Prelim Review CAMB 550 Section 1: Genetic Concepts 04-13-05, tjp 1. Mendelian segregation and basic terminology M. Sundaram locus: physical region or segment of a chromosome allele: one of a set of possible alternative forms at a given locus genotype: genetic constitution – or particular allele combos – of an organism phenotype: observable properties of an organism, influenced by genotype and environment wild type: “normal” phenotype, genotype or allele, as defined in the lab mutant: “abnormal” phenotype, genotype or allele, as defined in the lab haploid: single allele at each locus diploid: two alleles at each locus homozygote: diploid organism having two identical alleles at a given locus heterozygote: diploid organism having two different alleles at a given locus hemizygote: Monohybrid Cross: Y y Y YY Yy y Yy yy Punnett Square 1:2:1 Genotypic ratio 3:1 Phenotypic ratio Dihybrid Cross: Y R Y r y R y r Y R YY RR YY Rr Yy RR Yy Rr Y r YY Rr YY rr Yy Rr Yy rr y R Yy RR Yy Rr yy RR yy Rr y r Yy Rr Yy rr yy Rr yy rr 9:3:3:1 Phenotypic Ratio Probability of a particular Genotype: (from YyRr X YyRr cross) True breeding yellow peas True breeding green peas P0 YY X yy F1 “hybrids” Yy (All yellow: Yellow is dominant) green is recessive F2 1/4 YY Yellow, pure-breeding 1/2 Yy Yellow, hybrid 1/4 yy green, pure-breeding True breeding Yellow Round True breeding green wrinkled YY RR X yy rr F1 “dihybrids” Yy Rr Yellow, Round (interbreed) F2 9/16 1/16 YYRR Yellow, Round 2/16 YY Rr Yellow, Round 2/16 Yy RR Yellow, Round 4/16 Yy Rr Yellow, Round 3/16 1/16 YY rr Yellow, wrinkled 2/16 Yy rr Yellow, wrinkled 3/16 1/16 yy RR green, Round 2/16 yy Rr green, Round 1/16 1/16 yy rr green, wrinkled Color gene shape gene Probability of yy rr [1/2 X 1/2] X [1/2 X 1/2]= 1/16 1/4 1/4 Probability of Yy rr [(1/2 X 1/2) + (1/2 X 1/2)] X [1/2 X 1/2] = 2/16Backcross: Cross of progeny to one of the parents YyRr (F1) X YYRR(F0) gives offspring: 1/4 YYRR 1/4 YYRr 1/4 YyRr 1/4 YyRr 1:1:1:1 Genotypic ratio All have same phenotype (Y R) or YyRr (F1) X yyrr(F0) gives offspring: 1/4 YyRr 1/4 Yyrr 1/4 yyRr 1/4 yyrr 1:1:1:1 Genotypic and Phenotypic ratios Different phenotypes! Much more informative Test Cross: Cross of Unknown genotype with recessive homozygote Cross a Yellow Round pea (both dominant traits) with green wrinkled (recessive traits) Y _ R _ X yy rr Any green peas in F1? If so, genotype was Yy Any wrinkled peas? If so, genotype was Rr Outcross: Cross of mutagenized strain with wild-type strain to "clean up" genetic background T.H. Morgan - Drosophila, discovery of sex-linked inheritance = Chromosomal theory of inheritance Wild-type: Red Eyes Mutant: White-Eyes Morgan's observations: X-chromosome linkage explains the results: Experiment #1 Pure breeding red-eyed flies Pure breeding white-eyed flies female X male F1 All red (interbreed) F2 3 red:1 white, as expected BUT all the white-eyed flies were male! Experiment #2 Pure breeding red-eyed flies Pure breeding white-eyed flies male X female F1 All females red-eyed! All males white-eyed! Experiment #1 +/+ X w/Y (red-eyed) (white-eyed) F1 w/+ females and +/Y males (red-eyed) (red-eyed) F2 +/+ red-eyed females w/+ red-eyed females +/Y red-eyed males w/Y white-eyed malesSegregation of X-linked traits: Sons get an X from Mom and a Y (or 0) from Dad --> Sons of affected fathers do not inherit the trait --> Sons of het mothers have a 1/2 chance of being affected Daughters get an X from each parent --> Daughters of affected fathers are obligate hets --> Daughters of het mothers have a 1/2 chance of being het χχχχ square test: A statistical test used to evaluate some types of genetic data (including segregation ratios) Complications in the relationship between genotype and phenotype: Pleiotropy - A single gene mutation may be responsible for a number of distinct and seemingly unrelated phenotypic effects. Genetic Heterogeneity - Mutations in multiple different genes may all cause the same mutant phenotype. Redundancy - Mutations in two or more genes may need to be present before a mutant phenotype is observed. Suppression - A mutation in one gene may “mask” the effects of a mutation in another gene. Incomplete Penetrance and Variable Expressivity - Individuals with identical genotypes may have different phenotypes. The Complementation Test A test to determine whether or not two recessive mutations are allelic (e.g. Do two similar-looking mutant strains carry mutations in the same gene? Or in two different genes?) Experiment #2 +/Y X w/w (red-eyed) (white-eyed) F1 w/+ females and w/Y males (red-eyed) (white-eyed) F2 w/w white-eyed females w/+ red-eyed females w/Y white-eyed males +/Y red-eyed males χ2 = ‡” (expected – observed)2/expected, degrees of freedom = # classes – 1 True breeding wrinkled line #1 X True breeding wrinkled line #2 If F1 = wrinkled #1 and #2 “fail to complement” and are alleles of the same gene If F1 = Round #1 and #2 “complement” and are not alleles of the same gene Failure to Complement: One gene with three alleles: R (round), r 1(rec. wrinkled), r2 (rec. wrinkled) r 1 r1 x r2 r2 r1 r2 (wrinkled) and interbreeding of F1 would continue to give ~only wrinkled peas Complementation: Two genes with two alleles each: R (round), r (rec. wrinkled), S (round), s (rec. wrinkled) rr SS x RR ss Rr Ss (Round) and interbreeding of F1 could generate an RR SS true breeding Round line(Rare) Exceptions to the rule of Complementation Tests: Intragenic complementation: Two (non-null) alleles of the same gene complement each other. Intergenic non-complementation: Two alleles of different (but functionally related or physically linked) genes fail to
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