Gene

Molecular techniques that detect the presence and even the activity of genetic material are now a central part of forensic science. Exquisitely sensitive techniques can amplify and detect even small regions of deoxyribonucleic acid (DNA) that are present on objects such as cigarette butts or glass, or found underneath fingernails, as three examples.

Besides identifying the genetic material, modern-day forensic science techniques permit the detection of the fundamental unit of heritable genetic information (the gene), and can use genes to single out a person.

A gene is an individual element of an organism's genome and determines a trait or characteristic by regulating biochemical structure or a metabolic process.

Genes are segments of nucleic acid, consisting of a specific sequence and number of the chemical units of nucleic acids, the nucleotides. In most organisms the nucleic acid is DNA, although in retroviruses the genetic material is composed of ribonucleic acid (RNA). Some genes in a cell are active more or less all the time, which means that they are continuously transcribed and provide a constant supply of their protein product. These are the "housekeeping" genes that are always needed for basic cellular reactions. Others may be rendered active or inactive depending on the needs and functions of the organism under particular conditions. The signal that masks or unmasks a gene can come from outside the cell, for example, from a steroid hormone or a nutrient, or it can come from within the cell itself as a result of the activity of other genes. In both cases, regulatory substances can bind to the specific DNA sequences of the target genes to control the synthesis of transcripts.

In a paper published in 1865, Gregor Mendel (1823–1884), advanced a theory of inheritance dependent on material elements that segregate independently from each other in sex cells. Before Mendel's findings, inherited traits were thought to be passed on through a blending of the mother and father's characteristics, much like a blending of two liquids. The term "gene" was coined later by the Danish botanist Wilhelm Johannsen (1857–1927), to replace the variety of terms used up until then to describe hereditary factors. His definition of the gene led him to distinguish between genotype (an organism's genetic makeup) and phenotype (an organism's appearance). Before the chemical and physical nature of genes were discovered they were defined on the basis of phenotypic expression, and algebraic symbols were used to record their distribution and segregation. Because sexually reproducing, eukaryotic organisms possess two copies of an inherited factor (or gene), one acquired from each parent, the genotype of an individual for a particular trait is expressed by a pair of letters or symbols. Each of the alternative forms of a gene is also known as an allele. Dominant and recessive alleles are denoted by the use of higher and lower case letters. It can be predicted mathematically, for example, that a single allele pair will always segregate to give a genotype ratio 1AA:2Aa:1aa, and the phenotype ratio 2A:1aa (where A represents both AA and Aa since these cannot be distinguished phenotypically if dominance is complete).

In 1910, the American geneticist Thomas Hunt Morgan (1866–1945) began to uncover the relationship between genes and chromosomes. He discovered that genes were located on chromosomes and that they were arranged linearly and associated in linkage groups, all the genes on one chromosome being linked. For example, the genes on the X and Y chromosomes are said to be sex-linked because the X and Y chromosomes determine the sex of the organisms; in humans X determines femaleness and Y determines maleness. Nonhomologous chromosomes possess different linkage groups, whereas homologous chromosomes have identical linkage groups in identical sequences. The distance between two genes of the same linkage group is the sum of the distances between all the intervening genes. A schematic representation of the linear arrangement of linked genes, with their relative distances of separation, is known as a genetic map. In the construction of such maps, the frequency of recombination during crossing over is used as an index of the distance between two linked genes.

The molecular structure and activity of genes can be modified by mutations and the smallest mutational unit is now known to be a single pair of nucleotides, also known as a muton. Mutations used to be detected biochemically, typically by the failure of an organism to grow in a given food source due to the presence of the non-functional gene. Now, machines that automatically determine the arrangement of the nucleotide building blocks in the genetic material (a process called sequencing) allow mutations to be detected and, potentially, to match DNA with a victim or suspect.