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Mendelian Genetics Bio 250: Genetics Dr. Ramos Mendelian Genetics • • • • Gregor Mendel Insight into the mechanism behind inheritance. 1866- Rules of Inheritance Used Pisum savitarum as a model organism. Characteristics of a model organism Why was Mendel successful? Seven traits analyzed by Mendel Phenotype: physical appearance of a trait Gene: unit of inheritance Allele: alternative forms of a single gene. Determines phenotype. Genotype: genetic makeup of an individual. Homozygous: both alleles are the same Heterozygous: both alleles are different. Monohybrid cross • Mating of 2 parent strains (P1), each exhibiting one of 2 contrasting traits. • Offspring = F1 generation • F2 generation- observed after selfing (self-fertilizing) of F1 generation D = allele for Tall d = allele for dwarf DD = homozygous tall plant dd = homozygous dwarf plant Dd = heterozygous tall plant Monohybrid cross for stem length: P = parentals true breeding, homozygous plants: DD dd (tall) (dwarf) Dd F1 generation is heterozygous: (all tall plants) Mendel’s First Three Postulates • Unit factors exist in pairs. • There are dominant and recessive factors. • Paired unit factors segregate randomly. Using a Punnett Square • A useful tool to do genetic crosses • For a monohybrid cross, you need a square divided by four • STEPS: 1. Draw a p-square. 2. determine the genotypes of the parent organisms 3. write down your "cross" (mating) Parent genotypes: Dd and dd Cross Punnett square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) D DD dd d d Dd Dd D Dd Genotypes: 100% Dd Dd Phenotypes: 100% Tall plants • Determine the genotype and phenotype of the F2 generation. – Your plants self-fertilize. – The next monohybrid cross would be: Dd Dd (tall) (tall) Monohybrid cross: F2 generation D d D DD Dd d Dd dd Genotypes: 1 DD= Tall 2 Dd = Tall 1 dd = dwarf Genotypic ratio= 1:2:1 Phenotype: 3 Tall 1 dwarf Phenotypic ratio= 3:1 Testcross • Can you determine the genotype of the F1 generation if F2 consists of 4 purple plants? P = purple (dominant) p = white (recessive) Dihybrid crosses • • • • Two factor cross. Two contrasting traits. Yellow and round =dominant Green and wrinkled = recessive yr yr yr YyRr YR YyRr YyRr YyRr YyRr YyRr YyRr YyRr YR YyRr YyRr YyRr YyRr YR YyRr YyRr YyRr YyRr YR yr F1 = All yellow and round What happens to F2? YyRr x YyRr Possible gametes: • YR YR • Yr Yr • yR yR • yr yr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr yyRr 9/16 yellow and round 3/16 yellow and wrinkled Yyrr yyrr 3/16 green and round 1/16 green and wrinkled Glossary Sheet • Terms you should know from this lecture: • Terms you should know for the next lecture: Mitosis Meiosis Gamete Centromere Karyotype Haploid and Diploid
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