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Exercise 11 Mendelian Genetics Ji-Ung Maeng BIO 111 TAMU Objectives • Differentiate between homozygous and heterozygous genotypes • Explain the difference between genotype and phenotype • Describe the difference between monohybrid and dihybrid cross • Apply Mendel’s laws to several crosses • Demonstrate how chromosome alignment in metaphase I affects the alleles in haploid cells • Differentiate between simple, incomplete and co-dominance Gregor Mendel • 1865 Experiments in Plant-Hybridization – Foundation for genetics and inheritance – 7 pea plant traits through generations • Inherited in a predictable pattern/ratio • Some traits were hidden – “Factors” – genes • Affect inheritance • Alleles occur in pairs on homologous chromosomes form each parent 1 Terminology • Character (= a heritable feature), Trait (= each variant for a character) Ex) Eye color – blue, green, brown, etc • Gene = Unit of heredity, maternal and parental • Alleles = Form of gene, alternative version of gene • Loci = The specific site on a chromosome where alleles are located • Genotype = All alleles of the organism – Homozygous if both alleles are the same – Heterozygous if alleles are different • Phenotype = Expressed traits of the organism – DOMINANT = Allele that masks counterpart’s expression – Dominant does NOT mean better. It means just expression – recessive = Alleles that are masked 2 Terminology, cont’d • Monohybrid Cross = Crossing one trait • Dihybrid Cross = Crossing two traits • Hybrid = Organism that is the result of parents with different forms of the trait • Unlinked Genes = Genes are on separate chromosomes • Linked Genes = Genes occurring on the same chromosome Dihybrid Cross Mendel’s Laws of Inheritance • First Law -- Segregation – Each gamete has equal chance of possessing either of the two alleles (form of a gene) – Seperation of homologous chromosomes or Sister chromatids to poles • Second Law -- Independent Assortment – Alleles sort independently and are randomly distributed in gametes – Aligning at metaphase plate – Must be unlinked genes 3 Segregation Independent Assortment Steps to Solve Monohybrid Cross • Id paternal and maternal genotype • Set up squares – For each column or row • One parent’s gametes/alleles for each column/row • “Cross” or multiply the alleles – One from mom with one from dad • Look at the ratio of each geno- & phenotype 4 Monohybrid Example • A rancher wants to cross a brown (BB) horse with a white (ww) mare. Color is an unlinked gene and brown is dominant. • What are the genotypes and phenotypes for the F1 (first offspring) generation? B Parents = BB x ww B w Bw Bw w Bw Bw F1 = 100% Bw Brown? White? Monohybrid Example 2 • A rancher wants to cross a brown (Bb) horse with a white (ww) mare. Color is an unlinked gene and brown is dominant. • What are the genotypes and phenotypes for the F1 (first offspring) generation? • How does Mendel’s First Law apply? B Parents = Bb x ww b w Bw bw w Bw bw F1 = 50% Bw; Brown 50% bw; White Dihybrid Example • A heterozygous red tall plant crosses with a homozygous blue short plant. Red is dominant and Tall is dominant. Both traits are unlinked. • F1 genotypes and phenotypes? • How does Mendel’s Second Law apply? P = RrTt x rrtt RT rt Rt RrTt Rrtt Red & Tall? Red & Short? Blue & Tall? Blue & Short? 25% 25% 25% 25% rT rt rrTt rrtt RRTT? 0% RRTt? 0% RRtt? 0% RrTT? 0% RrTt? 25% Rrtt? 25% rrTT? 0% rrTt? 25% rrtt? 25% 5 Terminology • Transposons = “jumping genes” discovered by Barbara McClintock – Move around gene loci changing the phenotype • Purple Indian Corn Kernels • Incomplete Dominance = Blending of traits for an intermediate trait – Red + White = Pink • Co-Dominance = Equal expression of two alleles – Blood types; AB, A, B, O Incomplete Dominance Co-Dominance 6 Lethal Inheritance • If express a lethal trait, will usually abort during development – But if heterozygote then are a carrier of the trait – Can be passed recessively or dominantly • Not always expressed in development – Huntington’s Disease • Passed on dominant gene • Middle-age onset • Neural degeneration Post-lab Questions • Define genotype & phenotype • Define heterozygous & homozygous • Compare incomplete dominance and codominance • Explain the importance of Mendel’s Laws of Inheritance and Meiosis • Explain the role of heterozygosity in diseases such as Huntington’s • Set-up a cross between two heterozygous rabbits for fur color • Goggles next week! 7