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Overview of Chromosome and Gene Disorders


Nina N. Powell-Hamilton

, MD, Sidney Kimmel Medical College at Thomas Jefferson University

Reviewed/Revised Nov 2023
Topic Resources

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 or the code for functional ribonucleic... 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 or RNA molecule.

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 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 .)

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 ). 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 and Gene Disorders.) Chromosomes are structures within cells that contain DNA and... read more ) or have been duplicated.

Gene Abnormalities

Changes in one or more base pairs of the DNA in a gene (see figure 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 Sickle Cell Disease , 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 medications 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.

Testing for Chromosome and Gene Abnormalities

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.

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 Noninvasive Prenatal Tests for Genetic Abnormalities Prenatal testing for genetic disorders and birth defects involves testing a pregnant woman or fetus before birth (prenatally) to determine whether the fetus has certain abnormalities, including... read more . 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 , 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 and gene disorders but is not diagnostic. Doctors usually recommend further testing when any chromosome abnormality is detected.

Prevention of Chromosome and Gene Abnormalities

Drugs Mentioned In This Article

Generic Name Select Brand Names
Folacin , Folicet, Q-TABS
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