Genes create new variations through mutation. A gene controls a trait. Sometimes more than one gene controls a trait. Gene expression can also be regulated by the environment. Mammalian cells may contain over 50,000 genes. Each gene contains a certain number of nucleotides. The smallest gene has 75 and the largest gene has 40,000 nucleotides recorded.
The coding part of a gene is called an exon and the non-coding part is called an intron. Exons synthesize proteins. Sometimes exons are non-coding like introns. Such a gene is called a speit gene.