Reading assignment: Chapter 1, pages 1-17

Objective: This lecture seeks to provide an overview of modern genetics and the material that will be presented in this course. The objective is to provide a broad foundation that will allow each detailed topic in future lectures to be presented as a part of a larger picture. In the limited time available, we will examine the major aspects of contemporary genetics in terms of current knowledge and how earlier discoveries fit into the overall picture, with little concern about the chronology of discovery.

Chapter 1 of the textbook presents similar material in a rather different organizational framework that includes a brief historical perspective, as well as an analysis of the major branches of genetics, a description of various model systems used to study genetics, and a discussion the impact of modern genetics on society in general.

Terminology and concepts: A major part of any science course consists of learning new words. Common speech lacks the vocabulary necessary to describe scientific phenomena in detail. It has been claimed that the number of new words learned in a science course can be as large as in a beginning foreign language course. Please do not underestimate the importance of learning both the vocabulary of genetics and the correct spelling and usage of those terms.

Many (but probably not all) of the words and concepts presented today will be at least partially review. If you have trouble with any of the terminology in these notes or in the lecture, there is a glossary in the back of the book that includes almost all of the terms we will be using. Also, please keep in mind the diversity of nature. If you look hard enough, you can find exceptions to virtually all of the abbreviated generalizations presented below.

• Genetics -- the scientific study of inheritance and its underlying mechanisms.
• Gene -- the basic unit of heredity. Contains coded information for making one RNA, and in most cases, one polypeptide.
• Genome -- the total array of genes carried by an individual or a species.
• DNA (deoxyribonucleic acid) -- a polymer of deoxyribonucletides, the usual carrier of genetic information.
• Coding sequence -- a segment of DNA (or RNA) that contains the nucleotide sequence code for a polypeptide; this is the physical form of genetic information.
• DNA replication -- the process by which new DNA molecules (and thus new copies of genetic information) are synthesized.
• Transcription -- the templating process that results in synthesis of a strand of RNA containing a copy of specific genetic information found in DNA.
• RNA (ribonucleic acid) -- a polymer of ribonucleotides, usually containing genetic information transcribed from DNA.
• Translation -- synthesis of a polypeptide whose amino acid sequence is determined by the nucleotide sequence of a messenger RNA.
• Protein (polypeptide) -- a polymer of amino acids whose sequence has been determined by coded genetic information; the functional product of gene expression that has a structural, catalytic or regulatory role in the living organism.
• Chromosome -- an extended DNA molecule containing multiple genes and associated proteins. During mitosis and meiosis, condensed eukaryotic chromosomes form structures that are visible with high-powered light microscopes.
• Diploid -- having two complete sets of chromosomes, one from each parent.
• Haploid -- having only one complete set of chromosomes; usually occurs only in the gametes after meiosis in higher organisms, but can be the normal vegetative state in lower organisms.
• Gamete -- a haploid germ cell that has already undergone meiosis and is ready for fertilization (egg cells often do not complete meiosis until the fertilization process has been initiated).
• Zygote -- the diploid product of fusion of gametes; the initial diploid cell from which an entire diploid organism develops.
• Mutation -- a change in genetic information that is capable of being inherited; often (but not always) damaging to the fitness of the organism.
• Allele -- an alternative form of a gene, distinguishable in some manner from the "original" form it is being compared to.
• Genetic locus The site within a chromosome or DNA sequence where a particular coding sequence resides.
• Phenotype -- the morphological, biochemical, behavioral, or other traits of an organism that can be observed; often used to describe the overt expression of alleles of a particular gene.
• Genotype -- the full set of alleles carried by an organism, including recessive alles that are present, but not phenotypically expressed; usually described only in terms of alleles at the particular genetic locus (or loci) that are being studied.
• Homozygous -- carrying two identical copies of a particular gene in the diploid state.
• Heterozygous -- carrying two different alleles of a particular gene in the diploid state.
• Dominant allele -- an allele whose phenotypic expression can be observed even when it is only present in a single copy in a heterozygous organism.
• Recessive allele -- an allele whose phenotypic expression cannot be observed in the presence of a dominant allele in a heterozygous organism. The recessive phenotype can only be seen in an organism that is homozygous for the recessive allele.
• Independent assortment -- the pattern of inheritance that is observed for genes that are on different chromosomes; the distribution of each gene to the progeny is unaffected by the pattern of distribution of other genes; this is what Mendel observed in his original studies.
• Genetic linkage -- the pattern of inheritance that is observed for genes that are carried on the same chromosome; the genes tend to remain linked together in the combinations that are present in the gametes.
• Genetic recombination -- modification of genetic linkage resulting from recombining parts of homologous parental chromosomes during meiosis; recombination frequency is roughly proportional to the physical distance between the genes on the chromosome.
• Gene mapping -- identification of the relative positions of genes on chromosomes, including their relationship to observable physical and biochemical features of the chromosomes.
• Gene cloning -- the use of recombinant DNA technology to insert specific gene sequences into vectors that can then be replicated in bacteria or other host organisms.
• Population genetics analysis of the overall gene pool for a particular breeding population, including selective advantages and other factors that alter the pattern of gene frequency.

Time scale: Chapter 1 briefly summarizes some of the major discoveries in genetics. The following are a few of the particularly significant dates in the history of genetics:

• 1866 -- Mendel first published his discoveries.
• 1900 -- Mendel's observations were rediscovered and confirmed by others.
• 1910 -- Morgan presented experimental evidence that genes are carried on chromosomes.
• 1944 -- Avery et al. demonstrated that DNA was genetic material
• 1953 -- Watson and Crick determined the double helical nature of DNA.
• 1961 -- Nienberg et al. and Ochoa et al. deciphered the genetic code.
• 1977 -- Sanger et al. published a simple and rapid method for DNA sequencing
• 2000 -- This class; sequencing of entire human genome is close to completion.

Although 1866 may seem like a long time ago to someone who is not yet 20 years old, the total history of genetics actually corresponds to only about three professional careers. Your professor had just entered graduate school when Watson and Crick described the double helix 47 years ago. Morgan's confirmation that genes were carried on chromosomes occurred 43 years before that. Mendel's first (and only) publication of his work was 44 years earlier. Thus, a student at the time of Mendel would still be active at the time of Morgan, a student at the time of Morgan would still be active at the time of Watson and Crick, and a student at the time of Watson and Crick is still active at the present time.

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