It is a common knowledge that after sexual reproduction, the resulting offspring tends to look like their parents in one way or the other. This resemblance may be related to the immediate parents or even to the grand parents. This tendency of the offspring to look like their parents or grand parents is a phenomenon which goes on from generation to generation. This is due to certain characters or traits which the offspring have inherited from their parents this confirming one of the fundamental observations about heredity made by men several thousand years ago, that is “like tends to begat like”.
Very often in our homes such questions are asked about inheritance of traits such as complexion, colours of eyes and hair possess by individuals. Also, out of many children in a family, a particular child may be described as exhibiting some traits pertaining to one of the grand parents. When questions of this nature arises, one is actually asking genetic questions.
So what then is Genetics?
Genetics may then be described as the branch of biological science that deals with questions and answer on inheritance which include traits or characters such as height, complexion, colour of hair and eyes etc, which are transmitted from parents to offspring during sexual reproduction. The ways and manners through which these characters are inherited from parents is known as heredity.
History of Genetics
The study of genetics dates back to several years in man’s curiosity to unravel the mechanism for the transmission of traits from parents to offspring. A breeder called Kilreuter worked with tobacco plants about 1770. He made crosses with different varieties of plants and produce hybrids. He also recognized that parental characters were transmitted by both the pollen and the ovule. However, studies of heredity before 1866 were not conclusive. The results obtained by earlier investigations offered little explanation of the way inheritable features were transmitted from one generation to the other.
Work in genetics continued to draw the attention of natural scientists, and in the year 1866, an Australian monk who lived in the monastry in Brun, published two small treaties on the laws of heredity in the journal of the local natural science society. This man was Gregor Mendel. He later became the head of the monastery. His many added responsibilities prevented him from continuing his study of heredity. He died in 1886 without realising that his two little contributions could form the foundation for all the work that has since been done in genetics.
Gregor Mendel: The father of Genetics
It was not until 1900 that three investigators working independently discovered what they thought were new findings about heredity. These men were de Vries in Holland, Correns in Germany, and Von Tschermak in Australia. They discovered the phenomenon of regular, predictable bratios of the types of offspring produced by mating pure breeding parents. But, then, searching through literature in the field, they came across Mendel’s earlier reports on the same subject. They then realised bthat Mendel was the pioneer in these investigations and gave him full credit for his work by naming two of the fundamental principles of heredity Mendel’s laws. All the work that has been done in genetics has made use of Mendel’s basic discoveries, and so, today, he is known as the father of modern genetics. Great advances have been made in genetics, cytogenetics and related fields but Mendel’s two laws still remain the fundamental laws of heredity in genetics.
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Various Branches of Genetics
A sketch on the Branches of Genetics
Classical genetics: consists of the technique and methodologies of genetics that predate the advent of molecular biology. A key discovery of classical genetics in eukaryotes was genetic linkage. The observation that some genes do not segregate independently at meiosis broke the laws of Mendelian inheritance, and provided science with a way to map characteristics to a location on the chromosomes.
Quantitative genetics: s the study of continuously measured traits (such as height or weight) and their mechanisms. It can be an extension of simple Mendelian inheritance in that the combined effects of one or more genes and the environments in which they are expressed give rise to continuous distributions of phenotypic values.
Biochemical genetics: the study of the fundamental relationships between genes, protein, and metabolism.
This involves the study of the cause of many specific heritable diseases
Cytogenetics: is a branch of genetics that is concerned with the study of the structure and function of the cell,
especially the chromosomes
Behavioural genetics: is the field of study that examines the role
of genetics in animal (including human) behaviour.
Developmental genetics is the study of the process by which organisms grow and develop
Conservation genetics: is an interdisciplinary science that aims to apply genetic methods to the conservation
and restoration of biodiversity
Ecological genetics: is the study of genetics in natural populations.
Genetic engineering: is the direct manipulation of an organism’s genome using biotechnology.
New DNA may be inserted in the host genome by first isolating and copying the genetic material of interest using molecular cloning methods to generate a DNA sequence, or by synthesizing the DNA, and then inserting this construct into the host organism.
Metagenics: is the practice of engineering organisms to create a specific enzyme, protein, or other biochemicals from simpler starting materials. The genetic engineering of E. coli with the specific task of producing human insulin from starting amino acids is an example.
Genomics: is a discipline in genetics that applies recombinant DNA, DNA sequencing methods, and bioinformatics to sequence, assemble, and analyze the function and structure of genomes (the complete set of DNA within a single cell of an organism
Human genetics: is the study of inheritance as it occurs in human beings. Human genetics encompasses a
variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics and genetic counselling.
Medical genetics: is the specialty of medicine that involves the diagnosis and management of hereditary disorders. Medical genetics differs from Human genetics in that human genetics is a field of scientific research that may or may not apply to medicine, but medical genetics refers to the application of genetics to medical care.
Microbial genetics: This involves the study of the genotype of microbial species and also the expression system in the form of phenotypes. It also involves the study of genetic processes taking place in these micro organisms i.e., recombination etc.
Molecular genetics: is the field of biology and genetics that studies the structure and function of genes at a molecular level. Molecular genetics employs the methods of genetics and molecular biology to elucidate molecular function and interactions among genes. It is so-called to differentiate it from other sub fields of genetics such as ecological genetics and population genetics.
Population genetics: is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population structure. It attempts to
explain such phenomena as adaptation and speciation.
