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A body's genetic materials can be found within the nucleus of each of its cells. These genetic materials consist of coils of deoxyribonucleic acid or what is known as DNA. They are arranged in a complex way to form chromosomes of which there are 46 in pairs in each human cell. Of these pairs of chromosomes, one is the sex chromosome. The DNA molecule is a long double helix that when looked at under a microscope would remind one of a spiral stair case. The steps of this stair case are what determines a person's genetic code. These consist of pairs of four types of molecules which are called bases. In each of these steps guanine is paired with cytosine while thymine is paired with adenine. A person's genetic code is written in triplets, thus each step of the staircase codes the production of only one of the amino acids. These amino acids are the building blocks for proteins.

At any given time, when a part of the DNA molecule is controlling a function of the cell, its helix will split open along its length with one strand being inactive while the other strand acts as a template for a complementary strand of ribonucleic acid or RNA to form. The bases for the RNA are arranged in the same sequence as the bases of the inactive strand of DNA with the exception of the fact that RNA contains uracil and DNA contains thymine. This copy, which is called messenger RNA, will separate from the DNA leaving the nucleus and traveling into the cytoplasm of the cell. It then attaches to ribosomes which are the cell's factories for making proteins and instructs the ribosome as to the sequence of amino acids that are needed to construct a specific protein. These amino acids are brought to the ribosome by what are known as transfer RNA which is a much smaller type of RNA. Each molecule of the transfer RNA will bring one amino acid to be used in the growing chain of protein.

Each gene consists of a code that is needed to construct one protein. Depending on the size of the protein the genes will vary in size. All genes are arranged in a precise sequence on the chromosomes with the location of a specific gene called its locus. It is the two sex chromosomes that determine if a fetus will become male or female. Females will have two X chromosomes with only one being active while the male will have one X and one Y sex chromosomes. Since the Y chromosome carries very genes, one of which determines sex, in males all the genes on the X chromosome whether recessive or dominant are expressed. The genes on the X chromosome are called sex-linked or X- linked genes.

It is not uncommon for a person to have abnormalities of one or more gene and especially the recessive genes. Each human carries six to eight abnormal recessive genes but these genes do not cause cells to function abnormally except when two similar recessive genes are present. Generally the chance of a person having two similar recessive genes is minute but in the case of children of close relatives the chance becomes higher. This is also true with groups of people who inter-marry. The genetic make up of a person is known as a genotype. The expression of the genotype is known as the phenotype. Each inherited trait of a person is encoded by genes. In most cases characteristics such as hair color is not considered abnormal but any abnormal characteristics express by an abnormal gene can lead to hereditary disease.