CLO1 Part 4 PDF - Basic Genetics

Activity (5 min) Lecture (15 min) Activity (5 min) Lecture (15 min) Opening Question (5min) CLO1 BASIC GENETICS/GENETICS SSCG 2753/SSCY 2733 WELCOME ...

Activity (5 min) Lecture (15 min) Activity (5 min) Lecture (15 min) Opening Question (5min) CLO1 BASIC GENETICS/GENETICS SSCG 2753/SSCY 2733 WELCOME  CLO1 Opening Question WELCOME Identify an exception of the Mendel Laws, the chromosomal basis of inheritance particularly sex determination, nondisjunction in human being and chromosome variations Activity (5 min) Activity (5 min) Lecture (15 min) Lecture (15 min) Describe the principles of the Mendel laws and an exception cases CLO1 SMBB 2753 (5min) such as pleiotropy and epistasis (L2) BASIC GENETICS  Opening Question WELCOME What is epistasis? Describe six types of epistasis in genetics. Dominant Epistasis (12:3:1) Opening Question Activity (5 min) Activity (5 min) Lecture (15 min) Lecture (15 min) Recessive Epistasis (9:3:4) CLO1 SMBB 2753 Duplicate genes with cumulative effect (9:6:1) (5min) Duplicate Dominant Gene (15:1) Duplicate recessive genes (9:7) BASIC GENETICS Dominant and recessive interaction (13:3) Activity (5 min) Lecture (15 min) Activity (5 min) CLO1 Epistasis BASIC GENETICS SMBB 2753 WELCOME Lecture (15 min) Opening Question (5min) CLO1  EPISTASIS Dominant epistasis Duplicate dominant Duplicate recessive (12:3:1) genes (15:1) genes (9:7) Recessive Duplicate genes Dominant and epistasis (9:3:4) with cumulative recessive effect (9:6:1) interaction (13:3) Epistatic Gene Interactions Gene interactions occur when two or more different genes influence the outcome of a single trait Most morphological traits (height, weight, color) are affected by multiple genes Epistasis describes situation between various alleles of two genes Quantitative loci is a term to describe those loci controlling quantitatively measurable traits Pleiotropy describes situations where one gene affects multiple traits Epistasis Epistasis The effect of one gene pair (locus) masks or modifies the effect of another gene pair Examples Recessive alleles at one locus override expression of alleles at another locus. Alleles at 1st locus are said to be epistatic to the masked hypostatic alleles at the 2nd locus Allele(s) at one locus may require specific allele at another locus, these pairs are said to complement each other Epistatic Gene Interactions examine cases involving 2 loci (genes) that each have 2 alleles Crosses performed can be illustrated in general by AaBb X AaBb Where A is dominant to a and B is dominant to b If these two genes govern two different traits A 9:3:3:1 ratio is predicted among the offspring simple Mendelian dihybrid inheritance pattern If these two genes do affect the same trait the 9:3:3:1 ratio may be altered 9:3:4, or 9:7, or 9:6:1, or 8:6:2 or 12:3:1, or 13:3, or 15:1 epistatic ratios Epistatic Gene Interaction Complementary gene action Enzyme C and enzyme P cooperate to make a product, therefore they complement one another Enzyme C Enzyme P Colorless Colorless Purple precursor intermediate pigment Epistatic Gene Interaction Epistasis describes the situation in which a gene masks the phenotypic effects of another gene Epistatic interactions arise because the two genes encode proteins that participate in sequence in a biochemical pathway If either loci is homozygous for a null mutation, none of that enzyme will be made and the pathway is blocked Enzyme C Enzyme P Colorless Colorless Purple precursor intermediate pigment genotype cc genotype pp Enzyme C Enzyme P Colorless Colorless Purple precursor intermediate pigment A Cross Involving a Two-Gene Interaction Can Still Produce a 9:3:3:1 ratio Inheritance of comb morphology in chicken First example of gene interaction William Bateson and Reginald Punnett in 1906 Four different comb morphologies The crosses of Bateson and