Psychiatric genetics: is a subfield of behavioral neurogenetics, studies the role of genetics in psychological conditions such as alcoholism, schizophrenia, bipolar disorder, and autism. The basic principle behind psychiatric genetics is that genetic polymorphisms, as indicated by linkage to e.g. a single nucleotide
polymorphism (SNP), are part of the etiology of psychiatric disorders.
Importance of Genetics
Diseases and Treatments
Understanding the genetic basis behind human disease is one of the most important reasons for studying the human genome. While many genetic disorders are not treatable, early diagnosis can help improve the quality of life or even extend the lifespan of sufferers. Current clinical trials on genetic therapies for cystic fibrosis, hemophilia, and other genetic disorders offer the promise of eventual treatments that may give sufferers a life free of symptoms. Diagnostic tests can help couples decide whether to risk passing on specific disease-related genes to their children. Tests assist in vitro fertility doctors to specifically select embryos that do not carry the dangerous gene.
Studying human DNA and genetics can help scientists better understand where humans came from as a species. It can help elucidate the connections between different groups of people and give historians and anthropologists a clearer picture of historic human migration patterns. In some cases, a person’s genome can give clues to his personal ancestry and help him understand his genealogy. Genetic testing has been used to verify or rule out relatedness of individual persons or populations
Forensics and Legal Implications
The trial of O.J. Simpson in the 1990s brought to public light the use of human DNA in criminal cases, and the importance of human genetics in forensics has become even more important as techniques have improved. Human genetic information has been used to either match or rule out a suspect’s DNA to biological evidence found at a crime scene, to identify victims and to exonerate convicted individuals using newer genetic methods not available at the time of the initial conviction. Paternity testing is another common legal application of genetic testing.
Human genetic enhancement is a controversial topic, but research in this area holds some of the biggest promise for future applications. It will require a thorough understanding of human genetics before scientists can alter the human genome at the embryonic level, but once that is achieved, it may mean an end to certain incurable genetic diseases such as Down syndrome, congenital deafness and congenital heart defects. More controversial applications may include altering human DNA to enhance athletic ability, intelligence, or other characteristics.
Production of Improved Variety
Scientists have decoded the genomes of humans and various other plants and animals. This decoding would help in improving the yield, selecting better traits, producing disease resistant varieties which are the need of the hour to feed ever growing human population. Humans have about 24,000 different genes each made up a few hundred to a few thousand base pairs of DNA. All these genes are contained in 23 pairs of chromosomes, each of which carry thousands of different genes and millions of base pairs of DNA. When these genes are altered, there would be a direct effect on the synthesis of the corresponding proteins and consequently this will lead to diseases.
Application of Genetics
Taxonomy: Genetic characters like chromosome number and karyotypes are of taxonomic significance. Chromosome number helps in classification of plants. For example, the genus Triticum to which wheat belongs has been classified into three groups, viz., diploid, tetrapod and hexaploid.
Similarly, the genus Gossypium to which cotton belongs has been classified on the basis of chromosome number into two groups namely diploid and tetraploid. Karyotype suggests primitive or advanced feature of an organism.
A karyotype with large differences between the smallest and the largest chromosome of the set is known as asymmetric karyotype. Such karyotype is considered relatively advanced when compared with symmetrical karyotypes. The degree of chromosome homology is studied from chromosome pairing during meiosis. This gives an idea about the relationship of parental species. Higher the homology, closer is the relationship between the two species. Some species have B-chromosomes which help in the identification of such populations.
Agriculture: The contribution of genetics in the field of agriculture is remarkable in two ways, viz:
(i) In the improvement of crop plants and
(ii) In the improvement of domestic animals.
(i) Improvement of Crop Plants:
Various principles and methods of genetics have been applied for the development of plants useful to mankind. Controlled hybridization and artificial selection have increased usefulness of many plants.
Such applications include improvement in:
(iii) Maturity duration
(iv) Resistance to insects, diseases, salinity, drought, frost, lodging, etc. and
(ii) Improvement of Domestic Animals:
The usefulness of many domestic animals has been increased due to selective breeding. The milk production in cows and buffaloes, meat production in sheep’s, goats and pigs and egg production capacity in poultry have been significantly improved through the application of genetic principles. Moreover, many improved breeds of pet animals like horse, dogs, cats, pigeon and rabbits have been developed all over the world.
Medicine: The advancements made in the field of genetics have been useful in the field of Medicine in two main ways as given below:
(i) Detection of Hereditary Diseases:
Now hereditary diseases can be detected at an early stage of life when it is possible to provide secondary cures in some cases. Refined techniques such as amniocentesis (foetus test) and foetoscopy have made such cures possible. Moreover, genetic diseases can be prevented by advising future parents with the help of family pedigrees.
(ii) Production of Antibiotics:
Special genetic strains of fungi and bacteria have been isolated to greatly increase the yields of antibiotics and other drugs. Besides these, genetics also helps in settling the disputed case of children through blood group studies.
Evolution: Both natural and artificial selections have been responsible for evolution of various crop plants. However, selection is effective when sufficient amount of variability exists in the population in which selection has to be practiced.
Three genetic methods, viz:
(ii) Introgression, and
(iii) Mutagenesis have played significant role in the evolution of various crop plants by inducing additional genetic variability.
New plant species like Triticale have been evolved through the application of genetical principles. Genetics has also helped in understanding the genetic origin of various crop plants.
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