Lecture 2 April 5, 2012 “Mendelian Genetics Continued” I. Monohybrid Cross - The SS is Homozygous - This explains the Phenotypic ration of 3 smooth : 1 wrinkled o But in actuality its 1:2:1 Punneett Square F1 Male Gametes Genotypic Ratio  Phenotypic Ratio F1 Female Gametes F1 Male Gametes F1 Femal GametesF2 = SS + Ss + ss 1 : 2 : 1 Smooth 3 1 wrinked Phenotypic Classes Genotypic Classes Smooth : Wrinkled SS : Ss : ss 3:1 1 : 2 : 1 - Mendel wanted to put his model to test by using a TestCross - Testcross o Mating between an individual of unknown genotype and a homozygous recessive individual. - The test Cross helps us see what the P_ is. (Whether it’s a PP or a Pp) Testcross • A particular trait (phenotype) is determined by a factor (Gene).• There are two forms of this factor (alleles). • Principle of dominance: When T and t co-exist in a plant, one observes the T phenotype. T: dominant t: recessive • Principle of segregation: The two alleles, T and t, separate (segregate) during gamete formation, then unite at random, one from each parent, at fertilization. However when we discuss about human populations its more complicated because… • Most traits are affected by more than one gene and cannot be analyzed using simple Mendelian genetics • Many single gene traits that can be followed cause rare diseases, rather than common phenotypes • Genetics is not straight forward: No pure breeding humans, Generation time is long, No controlled matings, Siblings rarely mate to generate F2 The Rules of Probability Porbability: # times event is expected to happen # Opportunities (trials) Roll dice probability of getting 5 is 1/6. Number of time event is (the number 5 is expected to happen 1 time and the # of opportunities is expected to happen 6 times) - so its 1/6 Product Rule: The probability of independent events occurring together is the product of the probabilities of the individual events. Probability of (A and B) is P(AB) = P(A) X P(B) If they are independent and want them to happen together at the same time, have to multiply Independent Event: outcome event of one event doesn’t affect the outcome of the other event What is the probability of drawing an Ace of Hearts from this deck of cards? Drawing Ace = 4/52 Ace of Heart (Two independent event)) 4/52*1/4 = 1/52 Sum Rule: If A and B are independent events, the probability that A or B occurs, P(A or B) is P[A + B] = P(A) + P(B) - [P(A) X P(B)] What is the probability of drawing an Ace or a Heart from this deck of cards? 4/52 + ¼ - (1/52 x ¼)Important Special Case of the Sum Rule: If A and B are mutual exclusive events, the probability one of them occur, P(A or B) is P[A + B] = P(A) + P(B) *Mutual Exclusive: Like flipping a coin What is the probability of drawing an Ace or a King from this deck of cards? 1/13 + 1/13 = 2/13 Principle of Independent Assortment in Dihybrid Cross Two Genes: P X RRyy rrYY round green wrinkled yellow F1 RrYy round yellow 315 round yellow F2 108 round green 101 wrinkled yellow 32 wrinkled green • The 9:3:3:1 ratio of a dihybrid cross can be predicted because we can consider each trait separately.-Monohybrid phenotypes: 3/4 round and 1/4 wrinkled 3/4 yellow and 1/4 green -Dihybrid phenotypes (product rule): Round and yellow: R_Y_ = 3/4 X 3/4 = 9/16 Wrinkled and yellow: rrY_ = 1/4 X 3/4 = 3/16 Round and green: R_yy = 3/4 X 1/4 =3/16 Wrinkled and green: rryy = 1/4 X 1/4 = 1/16 Why 9:3:3:1? - Multiply ratio of one tree to the other ratio. Multiply Color and Shape (They are two independent event) Two hypotheses for gene assortment in a dihybrid cross: - (a) Hypothesis: Dependent assortment (b) Hypothesis: Independent Assortment o Dependent Assortment is WRONG because when you look at the two traits (Color vs Shape), there are Four Different Results (RY, rY, Ry, and ry) o Independent Assortment is the correct hypothesisDihybrid Test Cross: - If the Independent Assortment Hypothesis is right, how many types of gamete F1 plant RrYy can produce? In what ratio? - The dihybrid should make four types of gametes in equal numbers: RY rY Ry ry - Result  1 : 1 : 1 : 1 ratio if the unknown parent is RrYy Summary One Trait (one gene) 3:1 Monohybrid cross F2 phenotypic ratio 1:1 Test cross phenotypic ratio Two traits (two genes) 9 : 3 : 3 : 1 Dihybrid cross F2 phenotypic ratio 1 : 1 : 1 : 1 Test cross phenotypic ratio - A particular trait (phenotype) is determined by a factor (Gene). • There are two forms of this factor (alleles). - Principle of dominance: When T and t co-exist in a plant, one observes the T phenotype. T: dominant t: recessive - Principle of segregation: The two alleles, T and t, separate (segregate) during gamete formation, then unite at random, one from each parent, at fertilization. - Principle of independent assortment: The alleles of different genes segregate (assort) independently during gamete formation. Pedigree analysis: a tool to study human genetics - Mendel’s principles apply to the inheritance of many human traits - Rare traits can be studied by carefully analyzing families that are affected. In other words, the trait may be rare in the population but common in a particular family. - First Row is called Generation 1 - Second Row is called Generation 2 - Sphere is female while cube is male and diamond is Fetus or sex unspecified. - Dark Color means affected, light color means normal - Horizontal line is mating and vertical line is offspringFamily Pedigree - Shows the history of a trait in a family - Allows researchers to analyze human traits - Important assumption for now: When a trait is rare, and there is a random mating, you can assume that a person who marries into the affected family is not a carrier. - carrier: person with one copy of the allele for a recessive disorder and does not exhibit symptoms - Many human traits show simple inheritance patterns and are controlled by genes on autosomes (chromosomes other than sex chromosomes X & Y). or non sex chromosome - Dominant human genetic disorder; affected individuals have extra