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Mendelian Genetics. Chapter 2. Phenotype and Genotype. Genotype and Phenotype. Genotype – genetic constitution of an organism Phenotype – observable characteristic Genotype and environment Contribution of environment varies between genes Can be controlled by many genes
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Mendelian Genetics Chapter 2
Genotype and Phenotype • Genotype – genetic constitution of an organism • Phenotype – observable characteristic • Genotype and environment • Contribution of environment varies between genes • Can be controlled by many genes • Random developmental events
Stamen Carpel Mendelian Genetics • Modern genetics began with Gregor Mendel’s quantitative genetic experiments • Austrian monk • Mathematician • Numerical and observational data • Several generations
Mendelian Genetics • Heritable, obvious traits • Simple crosses at first • Used peas because: • Easy to grow and available • Many distinguishable characteristics • Self-fertilization • True breeding peas
Breeding Crosses • Initial cross is the P generation • Parents • Progeny of parents is first filial generation • F1 generation • Inbreeding of first generation creates second filial generation • F2 generation
Monohybrid Crosses • Cross between true-breeding individuals with one different trait • Mendel’s first crosses • Resembled only one of the parents • Planted progeny and allowed self-fertilization • Revealed both phenotypes
GENETIC MAKEUP (ALLELES) P PLANTS PP pp Gametes All P All p F1 PLANTS(hybrids) All Pp Gametes 1/2P 1/2p P P Eggs Sperm PP F2 PLANTS p p Pp Pp Phenotypic ratio3 purple : 1 white pp Genotypic ratio1 PP : 2 Pp : 1 pp Monohybrid Cross • Mendel determined that • Particulate factors for genes, each contains a set of two • Transmitted by both parents • Alternate forms called alleles • True breeding forms contains identical set
Monohybrid Cross • F1 generation had both alleles • Only one expresses • One allele masks • Dominant • Recessive • Identical alleles – homozygous • Different alleles - heterozygous
Principle of Segregation • Recessive characteristics are masked • Reappear in F2 • Members of a gene pair (alleles) segregated during gamete formation
How cells carry characteristics • Genes on chromosomes • At a specific loci • Homologous pairs carry the same genes at the same locus • Different versions • Separation of homologous chromosomes yields separation of alleles
Branch Diagrams • Punnett squares can become messy with more than one gene • Use branch diagram to figure out genotype and phenotype expected frequency
TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING All black 1 black : 1 chocolate Test Cross • Mendel did several crosses • Followed over several generations • Selfing also very important • Allowed plants to reveal their genotype and not just their phenotye
Recessive Alleles • Wild-type allele – functional allele • Predominates in population • Dominant allele • Loss-of-function mutations – causes protein product to be absent, partially functional, or nonfunctional • Recessive • Function of other in heterozygote is sufficient
Wrinkled Peas • SS type contains more starch and lower sucrose • Also more water • SBEI - starch-branching enzyme • Extra 800 bp piece in mutation
Dihybrid and Trihybrid Crosses and Mendel’s Principle of Independent Assortment
The Principle of Independent Assortment • Factors for different traits assort independently of one another • Genes are inherited independently of each other • Segregate randomly in gametes • Dihybrid Cross
Branch Diagram of Dihybrid Cross Phenotype
Statistical Analysis • Data from genetics is quantitative • Use statistics to show deviation of observed results from predicted results • Chance factors cause deviations • Null-hypothesis – no difference between the predicted and observed • If not accepted then have to come up with a new hypothesis for deviation
Chi-Square Test • Goodness of fit test • How much observed number deviates from the expected number
Pedigree Analysis • Inheritance patterns are studied using family trees • Pedigree analysis • Phenotypic records • Proband is where gene was discovered
Examples of Human Genetic Traits • Most genetic disorders are recessive • Due to lack of function • Homozygous recessive expression • Dominant usually selected out • Albinism
Characteristics of Recessive Inheritance Traits • Most have normal heterozygous parents • Heterozygotes have 3:1 ratio • When both parents have the trait then all progeny have the trait • Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs
Characteristics of Dominant Inheritance Traits • Gain of function mutations • New property of the mutant gene • No loss of function • Must have one parent with disease • Does not skip generations • Will transmit to half its progeny • Huntingtons disease, Marfan syndrome, achondroplasia