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Classical Genetics Humans have a long history of animal and plant breeding… but without an understanding of the underlying process Gregor Mendel Mendel conducted experimental crosses Classical Mendelian Genetics has a limitation: The requirement for observable phenotypic differences in different genotypes Mendel chose single gene mutants with extreme phenotypes to study. This made different genotypes recognizable and countable. Terminology • Genes and alleles • Genotype and Phenotype • Homozygote, Heterozygote, Hemizygote • Dominance • Meiosis and Syngamy (Fertilization) • Parents, Gametes, Offspring Genes and Alleles • A gene is a nucleotide sequence of a DNA molecule that codes for the primary structure of a protein or RNA molecule • Alleles are gene variants. They differ in their nucleotide sequences. Genotype and Phenotype • Genotype: An individual’s genetic constitution AA, Aa, aa are diploid genotypes • Phenotype: An organism’s appearance, reflecting genotypic and environmental influences blue yellow white Dominance • Many alleles are mutations whose gene products (proteins) work poorly or not at all (e.g., allele a). These alleles are recessive to normal alleles in the sense that they affect the phenotype only when there are no functional alleles present, i.e., in the homozygous recessive genotype aa. • Both homozygotes (e.g., AA) for the normal allele and heterozygotes (e.g., Aa) share the functional allele (A) and exhibit the normal phenotype. However, aa individuals are unable to perform the function that this gene is responsible for and they will have a different phenotype. • Operationally, one allele is said to be dominant over another if the heterozygote has the same phenotype as a homozygote (e.g., Aa and AA look alike). Homozygous for two normal alleles QuickTime™ and a Animation decompressor are needed to see this picture. Homozygous for two non-functional alleles QuickTime™ and a Animation decompressor are needed to see this picture. Heterozygous for a normal and a non-functional allele Dominance QuickTime™ and a Animation decompressor are needed to see this picture. Siamese Cats: An enzyme that catalyzes pigment synthesis is denatured under warmer physiological conditions, like warmer parts of the cat’s body. Only cooler extremities reveal intense pigmentation. Similarly, the enzyme can be deactivated not only under conditions that are too warm (below), but also under conditions that are cooler (above). Mendel figured out how to start a breeding experiment: A Classical Mendelian Research Program true breeding line “A” true breeding line “a” PA generation x aa Mendelian AA Research F1 generation all Aa Program backcross used as a test cross 1/2 Aa, 1/2 aa monohybrid cross F2 generation 1/4 AA, 2/4 Aa, 1/4 aa backcross used as a test cross The Three Steps of Classical Genetic Analysis Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle gametes Syngamy (fertilization) Meiosis multicellular body (parents and offspring) Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle gametes Syngamy (fertilization) Meiosis 1. Parental Genotypes Offspring Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle 2. Meiotic products = gametes Syngamy (fertilization) Meiosis 1. Parental Genotypes Offspring Rules for step 2: Diploid parents making haploid gamete genotypes AA parents produce all A gametes aa parents produce all a gametes but Aa parents produce 1/2 A and 1/2 a gametes MENDEL’S FIRST LAW Genetic Segregation is Based on Chromosomal Segregation Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle 2. Meiotic products = gametes Syngamy (fertilization) Meiosis 1. Parental Genotypes = start 3. Fertilization products = Offspring Fertilization: Sperm, Egg, and Zygote Predicting products of fertilization Step 1 parental genotypes Steps 2-3 predict gametes and combine them randomly haploid gametes haploid gametes diploid offspring Predicting products of fertilization: AA x AA Step 1 AA x AA Steps 2-3 predict gametes and combine them randomly All A gametes All A gametes All AA diploid offspring Genotypic ratio: all AA Phenotypic ratio: all “A” Predicting products of fertilization: AA x Aa Step 1 AA x Aa Steps 2-3 predict gametes and combine them randomly 1/2 A All A 1/2 AA 1/2 a 1/2 Aa Genotypic ratio: 1/2 AA and 1/2 Aa; 1:1 Phenotypic ratio: all “A” Predicting products of fertilization: aa x aa Step 1 aa x aa Steps 2-3 predict gametes and combine them randomly All a gametes All a gametes All aa diploid offspring Genotypic ratio: all aa Phenotypic ratio: all “a” Predicting products of fertilization: AA x aa Step 1 AA x aa Steps 2-3 predict gametes and combine them randomly All A gametes All a gametes All Aa diploid offspring Genotypic ratio: all Aa Phenotypic ratio: all “A” Predicting products of fertilization: Aa x aa Step 1 Aa x aa Steps 2-3 predict gametes and combine them randomly 1/2 A 1/2 a Test Cross All a gametes 1/2 Aa 1/2 aa Genotypic ratio: 1/2 Aa 1/2 aa Phenotypic ratio: 1/2 “A” 1/2 “a” Predicting products of fertilization: Aa x Aa Step 1 Aa x Aa Steps 2-3 predict gametes and combine them randomly 1/2 A 1/2 a Monohybrid 1/2 A 1/4 AA 1/4 Aa Cross 1/2 a 1/4 Aa 1/4 aa Genotypic ratio: 1/4 AA 2/4 Aa 1/4 aa Phenotypic ratio: 3/4 “A” 1/4 “a” Summary of the six diallelic crosses (with dominance) Mendel’s Experimental Results - Single Genes A Classical Mendelian Research Program true breeding line “A” true breeding line “a” PA generation x aa Mendelian AA Research F1 generation all Aa Program backcross used as a test cross 1/2 Aa, 1/2 aa monohybrid cross F2 generation 1/4 AA, 2/4 Aa, 1/4 aa backcross used as a test cross Only monohybrid and test crosses produce patterns in the progeny red 1/2 red x blue 1/2 blue red 3/4 red x red 1/4 blue Only monohybrid and test crosses produce patterns in the progeny Aa red 1/2 Aa red x aa blue 1/2 aa blue Aa red 3/4 A_ red x Aa red 1/4 aa blue Brain Teasers • Mother and father both find the taste of phenylthiourea very bitter, but three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer)? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes. • Mother finds the taste of phenylthiourea very bitter, but father and three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer) )? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes. Remember Monohybrid crosses provide the most information: Informing about both dominance and the number of genes ...and the parents in monohybrid crosses look alike Test crosses also produce different progeny phenotypes, but ...whereas the parents in test crosses look different Hints: • Each family produced both phenotypes in their children, so the matings must be either test crosses or monohybrid crosses. • Parents look alike in monohybrid crosses, but not in test crosses. Brain Teasers • Mother and father both find the taste of phenylthiourea very bitter, but three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer)? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes. Two progeny phenotypes, parents alike: Therefore a monohybrid cross, taster dominant: Taster(Aa) x Taster(Aa) 3 Non-taster (aa) and Taster (AA, Aa) Brain Teasers • Mother finds the taste of phenylthiourea very bitter, but father and three of their four children find it tasteless. Assuming that this difference is caused by a single gene with two alleles, is the non-taster phenotype dominant or recessive (circle the correct answer) )? What kind of cross is this? Be prepared to explain with a diagram of the cross that identifies phenotypes and their genotypes. Two progeny phenotypes, parents not alike: Therefore a test cross, but can’t resolve dominance relationships: Aa x aa 1/2 Aa and 1/2 aa No Dominance • Some heterozygotes have phenotypes unlike either homozygote. The alleles of these heterozygotes are said not to exhibit dominance. • In this case, each genotype has a unique phenotype. Incomplete Dominance white Summary of the six diallelic crossses (no dominance) Mendel’s Second Law Independent Assortment Two genes will be inherited independently of one another Classical genetic analysis involves 3 steps based on the structure of a eukaryotic life cycle 2. Meiotic products = gametes Syngamy (fertilization) Meiosis 1. Parental Genotypes = start 3. Fertilization products = Offspring Mendel’s Second Law 1 1/4 AB 1/4 aB 1/4 Ab 1/4 ab 2 3 Dihybrid Cross - Peas Dihybrid cross - Eye color BIG 1/4 AB 1/4 aB 1/4 Ab 1/4 ab and MESSY Using punnet squares can get cumbersome Forking Diagram 27/64 Sex Linkage The Human Chromosome Complement: 22 autosomes and a heteromorphic pair of sex chromosomes X X Human Y Chromosome Homogametic and Heterogametic Genotypes XX XY In our species XX = female, XY = male Other species XY = female, XX = male Sex Linkage Figure 1. Sex Linkage A centro mere X chromo some Y chromo some centro mere C pa iring reg ion di ffere ntia l reg ions Y L inka ge X L inka ge pseud o-autoso mal B Practice Hemophelia Victoria’s Clan Color Blindness normal color vision: XCXC, XCXc XCY color blindness: XcXc XcY Inheritance of White Eye wild type (red) eye white eye Inheritance of White Eyes Inheritance of White Eyes White eye revisted Some species have heterogametic females homogametic males heterogametic females