The members of a species give birth to their own kind. The offsprings show some basic similarities and some dissimilarities with their parents and with each other. The sexually reproducing organisms produce sex cells or gametes. The gametes constitute the link between one generation and the next and pass on the paternal and maternal characteristics to the offspring. This relationship that continues to exist between successive generations is referred to as heredity. Although the off spring inherits the characteristics of the parent and resembles them very closely, the resemblances are not complete in all respects. The differences are referred to as variations, which are necessary for organic evolution. The significance of variations shows up only if they continue to be inherited by the progeny for several generations. Thus both heredity and variations are fundamental factors in the process of organic evolution. The study of the mode of transmission of characters from one generation to the next is known as Genetics.

Mendel  Laws of Innertance

The Austrian monk Gregor Mendel is considered as the pioneer of modern genetics and appropriately called father of genetics. Actually Mendel himself did not postulate any genetically principles or laws. He simply gave conclusive theoretical and statistical explanation for his hybridization experiments in his research paper. However, it was Carrens who postulated three laws on the basis of the result of Mendel’s work

(1)   Law of dominance,

(2)           Law of segregation,

(3)           Law of independent assortment.

Gene

 Gene is a piece of the genetic material that determines the inheritance of a particular characteristic, or group of characteristics. Genes are carried by chromosomes in the cell nucleus and are arranged in a line along each chromosome. Every gene occupies a place, or locus, on the chromosome.

The genetic material is deoxyribonucleic acid, or DNA, a molecule that forms the “backbone” of the chromosome. Because the DNA in each chromosome is a single, long, thin, continuous molecule, the genes must be parts of that molecule; and because DNA is a chain of minute subunits known as nucleotide bases, each gene includes many bases. Four different kinds of bases exist in the chain—adenine, guanine, cytosine, and thiamine—and their sequence in a gene determines its properties.

Genes exert their effects through the molecules they produce. The immediate products of a gene are molecules of ribonucleic acid (RNA); these are copies of the DNA, except that RNA has the base uracil instead of thiamine. The RNA molecules from some genes play a direct part in the metabolism of the organism, but most are used to make protein. The sequence of bases in the RNA determines the sequence of amino acids in the protein by means of the genetic code. The sequence of amino acids in a protein dictates whether it will become part of the structure of the organism, or whether it will become an enzyme for promoting a particular chemical reaction. Thus, changes in the DNA can produce changes that affect the structure or the chemistry of an organism.

The nucleotide bases in DNA that code the structure of RNAs and proteins are not the only components of genes; groups of bases adjacent to the coding sequences affect the quantities and dispositions of gene products. In higher organisms (animals and plants, rather than bacteria and viruses), the noncoding sequences outnumber the coding ones by a factor of ten or more, and the functions of these noncoding regions are largely unknown. This means that geneticists cannot yet set precise limits to the sizes of animal and plant genes.

Chromosome

In cytology, tiny threadlike structure, composed of nucleic acids and proteins (chromatin), found in all plant and animal cells. The chromosome contains the nucleic acid DNA, which is divided into small units called genes. The genes determine the hereditary characteristics, such as color and size, of the cell or organism. Normally the body cells of each species contain a specific number of chromosomes; in higher plants and in animals chromosomes occur as pairs. Humans have 23 pairs of chromosomes. The reproductive cells of most higher plants and animals have only half the number of chromosomes of other cells. During fertilization the sperm and egg unite, thus making pairs of chromosomes in the new organism, half the number coming from each parent.

Autosomes and sex chromosomes: In some organisms, for example in Drosophila. Human beings etc. both male and female cells have the same number of chromosomes, but in one sex (male) one chromosome pair is heteromorphic. This old chromosome is known as ‘Y’ chromosome and its partner is ‘X’ chromosome, that is females are homogametic (XX) and males are heterogametic (XY). Non sex chromosomes are called autosomes which are similar in number as well as morphology. The distinction of maleness and femaleness is evolutionary tendency in lower organisms; there is no absolute distinction between male and female.

