Modifications of Mendelian Ratios - Biology Lecture Notes PDF

BIOL 239 Modifications of Mendelian Ratios Textbook 2.1-2.3 4.4 (last part only, “Autosomal genes contribute..”) Please read the weekly lesson pages Important! Online learning activities and SmartBook bonus are NOT the same thing  Lec...

BIOL 239 Modifications of Mendelian Ratios Textbook 2.1-2.3 4.4 (last part only, “Autosomal genes contribute..”) Please read the weekly lesson pages Important! Online learning activities and SmartBook bonus are NOT the same thing  Lecture recordings Are not guaranteed to be available or of good quality Please don’t waste your time! – Don’t listen to lectures over and over again Would you read a whole textbook chapter twice? – Do review specific things you missed the first time More study tips provided in the first lecture set and on LEARN  Mendel’s experiments Looked at traits that each have 2 variations Simple dominant-recessive alleles Most traits are more complex  This week Multiple alleles Co-dominance Incomplete dominance Pleiotropy (one gene, multiple traits) Lethal alleles Additive genes Epistasis Complex traits  Lesson-level learning objectives Explain the terms on the previous slide. Given an inheritance pattern, determine phenotypic ratios or probabilities of offspring and gametes. Given phenotypes of parents and offspring, determine possible inheritance patterns. Select breeding experiments to determine genotypes or inheritance patterns. Explain how ABO blood type is inherited. Define new key terms from this unit 7  Multiple alleles There can be more than 2 alleles for any 1 gene Note; When dealing with multiple alleles, the wild- type allele is designated with a superscript A+ A gene may have more than two alleles Reciprocal crosses can be conducted between pure- breeding lines representing all phenotypes, to establish the dominance relationships between all possible pairs of alleles. This reveals a dominance series, in which alleles are listed in order from dominant to recessive. The gene for coat colour in rabbits is a monomorphic gene -there is only 1 wildtype allele Breeds – selectively bred for rare mutant alleles  A dominance series: Four alleles for coat pattern Superscript + indicates wild-type allele Chinchillas are rodents related to rabbits from South America. The chinchilla coat colour in rabbits is named after these critters. Crosses using homozygotes to establish dominance series Pure-breeding lines! Dominance series: c+ > ch > c  ABO Blood types Gene I encodes enzyme that adds terminal sugar onto polymer chain Allele IA adds “A” sugar, allele IB adds “B” sugar, Allele i adds nothing Polymorphic gene, as 3 alleles are common  ABO Blood types IA IA or IA i = “A” sugar, type A IB IB or IB i = “B” sugar, type B IA I B = both “A” and “B”, type AB ii = none, type O  ABO Blood types IA and IB are each dominant over i IA and IB are codominant to each other  Co-dominance Contributions from both alleles are visible in the phenotype  Codominance Shows same genotype and phenotype ratio as incomplete dominance: 1:2:1 Neither is dominant or recessive to each other Dominance is not always complete ! Codominance: Alternative traits are both visible in the F1 hybrid Incomplete dominance: The F1 hybrid resembles neither purebred parent (intermediate phenotype)  Different dominance relationships Parent 1 phenotype Parent 2 phenotype AA aa Aa A is dominant to a Complete X a is recessive to A aa AA Aa A is dominant to a Complete X a is recessive to A A1A1 A2A2 A1A2 A1 and A2 are incompletely Incomplete X dominant A1A1 A2A2 A1A2 Codominant A1 and A2 are codominant X Incomplete dominance The heterozygote has an intermediate phenotype  Incomplete dominance Snap dragons  Incomplete dominance Snap dragons  Incomplete dominance Snap dragons Genotype and phenotype ratio is 1:2:1 Phenotype ratio is reflection of genotype ratio ! Now that we know the alleles are incompletely dominant, they can be re-designated to reflect this A1 A2 (as in textbook) or Rr Rw Where A is the gene for pigment Note You would never have to do a testcross to figure out genotype for phenotypes associated with incomplete dominance Because you can “see” the genotype heterozygote Incomplete dominance: The allele Rr allows production of the wild type red pigment The allele Rw produces white (lack of red pigment) When red (RrRr) plants are crossed with white (RwRw) plants the resulting F1 are pink (RrRw). This is because only one “dose” of red pigment is produced (from Rr ), resulting in pink offspring that do not resemble either of their parents. Note* it is proper to designate incompletely dominant or co-dominant alleles as both uppercase  Pleiotropy One gene influences multiple traits One gene may contribute to several visible characteristics Pleiotropy – Multiple phenotypic effects caused by a single gene Destruction of Circulatory Damage red blood cells blockages to organs Sickle cell syndrome: Anemia Kidney Spleen damage an example of failure Heart Physical pleiotropy weakness failure Brain Impaired damage mental function Paralysis  Lethal alleles A dominant or recessive allele can kill the organism  Recessive lethal alleles – Agouti gene Agouti allele (wildtype) = A (A+) -produces bands of yellow and black on each hair Yellow allele = AY -prevents deposition of black; hairs are yellow -yellow mice are always heterozygous Why? –because AYAY is lethal AY is a recessive lethal allele  Recessive lethal alleles – Agouti gene Phenotypic ratio of offspring from heterozygous parents is always 2:1 not 3:1; Homozygous AYAY die Allele for yellow coat is dominant for coat colour, but recessive for lethality (recessive lethal allele) Two heterozygotes for A+ and yellow (AY) produce only yellow and agouti offspring in 2:1 ratio  Manx cats - spinal development Homozygous fetus dies in utero All Manx cats are heterozygous for the Manx allele Manx allele is dominant for taillessness and recessive for lethality (recessive lethal allele) What happens if an allele is dominant for lethality? Both are examples of 1 allele affecting 2 different phenotypes (pleiotropy) Dominance for one phenotype does not mean it has to also be dominant for the others Don’t assume the wildtype allele is always dominant So far we looked at interactions between alleles of the same gene Now let’s look at interactions between different genes  Additive genes Two or more genes influence one trait  Epistasis One gene hides the effect of another gene Dominant epistasis e.g., Summer squash colour 12:3:1 phenotypic ratio indicates dominant epistatis C_ = white cc = yellow or green C (no colour) masks G (yellow) and gg (green) Allele C is epistatic to gene G  Recessive epistasis e.g., coat colour in Labrador retrievers black yellow chocolate Purebreeding Purebreeding x black yellow chocolate Purebreeding Phenotypic ratio 9:3:4 indicates recessive Epistasis B_ = black bb = brown E_ = expression of BE Be bE be gene B ee = no expression of BE BBEE BBEe BbEE BbEe gene B Be BBEe BBee BbEe Bbee ee is epistatic to bE BbEE BbEe bbEE bbEe gene B be BbEe Bbee bbEe bbee ee is always yellow, Phenotypic ratio 9:3:4 no matter what B/b is pale eyes and nose bbee BBee Can two blood type O parents have a type A child?  ABO Blood types Gene I encodes enzyme that adds terminal sugar onto polymer chain Gene H controls production of the polymer to which the A and B sugar attach!  Recessive epistasis e.g., h Bombay allele in blood type hh is epistatic to gene I IAiH_ H/h controls H allele production of lipid H to which Lipid A and B sugars stalk are attached A sugar (A antigen) hh is always B sugar (B antigen) No lipid stalk type O, no matter what I/i is ….YES Mother is IA_ hh type O (she has I A allele, but it does not express) Father is ii H_ type O Child is IAi Hh type A ….so much for paternity testing based on blood typing This can look like complementary gene action -phenotypic ratio of IAiHh x IAiHh = 9:7 type A to O but at molecular level, there is not just one product produced (like a pigment), there are 2 different products that are not working in a metabolic pathway e.g., the stalk and the sugar instead of a single pigment -you could not tell the difference without this information -your textbook calls complementation “reciprocal recessive epistasis” Complementary gene action Two or more genes can work in tandem, in the same biochemical pathway to produce a particular trait  Heterogeneous trait A mutation at any one of a number of genes can give rise to the same phenotype Complementary gene action - 2 genes Phenotypic ratio 9:7 Flower colour is a heterogeneous trait that can be complemented A B For heterogeneous traits: To determine if 1 gene or 2+ genes are involved in producing a particular phenotype, we must use complementation testing Complementation testing can also be used to determine if 2 individuals have mutations in the same gene  Mutations in different genes; complementation Parents Gene 1 Gene 2 Gene 1 Gene 2 X Offspring Gene 1 Gene 2 Gene 1 Gene 2 Mutation in the same gene; no complementation Parents Gene2 Gene2 A2 A3 X A2 A3 Offspring A2 A3  Complex traits A trait is determined by many genes or by the interaction between genes and environment Novel phenotypes can emerge from the combined action of the alleles of two genes e.g., comb shape in chickens  Comb Shape in Chickens Rose Pea Walnut Single Individual traits can be determined by more than one gene Phenotypic ratio 9:3:3:1 indicative of 2 genes responsible for comb shape acting independently in simple dominant and recessive manner Branching Pathway: F1 genotype = RrPp (all) F1 phenotype = All walnut F2 generation: RrPp x RrPp F2 phenotypic ratio F2 phenotypic ratio Combined F2 for Rr x Rr for Pp x Pp ratios 3/ 4 P_ 9/ 3/ 16 R_P_ = walnut 4 R_ 1/ 4 pp 3/ 16 R_pp = rose 3/ 3/ 1/ 4 P_ 16 rrP_ = pea 4 rr 1/ 1/ 4 pp 16 rrpp = single Notice: same phenotypic ratio as for dihybrid cross……. except only one trait (comb shape) is affected Here, two genes acting on two traits (phenotypes) Ratio of yellow (dominant) to green (recessive) = 12:4 or 3:1 Ratio of round (dominant) to wrinkled (recessive) = 12:4 or 3:1 Multifactorial inheritance – a phenotype arising from the action of two or more genes (polygenic), or from interactions between genes and the environment  The same genotype does not always result in the same phenotype influence of environment, modifier genes and chance Penetrance: percentage of the population with a particular genotype, that demonstrate the expected trait Expressivity: degree or intensity with which a particular genotype is expressed in a phenotype within a population The expression of the phenotype is NOT predictable!  Identical genotypes Complete penetrance Incomplete penetrance  Identical genotypes Constant expression Variable expression  Identical genotypes Incomplete penetrance and variable expression e.g., retinoblastoma - cancer of the retina - dominant allele - not all persons carrying the allele get disease - 75% penetrance (25% do not develop the disease) - of those who do get the disease, some get in it only one eye (expressivity) Sex-linked, sex-limited and sex- influenced traits X and Y chromosomes Sex-linked traits - due to genes ON the X or Y chromosome e.g., hemophilia and color blindness Due to genes that are NOT on sex chromosomes Sex-limited traits - affect a structure or process found in only one sex e.g., bright plumage in male birds, milk production, horns/antlers Sex-influenced traits - show up in both sexes but their expression may differ between the sexes e.g., patterned baldness  androgenic alopecia (male patterned baldness) - loss of hair from forehead and crown - affects ~70% of men, but not women - highly heritable, but not in a mendelian fashion - multiple autosomal and X-chromosome genes are associated with male patterned baldness Due to genes that are NOT (always) on sex chromosomes  Environment Factors such as temperature, light, and altitude can affect the phenotypic expression of a genotype The environment can affect the phenotypic expression of a genotype Himalyan coat pattern in cats, rabbits Enzyme non-functional Enzyme functional FYI: tyrosinase allele with temperature-sensitive expression Conditional lethality – when an allele is lethal under only certain conditions permissive vs. restrictive conditions (lives) (dies) Deciding between hypotheses  white brown True-breeding Deciding between different hypotheses using specific breeding tests  white brown True-breeding Deciding between different hypotheses using specific breeding tests New phenotype of F1 progeny indicates more than 1 gene is involved OR incomplete dominance of alleles in 1 gene white brown Deciding between different hypotheses using specific breeding tests X New phenotype of F1 progeny indicates more than 1 gene is involved OR incomplete dominance 4 of alleles in 1 gene  Deciding between different hypotheses using specific breeding tests X New phenotype of F1 progeny indicates more than 1 gene is involved OR incomplete dominance 4 of alleles in 1 gene Ratio indicates recessive epistasis Now that we know the inheritance pattern, what’s the actual genotype of an F2 white mouse? X 4 Solve for B (do a test cross) X 4 A pure-breeding white goat and a pure-breeding black goat are bred together several times and all offspring are grey. This result could indicate: a) complete dominance b) codominance c) incomplete dominance A pure-breeding white goat and a pure-breeding black goat are bred together several times and all offspring are grey. This result could indicate: a) complete dominance b) codominance c) incomplete dominance Is colouring in Zebras a result of codominance? The cow game What is the mode of inheritance of coat colour? Hint: -offspring has a different phenotype than the parents -the parents cannot be pure-breeding -parents must be heterozygous What is your hypothesis for the mode of inheritance? Hypothesis: Grey CWCB is heterozygous incomplete dominance phenotype; black is homozygous CBCB or Grey Gg is heterozygous dominant phenotype; black is homozygous recessive gg Hypothesis: Grey CWCB is heterozygous incomplete dominance phenotype; black is homozygous CBCB or Grey Gg is heterozygous dominant phenotype; black is homozygous recessive gg How can you prove which it is? (think about ratios) Could breed the parents several times and look at phenotypic ratios 3:1 = complete dominance (grey dom, black rec) 1:2:1 = incomplete dominance Can you do a back-cross? CWCB x CBCB = ??? Gg x gg = ??? Another case: What is the mode of inheritance of coat colour ? Hypothesis: Grey CWCB is heterozygous incomplete dominance phenotype or Grey Gg is heterozygous dominant phenotype or Greys are GG homozygous, pure-breeding parents What is the mode of inheritance of coat colour? What is the mode of inheritance of coat colour? New phenotype Note the phenotypic ratio Hypothesis: Grey CWCB is heterozygous incomplete dominance phenotype or Grey Gg is heterozygous dominant phenotype or Greys are GG homozygous, pure-breeding parents What is the mode of inheritance of coat colour?  What is the mode of inheritance of coat colour? 1:2:1 = codominance or incomplete dominance Which is it?  What is the mode of inheritance of coat colour? 1:2:1 = codominance or incomplete dominance Which is it?  HOWEVER An average cow will only produce 8 offspring in its life What if this one had not been born? You don’t always get the expected ratios if there are small numbers of offspring