Patient Education

Genetics is a branch of biology that deals with the heredity and variation of living things.

Cells are the structural / functional unit of all living things. Our bodies are made up of billions of cells. Inside each cell is a copy of our genetic material (DNA).

Chromosomes are thread like structures located inside each cell in the body that hold and organize genes. Humans have 23 pairs of chromosomes, one of each pair being from your mom and the other from your dad, totaling 46 chromosomes.

Sex Chromosomes are the chromosomes that determine a person’s sex. In humans, there are two sex chromosomes, X and Y. Females have two X chromosomes and males have an X and a Y chromosome.

Genes are segments of DNA that contain instructions for a person’s physical traits, growth, development, and processes in the body. Every person has two copies of each gene, one inherited from each parent. The Human Genome Project revealed that humans have approximately 20,500 genes.

A gene mutation is a permanent change in the DNA sequence that makes up the gene which may cause it to not work properly. Mutations range in size, affecting anywhere from a single DNA building block (base pair) to a large segment of a chromosome that includes multiple genes. A gene mutation may lead to disabilities or other medical concerns.

Gene mutations can be classified in two major ways:

• Hereditary mutations are inherited from a parent and are present throughout a person’s life in virtually every cell in the body.
• Acquired (or somatic) mutations occur at some time during a person’s life and are present only in certain cells, not in every cell in the body. These changes can be caused by environmental factors, such as ultraviolet radiation from the sun, or can occur if an error is made as DNA copies itself during cell division.

A pathogenic variant is a change in a gene (mutation) that increases an individual’s chance of developing a certain disease or disorder. When such a variant (also known as a disease-causing mutation) is present, development of symptoms is more likely.

A chromosomal abnormality is a permanent change in the DNA sequence that involves a long enough segment of DNA that it changes the structure, number, or arrangement of the chromosome. A chromosomal abnormality may lead to disabilities or other medical concerns.

Genetic disorders are caused by changes (mutations) in a person’s genes or chromosomes. Symptoms and severity of genetic disorders varies widely depending on the size and location of the mutation.

With autosomal recessive disorders, mutations on both gene copies are necessary to cause disease. To be affected, typically one mutation would have been inherited from each parent. Someone who has one working gene copy and one mutated gene copy typically does not have any symptoms. This person is then referred to as a “carrier” of the condition. Two carriers of the same recessive disorder have a 25% chance to have an affected child in each pregnancy.

X-linked disorders are caused by gene mutations on the X chromosome. Females, because they have two X chromosomes, tend to be asymptomatic “carriers”.  Carrier females have a 50% chance of having affected sons and a 50% chance of having “carrier” daughters. Men, because they have only one X chromosome, tend to be affected and show symptoms of the associated disorder. Daughters of affected men are always carriers. An affected father cannot pass on the disorder to his son (fathers can only pass on their Y chromosome to their son).

Aneuploidy is a genetic condition in which a person has an abnormal number of chromosomes because there are missing or extra chromosomes.

Sex chromosome aneuploidies are genetic conditions caused by an abnormal number of sex chromosomes due to either missing or extra copies of the X or Y chromosomes (Examples include Turner syndrome and Klinefelter syndrome).

Trisomy is a genetic condition which there is an extra chromosome (47 chromosomes in total instead of the typical 46).

A monosomy is a genetic condition which there is a missing chromosome (45 chromosomes instead of the typical 46).

Down syndrome is a chromosomal condition that is caused by having an extra copy of chromosome 21. Most cases of Down syndrome are caused by a random event and are not inherited. Down syndrome is associated with intellectual disability, a characteristic facial appearance, and weak muscle tone in infancy. All affected individuals experience some degree of intellectual disability. People with Down syndrome may have a variety of birth defects including, heart defects and digestive abnormalities. They also have an increased risk of developing several medical conditions. These include gastroesophageal reflux, celiac disease, hypothyroidism, and hearing and vision problems. Additionally, there is a higher chance of developing leukemia and Alzheimer’s disease.

