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Mendelelian Genetics copyright cmassengale 1 A Tale of Two Families Modes of inheritance are the rules explaining the common patterns of inheritance. Huntington’s Disease and Cystic Fibrosis demonstrate two important modes of inheritance. HD is autosomal dominant and CF is autosomal recessive. copyright cmassengale 2 Fig. 4.1 Autosomal Dominant – affects both sexes and appears every generation Fig. 4.2 Autosomal Recessive – affects both sexes and skip generations through carriers Gregor Mendel (1822-1884) Responsible for the Laws governing Inheritance of Traits copyright cmassengale 5 Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until the turn of the 20th century copyright cmassengale 6 Gregor Johann Mendel Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents Called the “Father of Genetics" copyright cmassengale 7 Site of Gregor Mendel’s experimental garden in the Czech Republic copyright cmassengale 8 Particulate Inheritance Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA copyright cmassengale 9 Genetic Terminology Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity copyright cmassengale 10 Types of Genetic Crosses Monohybrid cross - cross involving a single trait e.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height copyright cmassengale 11 Punnett Square Used to help solve genetics problems copyright cmassengale 12 copyright cmassengale 13 Designer “Genes” Alleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) copyright cmassengale 14 More Terminology Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) copyright cmassengale 15 Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes RR Rr rr Phenotypes RED RED YELLOW copyright cmassengale 16 Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid copyright cmassengale 17 Genes and Environment Determine Characteristics copyright cmassengale 18 Mendel’s Pea Plant Experiments copyright cmassengale 19 Why peas, Pisum sativum? Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially cross-pollinated copyright cmassengale 20 Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers copyright cmassengale 21 Mendel’s Experimental Methods Mendel hand-pollinated flowers using a paintbrush He could snip the stamens to prevent self-pollination Covered each flower with a cloth bag He traced traits through the several generations copyright cmassengale 22 How Mendel Began Mendel produced pure strains by allowing the plants to selfpollinate for several generations copyright cmassengale 23 Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) copyright cmassengale 24 copyright cmassengale 25 copyright cmassengale 26 Mendel’s Experimental Results copyright cmassengale 27 Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the theoretical ratio copyright cmassengale 28 Generation “Gap” Parental P1 Generation = the parental generation in a breeding experiment. F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P1 generation F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation copyright cmassengale 29 Following the Generations Cross 2 Pure Plants TT x tt Results in all Hybrids Tt Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt copyright cmassengale 30 Monohybrid Crosses copyright cmassengale 31 P1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr r r R Rr Rr R Rr Rr Genotype: Rr Phenotype: Round Genotypic Ratio: All alike Phenotypic Ratio: All alike copyright cmassengale 32 P1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F1 generation Genotypic & Phenotypic ratio is ALL ALIKE copyright cmassengale 33 F1 Monohybrid Cross Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr R r R RR Rr r Rr rr Genotype: RR, Rr, rr Phenotype: Round & wrinkled G.Ratio: 1:2:1 P.Ratio: 3:1 copyright cmassengale 34 F1 Monohybrid Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1 copyright cmassengale 35 What Do the Peas Look Like? copyright cmassengale 36 …And Now the Test Cross Mendel then crossed a pure & a hybrid from his F2 generation This is known as an F2 or test cross There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid copyright cmassengale 37 F2 Monohybrid Cross st (1 ) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr R r R RR Rr R RR Rr Genotype: RR, Rr Phenotype: Round Genotypic Ratio: 1:1 Phenotypic Ratio: All alike copyright cmassengale 38 F2 Monohybrid Cross (2nd) Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr R r r Rr Rr r rr rr Genotype: Rr, rr Phenotype: Round & Wrinkled G. Ratio: 1:1 P.Ratio: 1:1 copyright cmassengale 39 F2 Monohybrid Cross Review Homozygous x heterozygous(hybrid) Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr Phenotypic Ratio is 1:1 Called Test Cross because the offspring have SAME genotype as parents copyright cmassengale 40 Practice Your Crosses Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits copyright cmassengale 41 Mendel’s Laws copyright cmassengale 42 Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same copyright cmassengale 43 Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) copyright cmassengale 44 Law of Dominance copyright cmassengale 45 Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. copyright cmassengale 46 Applying the Law of Segregation copyright cmassengale 47 Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses. copyright cmassengale 48 Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel’s “Law of Independent Assortment” a. Each pair of alleles segregates independently during gamete formation b. Formula: 2n (n = # of heterozygotes) copyright cmassengale 49 Question: How many gametes will be produced for the following allele arrangements? Remember: 2n (n = # of heterozygotes) 1. RrYy 2. AaBbCCDd 3. MmNnOoPPQQRrssTtQq