A Brief Introduction to Genetics

One of my biggest interests lies in psychiatric genetics, the study of how particular variants in the human genome contribute to mental illness. For today's post, I thought it would be helpful to give a quick-and-dirty review of genetics, for those of us who can't quite remember much from high school biology.

The blueprint of every organism is contained in long molecules of an organic chemical called deoxyribonucleic acid (DNA). DNA is a polymer of nucleotides - in other words, DNA is made up of a long chain of four different nitrogenous bases (adenine, thymine, guanine, and cytosine) in a particular order. DNA is found in the nucleus of every human cell, where it is organized into larger structures called chromosomes. Humans have 23 pairs of chromosomes (one set from each parent). The composite of all the DNA in a human cell is called the human genome, and it is functionally organized into units called genes. A gene codes for a particular protein, although the relationship is a little more complicated than just that. The ultimate goal of the genome is to make proteins - proteins can control biological reactions, make and support structures, alter gene expression, and have many other jobs. A gene is located at a particular locus on the genome, and there can be multiple copies of that gene (referred to as alleles).

Of particular interest in the field of genetics these days are genetic polymorphisms. A polymorphism is a variable region of DNA in which the rarest variant cannot be maintained by mutation alone. What this means is that polymorphisms arise through mutations in the genome, and then are maintained as they are passed along to offspring. The simplest kind of polymorphism is the single nucleotide polymorphism (SNP). A SNP is a single nucleotide in the long chain of DNA that is variable between different people. Polymorphisms can alter the structure of proteins, making them either more or less functional. They can also affect the expression level of the protein product.

While polymorphisms tend to not be deterministic (meaning that they do not directly cause something to happen), they can modify or increase susceptibility to a particular disease. For example, a particular copy of the ApoE gene (which is involved in lipid transport and metabolism) can increased your likelihood of developing Alzheimer's disease.

One last thing I wanted to add is that genes do not necessarily determine the function of the organism. Just because you may have a family history of heart disease does not mean that you will die of heart disease. The environment is a large source of variation as well, and it can interact with genes through a mechanism called epigenetics to alter gene products. In almost all cases, you can overcome diseases you are genetic susceptible to if you just maintained a healthy environment around you.