Types of Genetic Diseases
There are several types of genetic disorders. The way in which the disorder is inherited can help determine the risks it will have on a pregnancy and the risk it will recur in future children. Risks for having a baby with a birth defect from a genetic abnormality may be increased when:
- the parents have another child with a genetic disorder.
- there is a family history of a genetic disorder.
- one parent has a chromosomal abnormality.
- the fetus has abnormalities seen on ultrasound.
The following are the different types of genetic diseases:
- chromosomal abnormalities
- single gene defects
- multifactorial problems
- teratogenic problems
Chromosomal abnormalities in the baby may be inherited from the parent or may occur with no family history. The following chromosomal problems are the most common:
- aneuploidy - more or fewer chromosomes than the normal number, including:
- Down syndrome (trisomy 21) - cells contain three #21 chromosomes.
- Turner syndrome - one of the two sex chromosomes is not transferred, leaving a single X chromosome, or 45 total.
- deletion - part of a chromosome is missing, or part of the DNA code is missing.
- inversion - when a chromosome breaks and the piece of the chromosome turns upside down and reattaches itself. Inversions may or may not cause birth defects depending upon their exact structure.
- translocation - a rearrangement of a chromosome segment from one location to another, either within the same chromosome or to another.
- balanced translocation - the DNA is equally exchanged between chromosomes and none is lost or added. A parent with a balanced translocation is healthy, but he/she may be at risk for passing unbalanced chromosomes in a pregnancy.
- Robertsonian translocation - a balanced translocation in which one chromosome joins the end of another.
- mosaicism - the presence of two or more chromosome patterns in the cells of a person, resulting in two or more cell lines (i.e., some with 46 chromosomes, others with 47).
These are also known as Mendelian inheritance disorders, from the first genetic work of Gregor Mendel. In these disorders, a single gene is responsible for a defect or abnormality. Single gene disorders usually have greater risks of inheritance. Single gene disorders can be:
- dominant - an abnormality occurs when only one of the genes from one parent is abnormal. If the parent has the disorder, the baby has a 50 percent chance of inheriting it. Examples include the following:
- achondroplasia - imperfect bone development causing dwarfism.
- Marfan syndrome - a connective tissue disorder causing long limbs and heart defects.
- recessive - an abnormality only occurs when both parents have abnormal genes. If both parents are carriers, a baby has a 25 percent chance of having the disorder. Examples include the following:
- cystic fibrosis - a disorder of the glands causing excess mucus in the lungs and problems with pancreas function and food absorption.
- sickle cell disease - a condition causing abnormal red blood cells.
- Tay Sachs disease - an inherited autosomal recessive condition that causes a progressive degeneration of the central nervous system which is fatal (usually by age 5).
- X-linked - the disorder is determined by genes on the X chromosome. Males are mainly affected and have the disorder. Daughters of men with the disorder are carriers of the trait and have a one in two chance of passing it to their children. Sons of women who are carriers each have a one in two chance of having the disorder. Examples include the following:
- Duchenne muscular dystrophy - a disease of muscle wasting
- hemophilia - a bleeding disorder caused by low levels, or absence of, a blood protein that is essential for clotting.
Some birth defects do not follow a single gene or chromosomal abnormality pattern. They may be due to several problems, or a combined effect of genes and the environment. It is difficult to predict inheritance of abnormalities caused by multiple factors. Examples include heart defects, cleft lip or cleft palate, and neural tube defects (defects in the spine or brain).
Certain substances are known to cause abnormalities in babies. Many birth defects occur when the fetus is exposed to teratogens (substances that cause abnormalities) during the first trimester of pregnancy when organs are forming. Some known teratogens include the following:
- certain medications (always consult your physician before taking any medications during pregnancy)
- high level radiation exposure
- certain infections (such as rubella)
Families at risk for genetic diseases may want to consult a specialized genetics counselor. A careful family pedigree (chart of members of the family) and history may help determine risks for certain problems. Counseling also helps parents understand the effects of a disorder and ways it may be prevented or treated.
It may be necessary to check each parent's DNA to learn about some genetic inheritance patterns. Prenatal testing is also available to check the fetus for problems. Testing may include ultrasound (using sound waves to look at internal structures), chorionic villus sampling (testing the tissues around the fetus), or amniocentesis (withdrawing a sample of the amniotic fluid).