Mendelian genetics Its all about jargon ratios and

Mendelian genetics It’s all about jargon, ratios, and nomenclature

Mendel’s peas fig. 2 -3 advantages of peas: self pollination and cross-pollination - chose characters with only two states, or phenotypes - created pure-bred lines

-cross-pollinated parental generation (P) to create first filial (F 1) generation -also did reciprocal crosses first filial (F 1) generation were all the same figs. 2 -4 and 2 -5

self-crossed F 1 to obtain second filial (F 2) generation – ‘missing’ type reappeared – all progeny in 3: 1 ratio deduced presence of dominant and recessive traits

Mendel’s postulates 1. there are hereditary ‘particles’ that determine traits (genes) 2. genes are in pairs (alleles) 3. members of a gene pair segregate equally into the gametes 4. each gamete carries only one allele 5. gametes combine at random with respect to allele type heterozygotes have different alleles homozygotes have same alleles

a bit of notation… dominant trait takes capital letter; recessive is lower case A/a letter is determined by phenotype of the dominant trait yellow peas are dominant, therefore Y/y

a bit of notation… dominant trait takes capital letter; recessive is lower case A/a letter is determined by phenotype of the dominant trait yellow peas are dominant, therefore Y/y Alternative notation wild type trait is +; mutant is or a+/aa’/a dominant genotype (when we don’t know what the second allele is) can be abbreviated a+/– this includes a+/a+ and a+/a- genotypes

What causes dominance? Genes code for proteins Proteins are either structural or functional (enzymes) An ‘error’ in the genetic code may yield a non-functional enzyme, or no protein, or a less efficient enzyme

Punnett square is used to work out progeny of a cross P Gametes F 1 Figure 2 -6 Overall F 2 ratio

genotypic ratio phenotypic ratio yellow Y green y yellow Y Y/y green y Y/y y/y ratio = 3 yellow : 1 green yellow Y green y yellow Y Y/y green y Y/y y/y ratio = 1 YY: 2 Y/y : 1 y/y note: Y/y = y/Y

Probability rules: Multiplication rule: the probability of two independent outcomes occurring simultaneously is equal to the product of each of the two outcomes taken separately. independent probabilities can be multiplied what is the probability of a litter of three being all male?

Probability rules: Multiplication rule: the probability of two independent outcomes occurring simultaneously is equal to the product of each of the two outcomes taken separately. independent probabilities can be multiplied what is the probability of a litter of three being all male? probability than any one offspring will be male = ½ probability that all three will be male – ½ x ½ = 1/8

Probability rules: Addition rule: the overall probability of any combination of mutually exclusive outcomes is equal to the sum of the probabilities of the outcomes taken separately. mutually exclusive probabilities can be added What is the probability that a litter of three will have at least one female? 0 females = ½ x ½ = 1/8 1 female = ½ x ½ = 1/8 x 3 = 3/8 2 females = ½ x ½ = 1/8 x 3 = 3/8 3 females = ½ x ½ = 1/8 sum of probabilities containing a female = 7/8

dihybrid cross (more ratios) F 1 – round, yellow X wrinkled, green = R/R, Y/Y X round, yellow R/r, Y/y r/r, y/y

dihybrid cross (more ratios) R/r, Y/y R/Y R/r, Y/y R/y r/Y r/y

dihybrid cross (more ratios) R/r, Y/y R/Y R/r, Y/y R/y r/Y r/y R/R: Y/Y R/R: Y/y R/r: Y/Y R/r: Y/y R/R: Y/y R/R, y/y R/r: Y/y R/r: y/y r/Y R/r: Y/y r/r: Y/Y r/r: Y/y round, yellow round, green wrinkled, yellow r/y R/r: Y/y R/r, y/y r/r: Y/y r/r: y/y wrinkled, green

dihybrid cross (more ratios) R/r, Y/y R/Y R/r, Y/y R/y r/Y r/y R/R: Y/Y R/R: Y/y R/r: Y/Y R/r: Y/y R/R: Y/y R/R, y/y R/r: Y/y R/r: y/y r/Y R/r: Y/y r/r: Y/Y r/r: Y/y round, yellow round, green wrinkled, yellow r/y R/r: Y/y R/r, y/y r/r: Y/y r/r: y/y wrinkled, green phenotype ratio: 9 round, yellow 3 round, green 3 wrinkled, yellow 1 wrinkled, green

