Genes are the basic units that determine the traits that we inherit from our parents. They contain coded information found in nearly every cell of the human body.
Our genes are made up of DNA, a molecule with this coded information. Hundreds to thousands of genes are found on each chromosome inside our cells. Together, the DNA, genes, and chromosomes make up the complete set of genetic instructions for every individual—referred to as a genome. These instructions include a person's sex, features, and risk of medical conditions.
The Human Genome Project, founded in 1990, mapped the entire human genome to understand how genes and chromosomes influence diseases and to find better ways to treat them. Based on information from the project, scientists have been able to discover over 1,800 genes that cause disease and, in turn, create tests and medicines to help diagnose and treat them.
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What Is a Genome?
A genome is the complete set of genetic instructions that determine the traits and characteristics of an organism. It contains all the information needed for an individual to develop and function.
There are three components to the genome of any living organism:
- DNA: This is a molecule that contains the genetic code that makes every individual unique.
- Genes: These are functional units of heredity made of up DNA. Every human has around 20,000 genes, half inherited from a person's mother and half inherited from a person's father.
- Chromosomes: These are structures within cells made up of around 1,000 genes each. In total, there are two sets of 23 chromosomes in nearly every cell of the body, one set inherited from a person's mother and the other from a person's father.
While the genome of each species is distinct, every organism within that species has its own unique genome. This is why no two people are exactly alike, including twins.
How Genomic Testing Predicts Health Risks
What Is DNA Made Of?
DNA (deoxyribonucleic acid) is a building block that makes up your genes.
Within DNA is a unique chemical code that guides your growth, development, and function. The code is determined by the arrangement of the following four chemicals known as nucleotide bases:
- Adenine (A)
- Cytosine (C)
- Guanine (G)
- Thymine (T)
The bases pair up with each other—A with T and C with G—to form units known as base pairs. The pairs are then attached to form what looks like a spiraling ladder, known as a double helix.
The specific order, or sequence, of these nucleotide bases determines which instructions are given.
Human DNA consists of around 3 billion of these bases, of which 99% are the same for all humans. The remaining 1% is what differentiates one human from the next.
What Are Genes Made Of?
Genes are composed of DNA and encoded for a specific purpose. How genes are encoded will determine how you look or how your body functions. Every person has two copies of a gene, each inherited from a parent.
Some genes provide instructions to produce specific types of proteins. Proteins are the molecules that not only make up tissues (like muscles and skin) but also play critical roles in the development and function of the body.
Other genes are encoded to produce RNA (ribonucleic acid), a molecule that converts the information stored in DNA to make protein.
Different versions of a gene are known as alleles.
The alleles you inherit from your parents may determine, for example, if you have brown eyes or blue eyes. Others may lead to congenital (inherited) disorders like cystic fibrosis or Huntington’s disease. Others still may not cause disease but may increase your risk of getting one (like cancer).
Genes only make up between 1% and 5% of the human genome. The rest is made up of non-coded DNA that doesn't produce protein but helps regulate how genes function.
Is Cancer Genetic?
What Are Chromosomes Made Of?
Genes are packaged into bundles known as chromosomes. Humans have 23 pairs of chromosomes for a total of 46 individual chromosomes. Chromosomes are contained within the nucleus of almost every cell of the body.
One pair of chromosomes, called the sex chromosomes, determines whether you are born male or female. Females have a pair of XX chromosomes, while males have a pair of XY chromosomes.
The other 22 pairs are called autosomal chromosomes. These determine the rest of your body’s makeup.
Certain genes within these chromosomes may either be dominant or recessive. This can determine which traits predominate and which don't. By definition:
- Autosomal dominant means that you need only one copy of an allele from one parent for a trait to develop (such as brown eyes or Huntington's disease).
- Autosomal recessive means that you need two copies of the allele—one from each parent—for a trait to develop (such as blue eyes or cystic fibrosis).
How Genetic Disorders Are Inherited
What Is Genetic Variation?
Genes are prone to coding errors. Many errors won't make any significant difference in the structure or function of a person's body, but some can.
Some genetic variations will directly cause a defect or disease, some of which may be congenital (seen at birth) and others that may only be seen later in life.
