Overview of Chromosome and Gene Disorders

(See also Chromosomal abnormalities Chromosomal abnormalities Genetic disorders are disorders caused by abnormalities in one or more genes or chromosomes. Some genetic disorders are hereditary and others are spontaneous. Hereditary genetic disorders are... read more .)

Chromosome abnormalities can affect any chromosome, including the sex chromosomes Sex chromosomes Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells... read more . Chromosome abnormalities affect the

Larger abnormalities may be visible with a microscope in a test called chromosome analysis or karyotyping. Smaller chromosome abnormalities can be identified using specialized genetic tests that scan a person's chromosomes for extra or missing parts. These tests include chromosomal microarray analysis (CMA) and fluorescent in situ hybridization (FISH). (See also Next-generation sequencing technologies Next-generation sequencing technologies Genetic diagnostic technologies are scientific methods that are used to understand and evaluate an organism's genes. (See also Genes and Chromosomes.) Genes are segments of deoxyribonucleic... read more .)

Numerical abnormalities occur when a person has one or more extra copies of a chromosome (for example, one extra is trisomy, and two extra is tetrasomy) or is missing a chromosome (monosomy). Trisomy can affect any of the 23 paired chromosomes, but the most common are trisomy 21 (Down syndrome Down Syndrome (Trisomy 21) Down syndrome is a chromosome disorder caused by an extra chromosome 21 that results in intellectual disability and physical abnormalities. Down syndrome is caused by an extra chromosome 21... read more ), trisomy 13 Trisomy 13 Trisomy 13 is a chromosomal disorder caused by an extra chromosome 13 that results in severe intellectual disability and physical abnormalities. Trisomy 13 is caused by an extra chromosome 13... read more , and trisomy 18 Trisomy 18 Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities. Trisomy 18 caused by an extra chromosome 18. Infants... read more , which affect both boys and girls. These abnormalities are visible with a microscope in karyotyping.

The older a pregnant woman is, the greater the chance that her fetus will have a whole extra chromosome or will be missing a chromosome ( see Table: What Is the Risk of Having a Baby With a Chromosomal Abnormality*? What Is the Risk of Having a Baby With a Chromosomal Abnormality*? ). The same is not true of a man. As a man gets older, the chance of conceiving a baby with a chromosome abnormality is only slightly increased.

Structural abnormalities occur when part of a chromosome is abnormal. Sometimes part or all of a chromosome incorrectly joins with another chromosome (called translocation). Sometimes parts of chromosomes are missing (called deletion― see Overview of Chromosomal Deletion Syndromes Overview of Chromosomal Deletion Syndromes Chromosomal deletion syndromes occur when part of a chromosome is missing. (See also Overview of Chromosome Disorders.) Chromosomes are structures within cells that contain DNA and many genes... read more ) or have been duplicated.

Some chromosome abnormalities cause the death of the embryo or fetus before birth. Other abnormalities cause problems such as intellectual disability Intellectual Disability Intellectual disability is significantly below average intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily... read more , short stature Growth Hormone Deficiency in Children , seizures, heart problems, or a cleft palate Cleft Lip and Cleft Palate A cleft is an opening that can form in the lip (cleft lip), roof of the mouth (cleft palate), or both if the tissue does not join together completely during pregnancy. Cleft lip and cleft palate... read more .

Changes in one or more base pairs of the DNA in a gene (see figure Structure of DNA Structure of DNA create a variant of that gene that may affect how the gene works. These changes do not affect the structure of the chromosomes and thus cannot be seen on karyotype analysis or other chromosomal tests. More specific genetic testing is required. Some variants in a gene cause no problems and some cause few or only mild problems. Other variants cause serious disorders such as sickle cell anemia Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin (the oxygen-carrying protein found in red blood cells) characterized by sickle (crescent)-shaped red blood cells and chronic... read more , cystic fibrosis Cystic Fibrosis (CF) Cystic fibrosis is a hereditary disease that causes certain glands to produce abnormally thick secretions, resulting in tissue and organ damage, especially in the lungs and the digestive tract... read more , and muscular dystrophy Introduction to Muscular Dystrophies and Related Disorders Muscular dystrophies are a group of inherited muscle disorders in which one or more genes needed for normal muscle structure and function are defective, leading to muscle weakness of varying... read more . Increasingly, medical scientists are finding specific genetic causes of children's diseases.