Fig. 2 -10

Gene interactions • one gene may affect several phenotypes (pleiotropy) - PKU (phenylketonuria): single gene for enzyme phenylalanine hydroxylase) that converts phenylalanine to tyrosine; - loss of function results in mental retardation, lower pigmentation, additional traits • several genes may produce the same phenotype - importance of duplicate genes

Gene interactions incomplete dominance – heterozygote has intermediate phenotype RR = red flower Rr = pink flower rr = white flower usually found when phenotype is a continuous trait (quantitative) e. g. , weight, height, fecundity, amount of enzyme produced alternative is discrete traits e. g. , round vs. wrinkled, yellow vs. green peas

Gene interactions co-dominance – intermediate phenotype is formed when two dominant alleles are present in heterozygote - characterized by having three phenotypes genotype A/A and A/i B/B and B/i i/i A/B blood type A produces A antigen B produces B antigen O produce neither antigen AB produce both antigens

Gene interactions Note that terms are somewhat arbitrary: depend on level of analysis RR = red flower Rr = pink flower rr = white flower Incomplete dominance if R allele produces pigment, r does not Co-dominance if R produces red pigment, r produces white pigment A better example is roan cattle: RR – red hair rr – white hair Rr – red AND white hairs

Gene interactions epistasis – two or more genes interact to form a phenotype 9 3 3 1 genotype flower color W/-, M/blue 9 W/-, m/m magenta 3 w/w, M/white w/w, m/m white 4 Fig. 4 -13

Gene interactions sickle-cell anemia: -single gene mutation (SNP) -affects hemoglobin configuration, which distorts RBCs -malaria parasite cannot digest altered hemoglobin genotype A/A A/S S/S malaria phenotype no anemia; normal blood cells no anemia; sickle only with low O 2 severe or fatal anemia with sickle cells sickle cell anemia

Gene interactions sickle-cell anemia: -single gene mutation (SNP) -affects hemoglobin configuration, which distorts RBCs -malaria parasite cannot digest altered hemoglobin genotype A/A A/S S/S phenotype no anemia; normal blood cells no anemia; sickle only with low O 2 severe or fatal anemia with sickle cells dominant: reccessive for expression of anemia incomplete dominance for cell shape codominance for production of hemoglobin

ratios so far: 1: 3 (phenotypic result of a mono-hybrid cross in dominance : recessive traits) 1: 2: 1 (genotypic result of a mono-hybrid cross) and phenotypic result for incomplete dominant traits 9: 3: 3: 1 (dihybrid cross in dominance : recessive traits) 1: 2 if lethal allele is present A/A ¼ A/a 2/4 a/a ¼ - but they are all dead 15: 1 (dihybrid cross with duplicate genes)

penetrance = whether genotype is expressed in phenotype due to modifiers, epistatic genes, suppression expressivity = degree to which genotype is expressed in the phenotype due to other alleles, or environment Fig. 4 -23

was Mendel honest? all dominant: recessive traits only two alleles at each gene no gene interactions no sex-linked traits all independent traits (no linkage; pea has 7 chromosomes)

How do we evaluate all this? ? (how close must the data be to the ratios? ) Chi square = Χ 2 = (observed – expected)2 expected Monohybrid cross with incomplete dominance: dev. from exp. phenotype observed expected (obs-exp) red 19 25 -6 pink 57 50 7 white 24 25 -1 total 100 degrees of freedom = 2 (= N – 1) (obs-exp)2 exp 1. 44 0. 98 0. 04 2. 46 = Χ 2

p = probability of obtaining the statistic by random chance