Other variations can lead to changes in the entire "gene pool" (the characteristic genes in a population) that will affect inheritance patterns in later generations.
There are three common types of genetic variation:
Genetic Mutations
A genetic mutation is a change in the sequence of DNA. This is often due to copying errors that occur when a cell divides. It can also be caused by outside forces like an infection, chemicals, or radiation that damage the structure of genes.
Genetic disorders like sickle cell disease, Tay-Sachs disease, and phenylketonuria are all caused by the mutation of a single gene.
Radiation-induced cancer is caused by genetic changes caused by excessive exposure to medical or occupational radiation.
Genetic Recombination
Genetic recombination is a process in which pieces of DNA are broken, recombined, and repaired to produce a new allele. Also referred to as "genetic reshuffling," recombinationoccurs randomly in nature during a normal event during cell division. The new allele can then be passed from parents to offspring.
Down syndrome is one such example of genetic recombination.
Genetic Migration
Genetic migration is an evolutionary process in which the addition or loss of people in a population changes the gene pool, making certain traits either less common or more common.
A theoretical example is the loss of red-haired people from Scotland, which over time may result in fewer and fewer Scottish children being born with red hair.
Genetic Testing
Based on the findings of the Human Genome Project, scientists have been able to create over 2,000 genetic tests to help diagnose genetic disorders or predict your risk of getting them. Genetic testing can be performed on blood, skin, hair, amniotic fluid, or other body tissues.
Genetic tests look for specific DNA mutations associated with different diseases. If you are suspected of having an inheritable disease or have a family history of one, genetic testing may be recommended.
Other reasons for testing include:
- To determine if certain parents are at risk of having a child with a genetic disease
- To determine if you are a "carrier" of a gene you can pass to a child
- To determine if an unborn child has a genetic disease
- To routinely screen newborns for up to 50 different genetic disease
- To see if a person is at an increased risk of developing a certain disease
- To determine if targeted drugs can be used based on the genetic makeup of the target cell
- To determine what dose of a drug is needed based on a person's genetic makeup
Some of these tests have a higher prognostic (predictive) value than others. While some test results are conclusive, delivering a positive or negative result, others may require a genetic counselor to help you understand what a result does and doesn't mean.
This is especially true if parents learn that they are at risk of having a baby with a potentially severe birth defect. A genetic counselor can help characterize the risk so that they can make an informed choice.
List of Genetic Disorders Detected by Genetic Testing
Today, there are not only lab-based tests to detect certain diseases but also home kits you can purchase at stores or online to help predict your risk. While the prognostic values of self-tests are improving, they are prone to inaccuracies.
Even so, a positive result may encourage you to see a healthcare provider for further testing (such as finding you have a BRCA mutation linked to an increased risk of breast cancer).
Lab-based genetic testing can help detect an ever-increase range of disorders and diseases, including:
- Acute lymphoblastic leukemia (ALL)
- Acute myeloid leukemia (AML)
- Alzheimer's disease (early-onset)
- Amyloidosis
- Aortic aneurysm (familial)
- Autoimmune diseases
- Breast cancer
- Chronic insomnia
- Chronic pancreatitis
- Color blindness
- Congenital heart defects
- Cystic fibrosis
- Down syndrome
- Epilepsy
- Glaucoma
- Hemophilia
- Huntington's disease
- Macular degeneration
- Multiple endocrine neoplasia syndrome (MENS)
- Muscular dystrophy
- Neural tube defects
- Obesity
- Parkinson's disease (early-onset)
- Phenylketonuria
- Polycythemia vera
- Retinoblastoma
- Rickets
- Sickle cell disease (SCD)
- Tay-Sachs disease
- Thalassemia
- Thrombocytopenia
- Tourette's syndrome
Summary
DNA is the building blocks that contain the coded instructions for building and maintaining a body. Genes are comprised of DNA and are tasked with making specific proteins that play a critical role in the structure and function of the body. Chromosomes containing thousands of genes are passed from parents to offspring and determine an individual's unique traits.
Together, DNA, genes, and chromosomes make up each organism's genome.
Genetic tests can detect mutations that may help diagnose or predict your risk of certain diseases. They can also be used to see if you or your partner are carriers of a gene you can pass to a child if you decide to get pregnant.
