Genetics Primer

Genetics at a glance

  • DNA is often called the blueprint for life because it contains the instructions for how an organism functions.
  • Changes in our DNA that cause disease are called mutations. Changes in DNA have varying (if any) effects on health depending on where they occur and whether they alter the function of essential proteins.
  • Mutations are inherited from our parents, or they occur spontaneously during development.
  • Genetic testing used during in vitro fertilization (IVF) can identify defects within embryos. This testing may help prevent certain diseases or disorders from being passed on to the child.

What are DNA, genes & chromosomes?

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.

Where Do Genes Come From?

How are genetic diseases inherited?

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:

  • Autosomal dominant inheritance: With an autosomal dominant condition, an alteration in one copy of the gene is sufficient to impair cell function, leading to disease. When a parent has a dominant trait, there is a 50 percent chance that any child will also inherit it. Autosomal dominant disorders effect men and women equally and tend to occur in every generation of an affected family.
  • Autosomal recessive inheritance: “Recessive” means that two copies of the gene are necessary to have the trait, one inherited from the mother and one from the father. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (referred to as carriers). Most people are unaware that they carry a recessive gene until they have a child with a disease. There is an increased risk among siblings to inherit a disease.
  • X-linked recessive inheritance: these conditions are caused by alterations or missing genes on the X chromosome. As males have only one X chromosome, if they have a gene alteration on their X chromosome they will develop the condition. Subsequently, males are primarily affected. X-linked recessive genes are expressed in females only if there are two copies of the gene (one on each X chromosome).
  • Mitochondrial inheritance: mitochondria, often called a cell’s “power house,” convert molecules into energy. Since only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial mutations to their children. Disorders resulting from mutations in mitochondrial DNA can appear in every generation of a family and can affect both men and women.

Trinucleotide repeat disorders

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.

Complex diseases

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:

  • Environmental influences such as a person’s lifestyle or exposure to carcinogens can increase or decrease the risk of the disease.
  • The disease occurs more frequently in one gender than in the other, but it is not sex-limited. In addition, first-degree relatives of individuals belonging to the more rarely affected gender have a higher risk of the disease.
  • The disease can occur in isolation, meaning that two unaffected parents can have affected children without a clear pattern.
  • The disease occurs more frequently in a specific ethnic group.

Overview of genetics & ethnicity

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.

What kind of genetic mutations can scientists screen for?

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:

  • Cystic fibrosis
  • Huntington’s disease
  • Sickle cell disease
  • BRCA1 mutations (a cancer causing gene)
  • Tay-Sachs disease
  • X-linked disorders
  • Myotonic dystrophy