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Overview of Chromosome and Gene Disorders

By

Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Last full review/revision Jul 2020
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Chromosomes are structures within cells that contain a person's genes.

Every normal human cell, except for sperm and egg cells, has 23 pairs of chromosomes for a total of 46 chromosomes. Sperm and egg cells have only one of each pair of chromosomes for a total of 23. Each chromosome contains hundreds to thousands of genes.

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 Sex chromosomes are one of the 23 pairs of chromosomes. There are 2 sex chromosomes, called X and Y. Females typically have two X chromosomes (XX) and males typically have one X chromosome and one Y chromosome (XY).

Structure of DNA

DNA (deoxyribonucleic acid) is the cell’s genetic material, contained in chromosomes within the cell nucleus and mitochondria.

Except for certain cells (for example, sperm and egg cells and red blood cells), the cell nucleus contains 23 pairs of chromosomes. A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct a protein.

The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar (deoxyribose) and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. In the steps, adenine is paired with thymine and guanine is paired with cytosine. Each pair of bases is held together by a hydrogen bond. A gene consists of a sequence of bases. Sequences of three bases code for an amino acid (amino acids are the building blocks of proteins) or other information.

Structure of DNA

Chromosome abnormalities

  • Number of chromosomes

  • Structure of chromosomes

Larger abnormalities may be visible with a microscope in a test called chromosome analysis or karyotyping. Smaller chromosome abnormalities can be identified using a specialized genetic test that scans 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 .)

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*? 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 ). 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.

Gene abnormalities

It remains unclear how most mutations occur. Most mutations are thought to appear spontaneously. Some substances or agents in the environment are capable of damaging and causing mutations 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 .

Testing for chromosome and gene abnormalities

A person's chromosomes and genes can be evaluated by analyzing a sample of blood. During pregnancy, doctors can use cells from amniocentesis 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 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 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 testing. 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 Down Syndrome (Trisomy 21) ), 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 Trisomy 13 , 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 Trisomy 18 and certain other chromosome disorders but is not diagnostic. Doctors usually recommend further testing when an increased risk of a chromosome abnormality is detected.

Prevention

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