copyright cmassengale 50 Answer: 1. RrYy: 2n = 22 = 4 gametes RY Ry rY ry 2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd aBCD aBCd abCD abCD 3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes copyright cmassengale 51 Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations copyright cmassengale 52 Dihybrid Cross RY Ry rY ry RY Ry rY ry copyright cmassengale 53 Dihybrid Cross RY RY RRYY Ry RRYy rY RrYY ry RrYy Ry rY ry RRYy RrYY RrYy RRyy RrYy Rryy RrYy rrYY rrYy Rryy rrYy rryy copyright cmassengale Round/Yellow: Round/green: 9 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 phenotypic ratio 54 Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1 copyright cmassengale 55 Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bbC__ x bbcc BB = brown eyes Bb = brown eyes bb = blue eyes CC = curly hair Cc = curly hair cc = straight hair bC b___ bc copyright cmassengale 56 Test Cross Possible results: bc bC b___ C bbCc bbCc or bc copyright cmassengale bC b___ c bbCc bbcc 57 Summary of Mendel’s laws LAW DOMINANCE SEGREGATION INDEPENDENT ASSORTMENT PARENT CROSS OFFSPRING TT x tt tall x short 100% Tt tall Tt x Tt tall x tall 75% tall 25% short RrGg x RrGg round & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods copyright cmassengale 58 Single-gene Inheritance in Humans 1. A Mendelian trait is caused by a single gene. 2. Mendelian human conditions are considered more rare than multifactorial ones 3. Mendelian inheritance in humans is more complex than in model organisms such as pea plants. copyright cmassengale 59 Eye color The inheritance of eye color is an example of a single gene trait copyright cmassengale 60 Modes of Inheritance Modes of inheritance – rules explaining common patterns of inheritance Genes may be autosomal or on a sex chromosome Alleles can be dominant or recessvie copyright cmassengale 61 copyright cmassengale 62 Criteria for an autosomal dominant trait: 1. Males & females can be affected; male to male transmission can occur 2. Both transmit the trait with equal frequency 3. Successive generations are affected. 4. Transmission stops after a generation where no one is affected. copyright cmassengale 63 Criteria for an autosomal recessive trait 1. Males & females can be affected 2. Both can transmit the gene, unless it causes death before reproductive age 3. The trait can skip generations 4. Parents of an affected individual are heterozygous or have the trait. copyright cmassengale 64 Modes of inheritance, cont. 1. Blood relatives that have children together have a much higher risk of having a child with a rare recessive disorder. 2. Marriage between relatives introduces consanguinity, literally meaning “shared blood” However genes are not passed in blood 3. Punnett squares apply Mendel’s first law to predict recurrence risks of inherited disorders or traits. 4. A Mendelian trait applies anew to each child. copyright cmassengale 65 On the Meaning of Dominance & Recessiveness At the biochemical level, recessive disorders often result from alleles that cause a loss of function or loss of a normal protein Dominant disorders can result from production of an abnormal protein that interferes with the function of a normal protein or imparts a gain of function. copyright cmassengale 66 Recessive disorders Recessive orders tend to be more severe producing symptoms earlier than dominant disorders Disease causing recessive alleles remain in populations because heterozygotes pass them to future generations. copyright cmassengale 67 Dominant disorders If a dominant mutation that harms early in life arises people who have the allele are too ill or don’t survive long enough to reproduce The allele becomes rare in the population unless it arises anew by mutation Dominant disorders that do not appear until adulthood or drastically disrupt health, remain in the population until after a person has reproduced. copyright cmassengale 68 Pedigree Analysis Pedigree charts depict family relationships and transmission of inherited traits. Squares represent males and circles represent females. Horizontal lines indicate parents, vertical lines show generations, and elevated lines depict siblings. Symbols for heterozygotes are half-shaded, and for an individual with a particular phenotype, completely shaded copyright cmassengale 69 copyright cmassengale 70 Pedigrees Then and Now Pedigrees have been in use since antiquity. They have been used throughout history to show family relationships, particularly royal bloodlines. copyright cmassengale 71 copyright cmassengale 72 Pedigrees Display Mendel’s Laws Pedigrees can reveal mode of inheritance, and can suggest molecular information, carrier status, and input from other genes and the environment. Interpretation of pedigrees can be inconclusive when more than one mode of inheritance can explain the pattern seen. copyright cmassengale 73 Solving Genetics Problems List genotypes and phenotypes for the trait Determine the genotypes of the parents Possible gametes Possible genotypes of offspring Repeat for successive generations Independent Events The probability of simultaneous independent events = the product of the probability of each event Example: If both parents are heterozygous (Bb)) what is the probability that they will produce a BB child? Probability of a sperm with B allele = ½ Probability of a ova with B allele = ½ Probability of a BB child is ½ X ½ = ¼ Dependent Events The probability of dependent events = the sum of probability of each event Example •Parents are heterozygous for a trait, R. •What is the chance that their child carries at least one dominant R allele? •Probability of child carrying RR = ¼ •Probability of child carrying Rr = ½ •Probability of child carrying R_ = ¼ + ½ = ¾