Trisomy 18, also called Edwards syndrome, is a chromosomal condition caused by having an extra copy of chromosome 18. Most cases of Trisomy 18 are not inherited, but occur as random events during the formation of eggs and sperm. Trisomy 18 is associated with abnormalities in many parts of the body. Individuals with Trisomy 18 often have slow growth before birth (intrauterine growth retardation) and a low birth weight. Affected individuals may have heart defects and abnormalities of other organs that develop before birth. Other features of Trisomy 18 include a small, abnormally shaped head; a small jaw and mouth; and clenched fists with overlapping fingers. Due to the presence of several life-threatening medical problems, many individuals with Trisomy 18 die before birth or within their first month. Five to 10 percent of children with this condition live past their first year, and these children often have severe intellectual disability.

Trisomy 13, also called Patau syndrome, is a chromosomal condition caused by having an extra copy of chromosome 13. Most cases of Trisomy 13 are not inherited and result from random events during the formation of eggs and sperm in healthy parents. Trisomy 13 is associated with severe intellectual disability and physical abnormalities in many parts of the body. Individuals with Trisomy 13 often have heart defects, brain or spinal cord abnormalities, very small or poorly developed eyes (microphthalmia), extra fingers or toes, an opening in the lip (a cleft lip) with or without an opening in the roof of the mouth (a cleft palate), and weak muscle tone (hypotonia). Due to the presence of several life-threatening medical problems, many infants with trisomy 13 die within their first days or weeks of life. Only five percent to 10 percent of children with this condition live past their first year.

A microdeletion syndrome is a genetic disorder caused by a small deletion (missing piece) on one of the chromosomes. Disorders caused by microdeletions vary in symptoms and severity. Examples include DiGeorge syndrome, Angelman syndrome, Prader-Willi syndrome, or Cri-du-chat syndrome.

Neural tube defects are abnormalities that can occur in the brain, spine, or spinal column, and are present at birth. These defects usually occur in the first month of pregnancy. There are several types of neural tube defects. Spina bifida is the most common type of neural tube defect. It occurs when the neural tube does not close completely. An infant born with spina bifida usually has paralysis of the nerves below the affected area of the spine, which can cause lifelong problems with walking and other difficulties.

Preeclampsia is a disorder that occurs only during pregnancy and the postpartum period and affects both the mother and the unborn baby. Affecting at least 5-8% of all pregnancies, it is a rapidly progressive condition characterized by high blood pressure and usually the presence of protein in the urine. Swelling, sudden weight gain, headaches and changes in vision are additional symptoms, however, some women with rapidly advancing disease report few symptoms. Early onset preeclampsia (EOPE) is defined as preeclampsia resulting in the delivery of the fetus before 34 weeks’ gestation.

Prenatal screening tests analyze a pregnant woman’s blood and/or findings from ultrasound examinations to assess the risk of having a baby with certain disorders. Prenatal screening includes first trimester screening, second trimester screening, and cell-free DNA screening.

Genetic Carrier Screening is done on parents before or during pregnancy. A blood sample is used to determine if a parent carries a gene mutation for certain autosomal recessive and X-linked disorders. It is used to help determine the risk of having a child who could be affected with the disorders.

What is the difference between prenatal screening tests and prenatal diagnostic testing?

Prenatal screening tests can tell you the chance that your baby has certain genetic disorders. Screening is considered non-invasive and usually performed through blood tests and/or ultrasounds. Prenatal diagnostic tests can tell you whether your fetus actually has certain genetic disorders. These tests are done through invasive procedures (amniocentesis or chorionic villus sampling) and come with a small risk of miscarriage.

Non-invasive prenatal screening is also known as cell-free DNA testing. Cell-free DNA is the small amount of DNA that is released from the placenta into a pregnant woman’s bloodstream. The cell-free DNA in a sample of a woman’s blood can be screened for Down syndrome, trisomy 13, trisomy 18, other autosomal chromosomal aneuploidies, sex chromosome aneuploidies, and certain microdeletion syndromes. This test can be done starting at 10 weeks of pregnancy.

A negative screening result means that your baby is at lower risk of having the disorder compared with the general population. It does not rule out the possibility that your fetus has the disorder.

A positive screening result means that your baby is at higher risk of having the disorder compared with the general population. It does not mean that your fetus definitely has the disorder. If you receive a positive result, your healthcare provider will review your options for next steps. For example, if you screen high risk for Down syndrome, your doctor may offer further screening or prenatal diagnostic testing.

In genetic counseling, specially-trained professionals help people learn about genetic conditions, find out their chances of being affected by or having a child or other family member with a genetic condition, and make informed decisions about testing and treatment.