**2.6 Dominant, recessive, and co-dominant genes**

The cell works via coded messages that send from both alleles of genes that involved in every set of chromosomes. Some of these genes appear dominant more than other in works [41]. The dominant occurs when one allele of gene is dominant, while the recessive appears in opposite to dominant within the pair. However, there is present other situation neither dominant nor recessive called homozygous [32]. Homozygous is an equal weight carrier combination of each allele in the gene pair and demonstrates phenotypic and physical characteristics between them [42].

### **2.7 The genetic keys**

The genetic information that endures on the genes order to produce specific protein will be converted to "switched on" position in a few specific specialized cells, and at the same time, other genes may be "switched off" position [43]. Cells differ in differentiation, so genes "switched on" in liver cells are completely different to those that at same position in brain cells [44].

Numerous cases are born with a defect in particular gene which related to a specific illness, and this does not mean you are more susceptible to it. At the same time, this raises the risk of appearing of the disease. So, the predisposition genetic effects such as occurrence of many types of cancer may be need to be triggered by environmental factors, and to reducing the risk achieved through decreasing or avoiding such triggers [45, 46].

In each gene, non-coding regions (introns) account more than 98% and as suggested previously, they do not have any function "junk," and do not involve any information of gene output in cells. The previous opinion about non-coding DNA regions is rejected despite that role is still unknown and appears to have very important roles to do in through gene expression and regulation in each cell [47].

## **2.8 Genes, mutation, and single nucleotide polymorphism**

As mentioned previously, about 23,000 genes in the human cell act as leader in growth and general health, which are responsible for everything in human life; in which the genetic code is a set of rules used by human being and every living system to translate the information encoded within DNA or RNA sequence to protein. Every three nucleotides called codon encode a certain amino acid in protein [48].

In this direction, any changes in the genetic code can lead to each person is exceptional in his behavior and health, that is to say, the alteration in the sequences of nucleotide in DNA can give a uniqueness characteristic for person. Mainly these changes are risk free, while others may have embroiled in proteins production either not properly, wrong sequence, or not produced totally [49]. Hence, the changes in genetic material cause inactive or disturbed gene called mutation. These mutations

occur in DNA sequence either by mistake during copy process or by environmental effectors. Sometimes mutations affect individuals directly or indirectly and are prone more susceptible to certain disease circumstances [50].
