Mendelian Genetics PDF

GENETICS Introduction to Genetics and heredity Gregor Mendel – a brief bio Genetic terminology (glossary) Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Introduction to Genetics GE...

GENETICS Introduction to Genetics and heredity Gregor Mendel – a brief bio Genetic terminology (glossary) Monohybrid crosses Patterns of inheritance Dihybrid crosses Test cross Beyond Mendelian Genetics – incomplete dominance  Introduction to Genetics GENETICS – branch of biology that deals with heredity and variation of organisms. Chromosomes carry the hereditary information (genes) Arrangement of nucleotides in DNA DNA 🡪 RNA 🡪 Proteins  Chromosomes (and genes) occur in pairs Homologous Chromosomes New combinations of genes occur in sexual reproduction – Fertilization from two parents  Gregor Johann Mendel Austrian Monk, born in what is now Czech Republic in 1822 Son of peasant farmer, studied Theology and was ordained priest Order St. Augustine. Went to the university of Vienna, where he studied botany and learned the Scientific Method Worked with pure lines of peas for eight years Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.  Mendel’s peas Mendel looked at seven traits or characteristics of pea plants:  In 1866 he published Experiments in Plant Hybridization, (Versuche über Pflanzen-Hybriden) in which he established his three Principles of Inheritance He tried to repeat his work in another plant, but didn’t work because the plant reproduced asexually! If… Work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered Mendel’s work.  Mendel was the first biologist to use Mathematics – to explain his results quantitatively. Mendel predicted The concept of genes That genes occur in pairs That one gene of each pair is present in the gametes  Genetics terms you need to know: Gene – a unit of heredity; a section of DNA sequence encoding a single protein Genome – the entire set of genes in an organism Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like ‘flavors’ of a trait). Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.  Homozygous – having identical genes (one from each parent) for a particular characteristic. Heterozygous – having two different genes for a particular characteristic. Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.  Genotype – the genetic makeup of an organisms Phenotype – the physical appearance of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.  Monohybrid cross Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant TT × tt  Monohybrid cross for stem length: P = parentals TT × tt true breeding, (tall) (dwarf) homozygous plants: F1 generation Tt is heterozygous: (all tall plants)  Punnett square A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. Looks like a window pane… We use the Punnett square to predict the genotypes and phenotypes of the offspring.  Using a Punnett Square STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t Cross TT × tt  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) T T TT × tt t Genotypes: Tt Tt 100% T t Phenotypes: t Tt Tt 100% Tall plants  Another example: Flower color For example, flower color: P = purple (dominant) p = white (recessive) If you cross a homozygous Purple (PP) with a homozygous white (pp): PP × pp Pp ALL PURPLE (Pp)  Mendel’s Principles 1. Principle of Dominance: One allele masks another, one allele was dominant over the other. 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.  Principle of Independent Assortment Based on these results…. 3. Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction  Beyond Mendelian Genetics: Incomplete Dominance Mendel was lucky! Traits he chose in the pea plant showed up very clearly… One allele was dominant over another, so phenotypes were easy to recognize. But sometimes phenotypes are not very obvious…  Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) R You get PINK flowers (Rr)! R × rr Genes show incomplete dominance when the heterozygous phenotype Rr is intermediate.  Incomplete dominance When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white Incomplete Dominance R r R R R Rr r Rr rr  Incomplete dominance What happens if you cross a pink with a white? × A pink with a red? ×  Summary of Genetics Chromosomes carry hereditary info (genes) Chromosomes (and genes) occur in pairs New combinations of genes occur in sexual reproduction Monohybrid vs. Dihybrid crosses Mendel’s Principles: – Dominance: one allele masks another – Segregation: genes become separated in gamete formation – Independent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation