A keen observer will readily recognize two basic aspects of the biological world. First, the progeny of an organism develop characteristics similar to that organism. In addition, there are always some similarities between the parents and their progeny. In contrast, the second aspect of the biological world concerns with the differences among the individuals belonging to a single species. In fact, ordinarily no two human beings are identical to each other in every respect. The science of genetics deals with the principles that explain the similarities between parents and their progeny and the differences among individuals of a single species. In other words, genetics is the science of inheritance and variation.
History of Genetics:
Understanding the law of origins has been a constant pursuit of human beings. Humans knew from the beginning that one of the prime causes of variation was veiled in sexual reproduction. They first tried to understand the variations that were naturally present in both animals and plants.
In 1858, the understanding of the origin of species and variation was revolutionized by the work of Darwin and Wallace. They proposed how natural selection takes place in order to evolve new forms. A few years later in 1866, Mendel, an Austrian Monk proposed a paper on the law of inheritance where he had quantitatively analyzed the inheritance pattern in garden peas (Pisum sativum). However, his paper lay forgotten in the stacks of many libraries till 1900 when it was rediscovered independently by Correns, de Vries, and Tschermack who themselves had obtained results similar to those of Mendel.
In 1903, Walter Sutton gave the theory of inheritance and they marked the birth of cytogenetics. In the same year, Johannsen used the term gene, genotype, and phenotype. In 1908, Nilsson-Ehle demonstrated the existence of multiple factors through his studies on the inheritance of seed color in wheat and oat. Morgan and his group in 1910, developed the techniques of chromosome mapping through linkage studies and Morgan was awarded the Nobel Prize in 1934 for these studies.
In 1944, Avery MacLeod and McCarty demonstrated that DNA (deoxyribonucleic acid) was the genetic material. However, in 1953, Watson and Crick proposed the double helix model of DNA and were awarded the Nobel Prize in 1962 for their work.
- Historical Landmarks: The link provides complete information on the historical events in the field of genetics
- Scientists’ contribution: The link lists the name of scientists who contributed in the genetics field.
- History of genetics: Read a complete guide on the history of genetics.
- Genetics & historical contributions: A list of links on the history of classical genetics to modern genetics.
Gregor Mendel and Mendelian Genetics:
Gregor Johann Mendel (1822-1884) was an Austrian Monk and a scientist who gained posthumous fame for his experimental work on pea plants. He is now considered the father of genetics.
Gregor Mendel performed the first quantitative study of inheritance in garden peas (Pisum sativum) Mendel selected pea plants for two main reasons: 1. Several varieties were available with different observable traits or characteristics such as yellow or green seed color.2. Peas are self-pollinated. However, Mendel selected 7 different varieties with respect to 7 traits. These seven traits were seed color, seed shape, flower color, pod color, pod shape, flower position and plant height.
Mendel’s experiments were performed in three stages. First, he confirmed that all the 14 varieties (7-pairs of contrasting forms) were true breeding. The second step was to hybridize or cross-pollinate plants with alternate forms of a trait. A cross where a single trait is brought together is called a monohybrid cross. The hybrid plant, which is generated in the next generation, is called the F1 generation or FIRST FILIAL generation. The third and the final step was to allow each F1 hybrid to self-pollinate in order to produce the Second Filial or F2 generation seeds and subsequently to F3, F4 and so on.
Mendel’s Findings from the Experiments:
Mendel found that F1 generation plants resembled only one parent. All the seven traits were examined and they gave similar results. This was true for reciprocal crosses also. For example, a cross between yellow and green seeds gave all yellow seeds in the first generation. However, in F2 generation, both dominant and recessive traits appeared in the ration of 3:1. On the basis of his experiment, Mendel proposed three Laws of inheritance:
1. Law of Dominance: Mendel observed that one of the traits in the F1 generation was always absent, but reappeared in the F2 generation. He proposed that some factors are attached with each trait and the one that is expressed in the F1 generation was dominant. For example, yellow color was considered as a dominant factor. However, the factor that remained recessive in the F1 plant, but reappeared in F2 plants was considered recessive.
2. The Principle of Segregation: Mendel in the F2 generation found a 1:2:1 genotypic ratio. According to his arguments, a plant contains two factors (now known as genes) for each trait. They can either be similar or dissimilar. If the two factors are similar, the plant is considered as pure, otherwise impure or a hybrid. When F1 plants are self-pollinated, segregation takes place during gamete formation. As a result, the genotypic ratio in the F2 generation comes as 1:2:1 (pure dominant: impure dominant i.e., hybrid: pure recessive).
3. Law of Independent Assortment: The law states that if two or more genes are considered for inheritance then their distribution in gametes and the subsequent generation remain independent of each other. Mendel considered two traits in each parent to explain this law. So, the law of independent assortment is the result of a dihybrid experiment. He used parents with round yellow seeds and wrinkled green seeds. All the plants in the F1 generation were having round yellow seeds. However, when self-pollinated, he got 9: 3: 3: 1 phenotypic F2 ratio (round yellow: round green: wrinkled yellow: wrinkled green).
- Mendel’s Genetics: Learn about Mendel’s contribution in the field of genetics.
- Mendelian Genetics: Read a brief guide on Mendel’s dihybrid experiment on pea plants.
- Mendel’s Law of Genetics: The site discusses Mendel’s Law in detail.
- Law of dominance: The link discusses the law of dominance. Including linkage, pedigree analysis, autosomal dominant inheritance, etc.
- Mendelian Inheritance: The link provides complete information on the law of independent assortment.
Molecular Genetics is the field of genetics that studies the agents that pass information from one generation to another. These molecules, or genes are long polymers of DNA. The building blocks of DNA are four bases i.e., adenine (A), guanine (G), thymine (T), cytosine (C) plus the sugar 2’-deoxyribose sugar and a phosphate group.
- Molecular genetics resources: A list of links containing articles and tutorials, replication, DNA to protein, etc.
- DNA & molecular genetics: The link provides comprehensive information on DNA structure, and its inheritance pattern.
- Prokaryotic genetics: Learn about the complete operon model in E.coli
In genetics, inheritance means the transmission of genetic-material from parents to their offspring. In humans, genetic materials are transferred through sexual reproduction when egg and sperm fertilize. However, in many animals and plants, other reproductive methods are possible.
- Inheritance Pattern: Read a complete guide on inheritance. Including single gene inheritance, multifunctional inheritance and mitochondrial inheritance, etc.
- Inheritance: Read a complete lecture note on inheritance.
DNA and Chromosomes:
DNA or deoxyribonucleic acid is the carrier of genetic information. It is a double helix structure that contains 2’-deoxyribose sugar, phosphate group and four bases i.e., A, T, G, and C. DNA not only replicates, it transcripts and translate as well.
Chromosomes are threadlike structures in the eukaryotic cell nucleus that carry genes and can be seen during cell division. Chromosomes consist of both DNA and proteins. Though DNA works as a hereditary material, proteins function largely to package DNA molecules.
- DNA structure: Learn more about DNA structure in 3D graphics form.
- DNA double helix: Read a complete guide on DNA double helix and answer the quiz.
- Chromosomes structure: Learn more about bacterial and eukaryotic chromosome structure.
- Chromosome structure & terminology: Learn about chromosome structure, banding pattern, etc.
Genetic Expression is the process through which genetic information is utilized for the synthesis of functional gene product. These products may be proteins or non-protein coding genes including rRNA or tRNA genes. During transcription, DNA is transcribed into RNA and RNA is translated into proteins. Each functional protein formed during translation is responsible for performing a particular function.
- Translation: A comprehensive guide on translation
- DNA to RNA transcription: A simple guide on DNA transcription.
- Structure of rRNA: A complete study on ribosomal RNA.
Genetic Mutations are defined as the change in the genetic sequence and are considered the main cause of diversity among organisms. Mutation refers to a change that alters the nucleic acid i.e., DNA and in viruses they are the building blocks of either DNA or RNA. Some mutations cause cancer and other genetic diseases, however, mutations also lead to evolution.
- Mutation: Read a complete lecture on mutation.
- What is mutation: Learn more about mutation, mutagens, and DNA repair.
- Mutation mechanism: Learn how mutation takes place on molecular basis.
Natural Selection and Evolution:
Darwin first proposed the theory of natural selection. The main components of natural selection include variation, inheritance, high rate of population growth and differential survival and reproduction. Evolution, on the other hand, is the process through which species possess the ability to adapt themselves to the environment where they live. Evolution is closely related to natural selection, but other factors such as mutations and spatial isolation also contribute to the same.
- Darwin’s Natural selection: Read a comprehensive guide on natural selection and evolution.
- Theory of evolution: Read a complete guide on evolution processes.
- Evidence of evolution: Read a complete topic on evolution and the evidences of the same.
Genetic Research and Technology:
Research is being done worldwide in the field of genetics. Recombinant DNA technology, gene therapy, cloning, DNA fingerprinting, Microaaray, and the Human Genome Project are some of the most important advancements that scientists have achieved in the 21st Century.