Four Basic Definitions in Genetics
When considering the complex and detailed genetic processes that occur in the nuclei of cells, there are some concepts that are a requisite for a accurate understanding: chromosomes, DNA, genes and alleles. These four notions will be briefly, but hopefully clearly, discussed here.
Chromosomes are double structures that are constituted by strains of chromatin, which contains DNA and protein. Human beings normally posses 46 chromosomes (or 23 pair) in the nucleus or regular cells in the body, called somatic cells. In contrast, gametes or sexual reproductive cells, contain 23 unpaired chromosomes. A chromosome has a short and a long arm, which are bound together by a centromere.
DNA, or deoxyribonucleic acid, is the chemical molecule that serves as genetic material. A strain of DNA is a long chain (or polymer) of nucleotides. Each nucleotide in the DNA contains a nitrogen containing base, a sugar with five carbon atoms called deoxyribose and a phosphate group. In DNA there are four kinds of nitrogen containing bases: adenine, thymine, cytosine and guanine. The double helix of DNA is coiled up and contained within the chromosomes.
Genes are located along the DNA strain and are constituted by chains of nucleotides. Some genes have an enormous number of nucleotides, while others only have a few. Human beings posses thousands of genes, located on different chromosomes, but on the same chromosome for every human being. For example, the gene for cystic fibrosis is, in human beings, always found on chromosome 7. The gene encoding for Huntington’s disease is found on chromosome 4 and the one that causes sickle cell anemia on chromosome 11. And this is true for all people. Yet, this does not mean that everybody will develop one or several of these conditions. Some genes are expressed, while others are oppressed.
Alleles are the different form of a characteristic. For example, the gene or genes for hair color are found on the same spot in all people. Still, people can have different hair colors, which are represented by different alleles. Let’s say that the gene for hair color can be found on chromosome 2 (this is just an imaginary example). This gene has different alleles, say one from the father who has dark brown hair and one from the mother who has blond hair. Since an allele for brown hair color is dominant over one for blond hair, the child will have brown hair.
Important to note here is that most human characteristics are polygenic, which means that many genes are involved in the expression of those traits. This, of course complicates the matter.