Punnett Dominant epistasis (12:3:1) The crosses of Bateson and Punnett F2 generation consisted of chickens with four types of combs 9 walnut : 3 rose : 3 pea : 1 single Bateson and Punnett reasoned that comb morphology is determined by two different genes R (rose comb) is dominant to r P (pea comb) is dominant to p R and P are codominant (walnut comb) rrpp produces single comb Dominant When the dominant allele (A) produces a certain phenotype regardless of the allele condition of another locus (B), A is said to be epistatic to B. Epistasis The dominant allele A is able to express itself in the presence of either B or b. (12:3:1) Only when the genotype of the individual is homozygous recessive (aa), then B or b can be expressed. A-B- & A-bb produce the same phenotype; aaB- & aabb produce 2 additional phenotypes. For example: Coat colors of dogs I- inhibit coat color pigment / expression B represents black color coat b represents brown color coat P: IiBb (white) X IiBb (white) F1: IiBb Dominant Epistasis (12:3:1) F2 IB Ib iB ib IIBB, IIBb, IiBB, IiBb, IB white white white white IIBb, IIbb, IiBb, Iibb, Ib white white white white IiBB, IiBb, iiBB, iiBb, iB white white black black IiBb, Iibb, iiBb, iibb, ib white white black brown Epistasis of Involving Sex-linked Genes Inheritance of the Cream-Eye allele in Drosophila a rare fly with cream-colored eyes identified in a true- breeding culture of flies with eosin eyes possible explanations 1. Mutation of the eosin allele into a cream allele 2. Mutation of a 2nd gene that modifies expression of the eosin allele The Hypothesis Cream-colored eyes in fruit flies are due to the effect of a second gene that modifies the expression of the eosin allele Testing the Hypothesis cream allele is recessive to +  Interpreting the Data Cross Outcome P cross: Cream-eyed male X F1: all red eyes wild-type female F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F2 generation contains males with eosin eyes This indicates that the cream allele is not in the same gene as the eosin allele  Interpreting the Data Cross Outcome P cross: Cream-eyed male X F1: all red eyes wild-type female F1 cross: F1 brother X F1 sister F2: 104 females with red eyes 47 males with red eyes 44 males with eosin eyes 14 males with cream eyes F2 generation contains – 151 + eye: 44 we eye: 14 ca eye a 12 : 3 : 1 ratio  Modeling the Data ◼ Cream phenotype is recessive therefore the cream allele is recessive allele (either sex-linked or autosomal) ◼ The mutated allele of the cream gene modifies the we allele, while the wt cream allele does not ◼ C = Normal allele ◼ Does not modify the eosin phenotype ◼ ca = Cream allele ◼ Modifies the eosin color to cream, does not effect wt or white allele of white gene.  Modeling the Data Putative genotypes in a cross P : w+/ w+; C/C x we/Y; ca/ca Male gametes CXw+ caXw+ c aY F1 :w+/ we; C/ca & w+/Y; C/ca CY F2 :¾ C/_ x ¾ w+/_ ¼ we/Y CXw+ CCXw+Xw+ CCXw+Y cacaXw+Xw+CcaXw+Y ¼ ca/ca x ¾ w+/_ ¼ we/Y Female gametes 9/16 C/_ ; + CXw-e CCXw+Xw-e CCXw-eY CcaXw+Xw-e CcaXw-eY 3/16 ca/ca; + red 3/16 C/_ ; we eosin 1/16 ca/ca; we cream caXw+ CcaXw+Xw+ CcaXw+Y cacaXw+Xw+ cacaXw+Y 12:3:1 caXw-e CcaXw+Xw-e CcaXw-eY cacaXw+Xw-ecacaXw-eY Recessive Epistasis (9:3:4)  If the recessive genotype at locus A (eg: aa) suppresses the expression of alleles at B locus, locus A exhibit recessive epistasis Recessive over locus B. Epistasis The alleles in locus B can only be expressed with the presence of dominant (9:3:4) alleles at locus A. Genotypes A-B- & A-bb produce 2 additional phenotypes. Recessive Epistasis (9:3:4) F2 AB Ab aB ab AB AABB, AABb, AaBB, AaBb, purple purple purple purple For example: Flower color of peas Ab AABb, AAbb, AaBb, Aabb, purple red purple red A- codes for color pigment B codes for color purple aB AaBB, AaBb, aaBB, aaBb, b codes for color red purple purple white white ab AaBb, Aabb, aaBb, aabb, P: AAbb (red) X aaBB (white) purple red white white F1: AaBb (purple) Duplicate genes with cumulative effect (9:6:1)  Duplicate Genes with Cumulative Effect (9:6:1) Occur when dominant allele (homozygous or heterozygous) at either locus (but not both) produces the same phenotype. Genotypes A-bb & aaB- produce one unit each and therefore have the same phenotype. Genotype aabb produces no pigment but in genotype A-B- the effect is cumulative and 2 units of phenotypes are produced. Duplicate Genes with Cumulative Effect (9:6:1) F2 RB Rb rB rb RRBB, RRBb, RrBB, RrBb, RB red red red red For example: Color of wheat kernels RRBb, RRbb, RrBb, Rrbb, Rb R-B- produce red color red brown red brown rrbb produce white color RrBB, RrBb, rrBB, rrBb, rB red red brown brown Any other combination produces brown color RrBb, Rrbb, rrBb, rrbb, rb P: RRBB (red) X rrbb (white) red brown brown white F1: RrBb (red) Quiz Time  Duplicate Dominant Gene (15:1)  Duplicate Dominant Gene (15:1) The 9:3:3:1 ratio is modified if the dominant alleles of both loci each produce the same phenotype without cumulative effect. For example: Flower color of peas aabb codes for color white any other combination produce color red P: AAbb (red) X aaBB (red) F1 : AaBb (red)  Duplicate Dominant Gene (15:1) F2 AB Ab aB ab AB AABB, red AABb, red AaBB, red AaBb, red Ab AABb, red AAbb, red AaBb, red Aabb, red aB AaBB, red AaBb, red aaBB, red aaBb, red ab AaBb, red Aabb, red aaBb, red aabb, white Gene Interaction Duplicate gene action Enzyme 1 and enzyme 2 are redundant They both make product C, therefore they duplicate each other Duplicate recessive genes (9:7) A Cross Producing a 9:7 ratio 9 C_P_ : 3 C_pp :3 ccP_ : 1 ccpp purple white Duplicate Recessive Genes (9:7) When identical phenotypes are produced by both homozygous recessive genotypes, the F1 ratio becomes 9:7. The genotype aaB-, A-bb & aabb produce one phenotype. Both dominant alleles, when present together, complement each other & produce a different phenotype. For example: Flower color of sweet peas A- codes for color pigment B codes for color purple b codes for color white P: AAbb (white) X aaBB (white) F1: AaBb (purple)  Duplicate Recessive Genes (9:7) F2 AB Ab aB ab AB AABB, purple AABb, purple AaBB, purple AaBb, purple Ab AABb, purple AAbb, white AaBb, purple Aabb, white aB AaBB, purple AaBb, purple aaBB, white aaBb, white ab AaBb, purple Aabb, white aaBb, white aabb, white  Dominant and recessive interaction (13:3) Dominant and Recessive Interaction (13:3) Only two F2 phenotypes result when a dominant genotype at 1 locus (A-) and the recessive genotype F2 AB Ab aB ab at another (bb) produce the same phenotypic effect. AB AABB, AABb, AaBB, AaBb, Genotype A-B-, aaB- & aabb produce one white white white white phenotype and genotype A-bb produce another in the ratio 13:3. Ab AABb, AAbb, AaBb, Aabb, white red white red For example: Flower color of peas aB AaBB, AaBb, aaBB, aaBb, A-bb codes for color red white white white white Any other combination codes for color white ab AaBb, Aabb, aaBb, aabb, P: AAbb (white) X aaBB (white) white red white white F1: AaBb (white)  Summary of Epistatic Ratios Genotype s A-B- A-bb aaB- aabb Classical ratio 9 3 3 1 Dominant epistasis 12 3 1 Recessive epistasis 9 3 4 Duplicate genes with cumulative effect 9 6 1 Duplicate dominant genes 15 1 Duplicate recessive genes 9 7 Dominant and recessive interaction 13 3 Categories of Inheritance Paterns Epistasis of aa over B- Epistasis of A- over bb Complementary action Generation of Epistatic Ratios Duplicate action Examples of Gene Interactions SUMMARY WELCOME TWO-MINUTE PAPER ❑ Take out a sheet of paper ❑ summarize the important points of concept/principle that was learned in class Activity (5 min) Question(5min) Activity (5 min) Lecture (15 min) Lecture (15 min) CLO1 Summary Opening SMBB 2753 (5min) BASIC GENETICS