Genotype and Phenotype

The genotype of an individual represents sum total of heredity whereas phenotype represents features, which are produced by interaction between genotype and environment. A genotype can thus exhibit different phenotypes under different conditions. Therefore, similar genotypes may not have the same phenotype. Conversely similar, phenotypes do not necessarily mean same genotype. In order to study the interaction of environment and heredity for the study of effect of different environments on a genotype would be to have individuals, which have same genotype. This can be done by using clones, pure lines or inbred lines. A clone is the progeny of a single plant reproduced asexually eg. Bacterial population derived from a single cell. A pure line is the progeny of a single plant obtained due to continuous self-fertilization. Similarly inbred lines are obtained in cross-fertilizing individuals due to fertilization among closely related lines.

Gene terminology

Photocopies: When two genotypes produce the same phenotypes due to different environments

Lethality: There are genes, which control certain phenotypic traits and at the same time also influence the viability of the individuals. Genes which cause the death of the individual are known as lethal genes. If the lethal effects is dominant over its normal allele all individuals carrying it will die and the gene cannot be transmitted to next generation such dominant lethal genes will therefore be lost in the same generation. On the other hand recessive lethals are carried in heterozygous condition and will express themselves only in the homozygous condition.

Complementary genes: The complementation between two genes meaning that both genes are necessary for the production of a particular phenotype.

Duplicate genes: When two different genes determine the same or nearly the same phenotype such genes are called duplicate genes.

Suppressor gene: Also called ‘Inhibitory genes’, the suppressor or inhibitory genes themselves do not directly cause the expression of characters but suppress the expression of other genes; such genes are involved in the expression of many coloration of plant leaf etc.

Multiple genes: There are some characters, which are controlled by more than 3 gene pairs. For example body colour of mice is determined by six pairs of genes.

Pleiotropism: It is assumed that a specific gene has a specific effect upon specific phenotypic traits of an organism that is each gene has its relation with a single phenytopic trait but mostly a single gene often influences more than one phenotype trait. These genes are called pleiotropic genes and this phenomenon of multiple effects of a single gene is called Pleiotropism.

Genetic Engineering[2]

The alteration of an organism’s genetic, or hereditary, material to eliminate undesirable characteristics or to produce desirable new ones. Genetic engineering is used to increase plant and animal food production; to diagnose disease, improve medical treatment, and produce vaccines and other useful drugs; and to help dispose of industrial wastes. Included in genetic engineering techniques are the selective breeding of plants and animals, hybridization (reproduction between different strains or species), and recombinant DNA.

Human Genome Project

It is an international scientific collaboration, the goal of which is to gain a basic understanding of the entire genetic blueprint of a human being. The ultimate goal of genomic mapping and sequencing is to associate specific human traits and inherited diseases with particular genes at precise locations on the chromosomes. The successful completion of the genome project will provide an unparalleled understanding of the fundamental organization of human genes and chromosomes. It promises to revolutionize both therapeutic and preventive medicine by providing insights into the basic biochemical processes that underlie many human diseases.

The idea of undertaking a coordinated study of the human genome arose from a series of scientific conferences held between 1985 and 1987. The Human Genome Project began in earnest in the United States in 1990 with the expansion of funding from the National Institutes of Health (NIH) and the Department of Energy (DOE). One of the first directors of the U.S. program was American biochemist James Watson, who in 1962 shared the Nobel Prize in physiology or medicine with British biophysicists Francis Crick and Maurice Wilkins for the discovery of the structure of DNA. Many nations have official human-genome research programs as part of this informal collaboration, including France, Germany, Japan, the United Kingdom, and other members of the European Union. The projected cost of the U.S. component of the project is $3 billion over a 15-year period ending in 2005.

The Human Genome

A genome is the complete collection of an organism’s genetic material. The human genome is composed of about 50,000 to 100,000 genes located on the 23 pairs of chromosomes in a human cell. A single human chromosome may contain more than 250 million DNA base pairs, and it is estimated that the entire human genome consists of about 3 billion base pairs.

The DNA being analyzed in the Human Genome Project typically comes from small samples of blood or tissue obtained from many different people. Although the genes in each person’s genome are made up of unique DNA sequences, the average variation in the genomes of two different people is estimated to be much less than 1 percent. Thus the differences between human DNA samples from various sources are small in comparison to their similarities.

