what is a gene

A gene is the basic physical and functional unit of heredity. It is a segment of DNA that contains the instructions for a specific molecule, usually a protein. Genes are passed from parents to offspring and contain the information needed to specify physical and biological traits. There are two types of genes: the Mendelian gene, which is a basic unit of heredity, and the molecular gene, which is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. During gene expression, the DNA is first copied into RNA, which can be directly functional or be the intermediate template for a protein that performs a function. Some genes act as instructions to make molecules called proteins, while many genes do not code for proteins. Humans have approximately 20,000 protein-coding genes. Genes come in different forms, called alleles, and in humans, alleles of particular genes come in pairs, one on each chromosome. An individual’s phenotype is determined by the combination of alleles they have.