<P> The concept of a gene continues to be refined as new phenomena are discovered . For example, regulatory regions of a gene can be far removed from its coding regions, and coding regions can be split into several exons . Some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs . Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism's traits by being expressed as a functional product or by regulation of gene expression . </P> <P> The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1905 . It is inspired by the ancient Greek: γόνος, gonos, that means offspring and procreation . </P> <P> The existence of discrete inheritable units was first suggested by Gregor Mendel (1822--1884). From 1857 to 1864, in Brno (Czech Republic), he studied inheritance patterns in 8000 common edible pea plants, tracking distinct traits from parent to offspring . He described these mathematically as 2 combinations where n is the number of differing characteristics in the original peas . Although he did not use the term gene, he explained his results in terms of discrete inherited units that give rise to observable physical characteristics . This description prefigured Wilhelm Johannsen's distinction between genotype (the genetic material of an organism) and phenotype (the visible traits of that organism). Mendel was also the first to demonstrate independent assortment, the distinction between dominant and recessive traits, the distinction between a heterozygote and homozygote, and the phenomenon of discontinuous inheritance . </P> <P> Prior to Mendel's work, the dominant theory of heredity was one of blending inheritance, which suggested that each parent contributed fluids to the fertilisation process and that the traits of the parents blended and mixed to produce the offspring . Charles Darwin developed a theory of inheritance he termed pangenesis, from Greek pan ("all, whole") and genesis ("birth") / genos ("origin"). Darwin used the term gemmule to describe hypothetical particles that would mix during reproduction . </P>

Dna that contains genes from more than one source is called