not so much!

Mendelian Inheritance 23 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

Mendelian Inheritance 24 Try Mendel’s Classic Dihybrid Cross Cross two heterozygous tall, heterozygous green pod producing plants. Use a punnett square to show expected offspring and complete a phenotype ratio. Key: T = tallG = green pods t = shortg = yellow pods

25 Two-Trait (Dihybrid) Cross

26 Hartford’s example

27 Rat Answers

28 Two-Trait Test Cross

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32 SOLVING GENETICS PROBLEMS Rules of Probability *segregation and independent assortment of alleles during meiosis and fertilization are random events *if we know genotype of parents we can predict the EXPECTED genotypes of offspring using rules of probability

33 Probability Scale Scale range: 0-1Scale range: 0-1 The probability of all possible outcomes for an event must add up to 1The probability of all possible outcomes for an event must add up to 1

34 Common Examples EVENTPROBABILITY *tossing heads w/normal coin………………1/2 *tossing tails w/normal coin………………1/2 1

35 Try a common one Probability of rolling 3 on a six-sided die? 1/6 Probability of rolling a number other than 3? 5/6

36 Random events are independent of one another! If a couple has 6 sons and tries for the little girl: there is still a ½ chance of getting their wish. Regardless of the past events!

37 LAWS OF PROBABILITY * Rule of multiplication *Rule of addition

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39 Ex 3 - monohybrid Q: In a Mendelian cross between two heterozygous pea plants (Tt), what is the probability that the offspring will be homozygous recessive? A: egg w/ t = ½sperm w/ t = ½ overall probability that two will unite is ½ x ½ = ¼ overall probability that two will unite is ½ x ½ = ¼

40 Ex 4. Dihybrid cross Q: For YyRr x YyRr, what is the probability of an F2 plant having genotype YYRR? A: prob. of egg w/ YR = ½ x ½ = ¼ prob. of sperm w/ YR = ½ x ½ = ¼ prob. of sperm w/ YR = ½ x ½ = ¼ Overall prob. for offspring: ¼ x ¼ = 1/16 ¼ x ¼ = 1/16

41 Ex. 5 Trihybrid Q: What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc produce an AABBCc offspring? A: Look at each allele pair separately Aa x Aa: prob. of AA = ¼ Bb x Bb: prob. of BB = ¼ Cc x Cc: prob. of Cc= ½ Overall that this will occur simultaneously: ¼ x ¼ x ½ = 1/32

42 Rule of Addition The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways. Think heterozygotes!

43 ¼ + ¼ = 1/2

44 Ex. 2: monohybrids Q: In a Mendelian cross between pea plants that are heterozygous (Pp), what is the probability of the offspring being heterozygous? A: there are two different ways for this to occur- 1) sperm gives P and egg gives p OR 2) egg gives P and sperm gives p

45So… 1)sperm gives P and egg gives p ½ X ½ = ¼ 2) egg gives P and sperm gives p ½ X ½ = ¼ OVERALL PROB. of way 1 or way 2 OVERALL PROB. of way 1 or way 2 ¼ + ¼ = ½

46 EX 3 - Dihybrid What is the probability that two parents heterozygous for both height and flower color will produce HETEROZYGOUS tall offspring with purple flowers?

47 Ex 4 Trihybrid Q: What is the probability that a trihybrid cross between two organisms with genotypes AaBbCc and AaBbCc produce an aabbcc offspring? A: Look at each allele pair separately Aa x Aa: prob. of aa = ¼ Bb x Bb: prob. of bb = ¼ Cc x Cc: prob. of cc= ¼ Overall that this will occur simultaneously: ¼ x ¼ x ¼ = 1/64

48 Ex. 5 Trihybrid Q: PpYyRr x Ppyyrr What is the chance that the offspring will show at least two recessive phenotypes out of the three traits? A: break down the possibilities of all the genotypes that are homozygous recessive for at least two traits: ppyyRr, ppYyrr, Ppyyrr, PPyyrr, and ppyyrr

49 PpYyRr x Ppyyrr ppyyRr¼ x ½ x ½ = 1/16 ppYyrr¼ x ½ x ½ = 1/16 Ppyyrr½ x ½ x ½ = 2/16 PPyyrr¼ x ½ x ½ = 1/16 ppyyrr¼ x ½ x ½ = 1/16 6/16 or 3/8 chance of an offspring showing at least 2 of the recessive traits

50 ANALYZING GENETIC DISORDERS CHAPTER 11 – autosomal disorders CHAPTER 12 – sex-linked and gene- linked and trisomy/monosomy

Mendelian Inheritance 51 Human Genetic Disorders Autosome - Any chromosome other than a sex chromosome 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

52 Autosomal Recessive Pedigree Chart

53 Example: albinism

54 Autosomal Dominant Pedigree Chart

Mendelian Inheritance 55 Autosomal Recessive Disorders Tay-Sachs Disease  Progressive deterioration of psychomotor functions Cystic Fibrosis  Mucus in bronchial tubes and pancreatic ducts is particularly thick and viscous Phenylketonuria (PKU)  Lack enzyme for normal metabolism of phenylalanine

56 Cystic Fibrosis Therapy

Mendelian Inheritance 57 Autosomal Dominant Disorders Neurofibromatosis  Tan or dark spots develop on skin and darken  Small, benign tumors may arise from fibrous nerve coverings Huntington Disease  Neurological disorder  Progressive degeneration of brain cells ­Severe muscle spasms ­Personality disorders

58 A Victim of Huntington Disease

59 Huntington Disease: Normal and Diseased Brain

60 Sex-linked – disorders carried on the X chromosome -Colorblindness -Hemophilia -Baldness (?)

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Mendelian Inheritance 62 Incomplete and CoDominance Incomplete - Heterozygote has phenotype intermediate between that of either homozygote (think pink) Phenotype reveals genotype without test cross ADD to Notes – CoDominance – heterozygote phenotype shows both alleles, equal dominance (think spots)

63 Incomplete Dominance

64 ROAN COW - MOO

Mendelian Inheritance 65 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 Phenotype (Blood Type) Genotype A (actually AA or AO) I A I A or I A i B (actually BB or BO) I B I B or I B i AB IAIBIAIBIAIBIAIB O (actually OO) ii

66 Inheritance of Blood Type

67 Blood Type Mystery

68 QUICK REVIEW Incomplete vs. Codominance Mendel proposed independent assortment (not always the case) * linked genes (w/crossing over at times) Mendel proposed complete dominance (not always the case) * incomplete and codominance occurs

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70 AB blood type is also example of codominance

71 Blood Type: good for complete dom., codominance and multiple alleles

72 Complete the chart

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Mendelian Inheritance 76 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

77 Height in Human Beings

78 Frequency Distributions in Polygenic Inheritance

Mendelian Inheritance 79TerminologyPleiotropy  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)

Mendelian Inheritance 80Pleiotropy

Mendelian Inheritance 81 Epistasis – disrupts the expected

82 Environment and Phenotype: Himalayan Rabbits

Mendelian Inheritance 83Review Blending Inheritance Monohybrid Cross  Law of Segregation Modern Genetics  Genotype vs. Phenotype  Punnett Square Dihybrid Cross  Law of Independent Assortment Human Genetic Disorders

Mendelian Inheritance Ending Slide Chapter 11