It remains unclear how most variants occur, but most are thought to appear spontaneously. Some substances or agents in the environment are capable of damaging and causing variation in genes. These substances are called mutagens. Mutagens, such as radiation, ultraviolet light, and certain drugs and chemicals, can cause some cancers and birth defects Overview of Birth Defects Birth defects, also called congenital anomalies, are physical abnormalities that occur before a baby is born. They are usually obvious within the first year of life. The cause of many birth... read more .

A variant in the genes in a sperm or egg can be passed from parent to child. Variants of genes in other cells may cause disease that is not passed down to children (because the sperm or eggs are not affected). Having two copies of an abnormal gene can lead to serious diseases or conditions, such as cystic fibrosis Cystic Fibrosis (CF) Cystic fibrosis is a hereditary disease that causes certain glands to produce abnormally thick secretions, resulting in tissue and organ damage, especially in the lungs and the digestive tract... read more or Tay-Sachs disease Tay-Sachs Disease and Sandhoff Disease Tay-Sachs disease and Sandhoff disease are types of lysosomal storage disorder called sphingolipidoses and are caused by a buildup of gangliosides in the tissues in the brain. These diseases... read more . Sometimes disorders can occur even when a person has just one copy of an abnormal gene.

A person's chromosomes and genes can be evaluated by analyzing a sample of blood as well as cells from other parts of the body such as from a swab of the inside of the cheek. During pregnancy, doctors can use cells from amniocentesis Amniocentesis Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more or chorionic villus sampling Chorionic Villus Sampling Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more to detect certain chromosome or gene abnormalities in a fetus. If the fetus has an abnormality, further tests may be done to detect specific birth defects. More recently, a screening test has been developed in which a sample of a pregnant woman's blood is analyzed to determine whether her fetus has an increased risk of certain genetic disorders. This test is based on the fact that the mother's blood contains a very small amount of DNA from the fetus. This test is called noninvasive prenatal screening (NIPS) or cell-free fetal DNA analysis. NIPS can be used to detect an increased risk of trisomy 21 (Down syndrome Down Syndrome (Trisomy 21) Down syndrome is a chromosome disorder caused by an extra chromosome 21 that results in intellectual disability and physical abnormalities. Down syndrome is caused by an extra chromosome 21... read more ), trisomy 13 Trisomy 13 Trisomy 13 is a chromosomal disorder caused by an extra chromosome 13 that results in severe intellectual disability and physical abnormalities. Trisomy 13 is caused by an extra chromosome 13... read more , or trisomy 18 Trisomy 18 Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities. Trisomy 18 caused by an extra chromosome 18. Infants... read more and certain other chromosome disorders but is not diagnostic. Doctors usually recommend further testing when an increased risk of a chromosome abnormality is detected.

Although chromosome and gene abnormalities cannot be corrected, some birth defects can sometimes be prevented, for example, by taking folate [folic acid] to prevent neural tube defects Neural Tube Defects and Spina Bifida Neural tube defects are a certain type of birth defect of the brain, spine, and/or spinal cord. Neural tube defects can result in nerve damage, learning disabilities, paralysis, and death. The... read more or screening parents for carrier status Carrier Screening Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder. Hereditary genetic disorders are disorders of chromosomes or... read more of certain genetic abnormalities. An embryo conceived through in vitro (test tube) fertilization In vitro (test tube) fertilization (IVF) Assisted reproductive technologies involve working with sperm and eggs or embryos in a laboratory (in vitro) with the goal of producing a pregnancy. (See also Overview of Infertility.) If treatment... read more (IVF) can also be tested for a genetic abnormality before it is transferred into the woman’s uterus (see Preimplantation Genetic Diagnosis Preimplantation Genetic Testing Prenatal diagnostic testing involves testing the fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including certain hereditary or spontaneous genetic... read more ).