Mapping and Sequencing

There are two main categories of gene-mapping techniques: linkage or genetic mapping, a method that identifies only the relative order of genes along a chromosome; and physical mapping, a group of more precise methods that can place genes at specific distances from one another on a chromosome. Both types of mapping use genetic markers, detectable physical or molecular characteristics that differ among individuals and that are passed from one generation to the next.

Linkage mapping was developed in the early 1900s by American biologist and geneticist Thomas Hunt Morgan. By observing how frequently certain characteristics were inherited in combination in numerous generations of fruit flies, he concluded that traits that were often inherited in combination must be associated with genes that were near one another on the chromosome. From his studies, Morgan was able to create a rough map showing the relative order of these associated genes on the chromosomes, and in 1933 he was awarded the Nobel Prize in physiology or medicine for his work.

Human linkage maps are created mainly by following inheritance patterns in large families over many generations. Originally, these studies were limited to inherited physical traits that could be observed easily in each family member. Today, however, sophisticated laboratory techniques allow researchers to create more detailed linkage maps by comparing the position of the target gene relative to the order of genetic markers, or specific known segments of DNA.

Physical mapping determines the physical distance between landmarks on the chromosomes. The most precise physical mapping techniques combine robotics, lasers, and computers to measure the distance between genetic markers. For these maps, DNA is extracted from human chromosomes and randomly broken into many pieces. The DNA fragments are then duplicated numerous times in the laboratory so that the resulting identical copies, called clones, can be tested individually for the presence or absence of specific genetic landmarks. Those clones that share several landmarks are likely to come from overlapping segments of the chromosome. The overlapping regions of the clones can then be compared to determine the overall order of the landmarks along the chromosome and the exact sequence in which the cloned pieces of DNA originally existed in the chromosome.

Very detailed physical maps that indicate the precise order of cloned pieces of a chromosome are required to determine the actual sequence of nucleotides. The Human Genome Project most commonly uses the DNA sequencing method developed by British biochemist and two-time Nobel laureate Frederick Sanger. In Sanger’s method, specific pieces of DNA are replicated and modified so that each ends in a fluorescent form of one of the four nucleotides. In modern automated DNA sequencers, the modified nucleotide at the end of such a chain is detected with a laser, and the exact number of nucleotides in the chain is determined. This information is then combined by computer to reconstruct the sequence of base pairs in the original DNA molecule.

Duplicating DNA accurately and quickly is of critical importance to both mapping and sequencing. Scientists first replicated fragments of human DNA by cloning them in single-celled organisms that divide rapidly, such as bacteria or yeast. This technique can be time consuming and labor-intensive. In the late 1980s, however, a revolutionary method of reproducing DNA, known as the polymerase chain reaction (PCR), came into widespread use. PCR is easily automated and can copy a single molecule of DNA many millions of times in a few hours. In 1993 American biochemist Kary Mullis was awarded the Nobel Prize in chemistry for originating this technique.

Bioinformatics

When completed, the Human Genome Project will have generated a catalog describing 50,000 to 100,000 human genes at some level of detail; high-resolution maps of the chromosomes, including hundreds of thousands of landmarks; and billions of base pairs of DNA-sequence information. Laboratory information-management systems, robotics, database-management systems, and graphical user interfaces are among the computing tools required to help genome researchers make sense of this flood of data.

A new field of research, bioinformatics, has developed to address the computing challenges raised by the project. Researchers in bioinformatics have developed public databases connected to the Internet to make genome data available to scientists worldwide. For example, the results of human-gene-mapping research is stored in the Genome Database, and DNA-sequence information is stored in several databases, including the NIH’s GenBank, the European Molecular Biology Laboratory’s Nucleotide Sequence Database, the DNA Databank of Japan, and the DOE’s Genome Sequence Database.

The genes associated with hereditary diseases such as cystic fibrosis, muscular dystrophy, and Huntington’s disease have been identified in recent years. This is the first step in developing better genetic screening tests, new drugs, and genetic therapies to fight these illnesses. The ability to correct fatal flaws in human genetic heritage may dramatically change the approach to human disease.

Increased knowledge of the human genome also has many controversial ethical, legal, and social implications. The project’s early findings have already sparked worldwide debate on the appropriateness of patenting human-gene sequences for commercial use, making genetic information available to insurance companies and employers, and correcting genetic defects in ways that would be passed from one generation to the next.