DNA (deoxyribonucleic acid) is often called the blueprint for life because it contains the instructions for how an organism functions. DNA is wrapped and organized into tightly packed structures called chromosomes. When functioning properly, this unique packaging helps DNA fit into cells and remain intact, even as cells divide.
Chromosomes are further organized into short segments called genes, the basic unit of genetics. They are passed down from parents to children, coding what we look like on the outside and how we work on the inside.
Every person has two copies of each gene, one from each parent, but the copies can vary slightly. Individual copies of a gene are referred to as alleles.
Humans have about 20,000 sets of genes and 23 pairs of chromosomes. The physical differences between people come from variations in these genes.
Changes in our DNA that cause disease are called mutations. DNA changes have varying (if any) effects on health depending on where they occur and whether they alter the function of essential proteins. Mutations can be inherited from our parents, or they occur spontaneously during development.
A person’s complete set of DNA, including all genes, is his or her genome. A genome contains all the information needed for development and lifelong biological maintenance.
Each genome more than 3 billion DNA base pairs, and every cell with a nucleus contains a copy of the entire genome.
A genetic disease or disorder is the result of mutations or alterations in an individual’s DNA. There are more than 10,000 different genetic conditions. Common ways to inherit a genetic condition are:
DNA contains three-letter code words known as “trinucleotide repeats.” Many genes typically contain a number of these repeats. While once thought benign in the genome, we now know these mutating stretches of DNA can sometimes expand over time and — with high enough numbers — can alter DNA and cause disease.
Repeat expansions can increase with each cell division and over successive generations in a family carrying the expansion mutation. Sometimes, a person may have more than the usual number of copies, but not enough to alter the function of the gene.
These individuals are referred to as “premutation carriers.” Over time, however, a carrier can have a gene that is not functioning properly (if at all). One of the most recognizable features of these disorders is an increase in disease severity through multiple generations.
An example of a trinucleotide repeat disorder is Fragile X syndrome, a disorder primarily affecting boys and causing intellectual disabilities and social anxieties, as well as physical characteristics such as over-sized ears and testes and an elongated face.
For the most part, complex inherited diseases are caused by a combination of genetic, lifestyle, and unknown environmental factors. Some disorders, such as Huntington’s disease and cystic fibrosis, are caused by a single gene mutation; most, however, are much more complex.
The vast majority of major health concerns — such as heart disease, asthma, diabetes, Alzheimer’s disease, and obesity — are complex diseases. This type of genetic inheritance is also referred to as multifactorial.
Due to their complexity, science does not yet fully understand the multitude of genetic and environmental factors involved in these conditions. Although often clustered in families, these disorders do not have a clear-cut pattern of inheritance, making it difficult to determine a person’s risk of inheriting or passing along the disease.
There are, however, a number of distinctive characteristics for multifactorial disease. Namely:
A person’s ethnicity influences his or her health through a complex interplay of social, environmental, biological, and genetic factors. Race alone can be a significant factor in the odds of any single person inheriting a specific genetic disease.
For example, sickle cell anemia — an inherited blood disorder occurring when red blood cells are unable to carry sufficient oxygen throughout the body — is most common in people of African, Caribbean, Middle Eastern, Mediterranean, South American and Central American heritage. In fact, sickle cell disease is so prevalent in those geographical locations because having one copy of the sickle cell gene mutation was and is a biological protection to malaria.
However, from a doctor’s or scientist’s perspective, disease incidences and drug responses sometimes vary among populations, just as they often vary by age or sex.
However, for almost all traits influenced by genetics, ethnicity is not a reliable predictor of any single person’s characteristics. While genetic variants differ in frequencies among populations, very few are found exclusively and commonly in only one specific population or race.
When averaged over the entire genome, about 85 to 90 percent of the genetic diversity in the human species is found in any group of humans. Therefore, two individuals from different continents probably differ genetically by only 10 to 15 percent more than two individuals chosen at random within the same continent.
Genetic defects in human embryos, whether from genetic or chromosome disorders, can be major barriers to having a healthy baby.
Two nearly identical techniques can be used in combination with in vitro fertilization (IVF) to identify these defects within embryos: preimplantation genetic diagnosis (PGD), which helps identify genetic mutations that could lead to an inherited disease, and preimplantation genetic screening (PGS), which helps intended parents identify chromosome problems that could result in a miscarriage. For more on these techniques, see the article “Preimplantation Genetic Testing” on this website.
Common disorders that PGD can test for include: