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G`E`N`E`T`I`C`S

GENETICS, a discipline of biology, is the science of heredity and variation in living organisms. Knowledge of the inheritance of characteristics has been implicitly used since prehistoric times for improving crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of Gregor Mendel in the mid-1800s. Although he did not know the physical basis for heredity, Mendel observed that inheritance is fundamentally a discrete process with specific traits that are inherited in an independent manner — these basic units of inheritance are now called genes.
Following the rediscovery of Mendel's observations in the early 1900s, research in 1910s yielded the first physical understanding of inheritance — that genes are arranged linearly along large cellular structures called chromosomes. By the 1950s it was understood that the core of a chromosome was a long molecule called DNA and genes existed as linear sections within the molecule. A single strand of DNA is a chain of four types of nucleotides; hereditary information is contained within the sequence of these nucleotides. Solved by Watson and Crick in 1953, DNA's three-dimensional structure is a double-stranded helix, with the nucleotides on each strand complementary to each other. Each strand acts as a template for synthesis of a new partner strand, providing the physical mechanism for the inheritance of information.
The sequence of nucleotides in DNA is used to produce specific sequences of amino acids, creating proteins — a correspondence known as the "genetic code". This sequence of amino acids in a protein determines how it folds into a three-dimensional structure, this structure is in turn responsible for the protein's function. Proteins are responsible for almost all functional roles in the cell. A change to DNA sequence can change a protein's structure and behavior, and this can have dramatic consequences in the cell and on the organism as a whole.
Although genetics plays a large role in determining the appearance and behavior of organisms, it is the interaction of genetics with the environment an organism experiences that determines the ultimate outcome. For example, while genes play a role in determining a person's height, the nutrition and health that person experiences in childhood also have a large effect.

History of Genetics

Although the science of genetics has its origins in the work of Gregor Mendel in the mid-1800s, various theories of inheritance preceded Mendel. These theories generally assumed that there existed an inheritance of acquired characteristics: the belief that individuals inherit traits that have been strengthened in their parents. Today, the theory is commonly associated with Jean-Baptiste Lamarck, who used this pattern of inheritance to explain the evolution of various traits within species (these changes are now understood to be the product of natural selection rather than a product of soft inheritance).

Mendelian and classical genetics
The modern science of genetics traces its roots to the observations made by Gregor Johann Mendel, a German-Czech Augustinian monk and scientist who made detailed studies of the nature of inheritance in plants. In his paper "Versuche über Pflanzenhybriden" ("Experiments on 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.
The significance of Mendel's observations 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. The word "genetics" itself was coined in 1905 by William Bateson, a significant proponent of Mendel's work, in a letter to Adam Sedgwick. (The adjective "genetic", derived from the Greek word "genno" γεννώ: to give birth, predates the noun and was first used in a biological sense in 1860.) Bateson publicly promoted and popularized usage of word "genetics" to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.
In the decades following rediscovery and popularization of Mendel's work, numerous experiments sought to elucidate the molecular basis of DNA. In 1910 Thomas Hunt Morgan argued that genes reside on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies. In 1913 his student Alfred Sturtevant used the phenomenon of genetic linkage and the associated recombination rates to demonstrate and map the linear arrangement of genes upon the chromosome.

Features of Inheritance

Discrete inheritance and Mendel's laws

At its most fundamental level, inheritance in organisms occurs by means of discrete traits, called "genes". This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants. In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or white — and never an intermediate between the two colors. These different, discrete versions of the same gene are called "alleles".
In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent. Many organisms, including humans, have this pattern of inheritance. Organisms with two copies of the same allele are called "homozygous", while organisms with two different alleles are "heterozygous".
The set of alleles for a given organism is called its genotype, while the observable trait the organism has is called its "phenotype". When organisms are heterozygous, often one allele is called "dominant" as its qualities "dominate" the phenotype of the organism, while the other allele is called "recessive" as its qualities "recede" and are not observed. Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.
When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as "Mendel's first law" or the "Law of Segregation".

Assortment and Interactions of Multiple Genes

Organisms have thousands of genes, and in sexually reproducing organisms assortment of these genes are generally independent of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "Mendel's second law" or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. (Some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article.)
Often different genes can interact in a way that influences the same trait. In the blue-eyed Mary, for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all: color or white. When a plant has two copies of this white allele, its flowers are white — regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called "epistasis", with the second gene epistatic to the first.
Many traits are not discrete features (eg. purple or white flowers) but are instead continuous features (eg. human height and skin color). These "complex traits" are the product of interactions of many genes. The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called "heritability". Measurement of the heritability of a trait is relative, though — in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a complex trait with a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.