Genetics for Plain People
What is DNA and what is a gene?
The structure and composition of living organisms varies greatly - from single celled bacteria to complex multi-cellular organisms with differentiated cell types and interconnected organ systems. Regardless of the complexity, every living entity contains a blueprint for its construction in the form of a double-helical chain of molecules called deoxyribonucleic acid (DNA). In humans, DNA is housed inside a nucleus, a separate membrane-bound compartment inside our cells. The DNA is arranged in long linear strings but is tightly wound around a complex of proteins that together form a chromosome. This compacted form makes the chromosome visible under the microscope. Each cell contains 46 chromosomes, of which 23 are contributed by each parent.
Every one of us has a unique genetic code carried by our DNA (deoxyribonucleic acid). This code is sometimes referred to as our genome. DNA itself is comprised of only four different molecules, called ‘nucleic acids’ or bases (adenine, guanine, cytosine and thymine—usually abbreviated to A, G, C and T). These are connected linearly into long strings, which are compacted and coiled in a highly complex manner within the nucleus. The 'function' of DNA is to make proteins which are composed of amino acids, also attached to each other in linear fashion creating proteins. The ‘genetic code’ consists of triplets of these bases (i.e., specific sequences such as CGT, or AAA) each of which selects a specific amino acid that contributes to the construction of a protein. Proteins perform most cell functions and even make up the majority of cellular structures.
A gene is the basic physical and functional unit of heredity that carries the code (i.e., connected DNA triplets) to make a specific protein product. The gene then, is simply a string of these DNA triplets, whose order determines the order of amino acids that together make up a protein such as insulin, for example. Each of our trillions of cells contains about three billion DNA bases that make up about 25,000 genes. Together, these direct the synthesis of many thousands of proteins that make up our body tissues and regulate the biochemical processes necessary to sustain life.
To learn more about the structure of DNA, and to see how DNA encodes proteins, see: http://learn.genetics.utah.edu/content/begin/dna/
What are genetic disorders?
A genetic disorder is a disease caused by abnormalities in an individual’s genetic material (genome). A ‘single-gene’ disorder (also called Mendelian or monogenic disease) is a type of inherited disease that is caused by changes (or mutations) that occur in the DNA sequence of a single gene. By changing a gene’s instructions for making a protein, a mutation can cause the protein to malfunction or to be missing entirely. When a mutation alters a protein that plays a critical role in the body, it can disrupt normal development or cause disease, or what is sometimes called a genetic disorder. Single-gene disorders are inherited in recognizable patterns (modes of inheritance): autosomal dominant, autosomal recessive, and X-linked. When a mutation of only one member of a paired gene—one from each parent—is sufficient to cause disease, we call the inheritance pattern “autosomal dominant.” Other conditions, however, require the mutation to be present in both members of the pair, which results in an “autosomal recessive” disease.
Why do we have genes that cause genetic disorders?
Sometimes genes are named after the disorders to which they have been shown to be linked. This can be very confusing. For example, the gene that causes hereditary hemochromatosis is called the “hemochromatosis gene.” This name implies that the gene exists for the sole purpose of causing disease. This of course is not the case as the normal function of a gene is to encode a protein, not to cause an illness. The genetic disease occurs because a particular gene is unable to work properly. The hemochromatosis gene actually encodes a membrane protein that works alongside other proteins to regulate iron absorption in cells. Like other single-gene disorders, hemochromatosis occurs when the heochromotosis gene is mutated in a way that prevents it from encoding a normal, functional protein.
Mutations are simply mistakes in the DNA code. We all carry some, most of which do not directly cause disease fortunately. Some mutations arise spontaneously during normal cell division. Most cells in our bodies constantly divide to replace those that die off and when the new DNA is formed, it may not be an exact duplicate of the original. Mutations may also arise through exposure to toxins or radiation in our environment which damages or changes the DNA bases. When such changes occur in reproductive cells and we pass them to our children, a genetic condition may result.