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Overview Discontinuous variation Hereditary material is DNA, a double helix of complementary polynucleotides. Genes are segments of DNA encoding the amino acid sequence of a polypeptide. Hereditary variation is caused by variant forms of genes known as alleles. Alleles can be studied at many levels. Each species has its own distinctive pool of genes. Evolution is a consequence of genetic changes in a population over time. Relatively rare compared to continuous variation Variation falls into distinct categories Usually due to differences between two alleles Š designated by letters, e.g., A and a Š A dominant to a Polymorphism: multiple alleles of single gene Mutants: rare, exceptional variants Š normal phenotype called wild-type Alleles arise through mutation, DNA change Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Genetic methodology Continuous variation Isolation of mutations (natural or induced) Analysis of progeny of controlled matings (crosses) Biochemical analysis of underlying cellular processes Microscopic analysis of chromosomes (cytogenetics) and phenotypes Direct analysis of DNA Unbroken range of phenotypes; no discrete classes Intermediate phenotypes more common than extreme ones A result of a combination of genetic and environmental variation Underlying genetic basis often difficult to study Important in plant and animal breeding Š genomics: sequencing genome Š bioinformatics: extraction of information from DNA Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Genes, environment, organism Model 1: genetic determination Š characters specified by genes Model 2: environmental determination Often, there are multiple inputs into phenotype. Š characters determined by environment Model 3: genotype-environment interaction Š gene expression is conditioned by environment Norm of reaction: set of environment-genotype relations for given genotype Š single genotype may have several phenotypes, depending upon the environment Developmental noise: random events in development leading to phenotypic variation Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Genes and evolution Darwin recognized role of hereditary variation in evolution (but was unaware of true mechanism of heredity) Genetic variation (product of mutation) is raw material for evolutionary change Natural selection: differential reproduction of individuals with different alleles Random genetic drift: change in frequencies of genetic variants resulting from random, non-selective processes Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company 1 Genetics and human affairs >1000 inherited genetic diseases in humans Cancer is caused by mutation in somatic cells Genetics pertains to social policy Š debate over role of genetics and IQ Š genetics of sexual orientation Biotechnology and genetic engineering Š new pharmaceuticals Š new varieties of plants and animals Š concerns over ethics and safety Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Genetics Experimental science of heredity Grew out of need of plant and animal breeders for greater understanding of inheritance of economically important characters Gregor Mendel: discovered principles of heredity Today, genes are explained in molecular terms Chapter 1: Genetics and the organism From gene to protein Physical and chemical basis Genome: basic complement of DNA of an organism Š haploid: one copy of genome (fungi, algae, bacteria) Š diploid: two copies of genome (plants, animals) Genes: regions of chromosomal DNA encoding polypeptides Š different genes on each chromosome Š homologs: chromosomes with same genes Š in diploids, one homolog inherited from each parent DNA: complementary polynucleotide chains Š A-T and G-C base pairs Š double helix Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company © 2002 by W. H. Freeman and Company Protein Š linear chain of amino acids (polypeptide) encoded by gene subject to variation Š folds into 3 dimensional structure Š may associate with other proteins DNA → mRNA → polypeptide Š DNA → mRNA : transcription Š mRNA → polypeptide : translation genetic code ribosomes tRNA Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company Genetic variation Alleles Š alternative forms of a gene encoding proteins with altered amino acid sequence Š located at same position (locus) on chromosome Phenotype: appearance or physiological expression of gene Genotype: alleles present in individual Discontinuous variation (qualitative) Continuous variation (quantitative) Chapter 1: Genetics and the organism © 2002 by W. H. Freeman and Company 2