An Introduction to Genetics

I. An Introduction to Genetics

Genetics is the science of heredity. The discipline has a rich history and involves investigations of molecules, cells, organisms, and populations, using many different experimental approaches. Not only does genetic information play a significant role during evolution, its expression influences the functioning of individuals at all levels. Genetics thus unifies the study of biology and has had a profound impact on human affairs.

1. Definition:

Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. The word genetics was first suggested to describe the study of inheritance and the science of variation by the British scientist William Bateson in a personal letter to Adam Sedgwick, dated April 18, 1905. Bateson first used the term genetics publicly at the Third International Conference on Genetics (London, England) in 1906.

Humans applied knowledge of genetics in prehistory with the domestication and breeding of plants and animals. In modern research, genetics provides important tools for the investigation of the function of a particular gene, e.g., analysis of genetic interactions. Within organisms, genetic information generally is carried in chromosomes, where it is represented in the chemical structure of particular DNA molecules.

Genes encode the information necessary for synthesizing the amino-acid sequences in proteins, which in turn play a large role in determining the final phenotype of the organism. In diploid organisms, a dominant allele on one chromosome will mask the expression of a recessive gene on the other. The phrase to code for is often used to mean a gene contains the instructions about how to build a particular protein, as in the gene codes for the protein. The "one gene, one protein" concept is now known to be simplistic. For example, a single gene may produce multiple products, depending on how its transcription is regulated. Genes code for the nucleotide sequences in mRNA, tRNA and rRNA, required for protein synthesis.

Genetics determines much (but not all) of the appearance of organisms, including humans, and possibly how they act. Environmental differences and random factors also play a part. Monozygotic ("identical") twins, a clone resulting from the early splitting of an embryo, have the same DNA, but different personalities and fingerprints. Genetically-identical plants grown in colder climates incorporate shorter and less-saturated fatty acids to avoid stiffness.

2. History

In his paper "Versuche ÑŒber Pflanzenhybriden" ("Experiments in Plant Hybridization"), presented in 1865 to the Brunn Natural History Society, Gregor Mendel traced the inheritance patterns of certain traits in pea plants and showed that they could be described mathematically. Although not all features show these patterns of Mendelian inheritance, his work suggested the utility of the application of statistics to the study of inheritance. Since that time many more complex forms of inheritance have been demonstrated.

The significance of Mendel's work was not understood until early in the twentieth century, after his death, when his research was re-discovered by other scientists working on similar problems.

Mendel did not understand the nature of inheritance. We now know that some heritable information is carried in DNA. (Retroviruses, including influenza, oncoviruses and HIV, and many plant viruses, carry information as RNA.) Manipulation of DNA can in turn alter the inheritance and features of various organisms.

Timeline Of Notable Discoveries

1859 Charles Darwin publishes The Origin of Species

1865 Gregor Mendel's paper, Experiments on Plant Hybridization

1903 Chromosomes are discovered to be hereditary units

1905 British biologist William Bateson coins the term "genetics" in a letter to Adam Sedgwick

1910 Thomas Hunt Morgan shows that genes reside on chromosomes

1913 Alfred Sturtevant makes the first genetic map of a chromosome

1918 Ronald Fisher publishes On the correlation between relatives on the supposition of Mendelian inheritance - the modern synthesis starts.

1913 Gene maps show chromosomes containing linear arranged genes

1927 Physical changes in genes are called mutations

1928 Frederick Griffith discovers a hereditary molecule that is transmissible between bacteria

1931 Crossing over is the cause of recombination

1941 Edward Lawrie Tatum and George Wells Beadle show that genes code for proteins; see the original central dogma of genetics

1944 Oswald Theodore Avery, Colin McLeod and Maclyn McCarty isolate DNA as the genetic material (at that time called transforming principle)

1950 Erwin Chargaff shows that the four nucleotides are not present in nucleic acids in stable proportions, but that some general rules appear to hold (e.g., that the amount of adenine, A, tends to be equal to that of thymine, T). Barbara McClintock discovers transposons in maize

1952 The Hershey-Chase experiment proves the genetic information of phages (and all other organisms) to be DNA

1953 DNA structure is resolved to be a double helix by James D. Watson and Francis Crick, with the help of Rosalind Franklin

1956 Jo Hin Tjio and Albert Levan established the correct chromosome number in humans to be 46

1958 The Meselson-Stahl experiment demonstrates that DNA is semiconservatively replicated

1961 The genetic code is arranged in triplets

1964 Howard Temin showed using RNA viruses that Watson's central dogma is not always true

1970 Restriction enzymes were discovered in studies of a bacterium, Haemophilius influenzae, enabling scientists to cut and paste DNA

1977 DNA is sequenced for the first time by Fred Sanger, Walter Gilbert,