Genetic Disorders

B

Chromosomal Disorders

Chromosomal disorders are caused by the presence of an extra or missing whole or partial chromosome. In some cases, whole chromosomes or pieces of chromosomes are attached to one another in abnormal ways, which cause a person or their offspring to have an incorrect amount of chromosomal material. Chromosomal disorders are sometimes caused by an error in a type of cell division called meiosis, which occurs during the formation of eggs and sperm. Chromosomal disorders disrupt the biological functions of many genes. They produce multiple problems in the affected individual, often including mild or severe mental retardation. More than 600 chromosomal syndromes have been identified.

Down syndrome is the most common chromosomal disorder, affecting about 1 in 800 newborns. People with Down syndrome characteristically have three copies of the autosomal chromosome known as number 21 instead of the normal pair of number 21 chromosomes. For this reason, Down syndrome is commonly called trisomy 21. People with Down syndrome usually have mild to severe learning disabilities and physical symptoms that include a small skull, an extra fold of skin at the inner corner of each eye, and a flattened bridge of the nose. They also may have heart defects and other serious health problems.

Some chromosomal disorders involve the sex chromosomes. In many instances, an extra or missing sex chromosome is less life threatening than an extra or missing autosome. A person with Klinefelter syndrome, which affects about 1 in 500 males, has two X chromosomes and one Y chromosome. Males with Klinefelter syndrome are typically tall, and they may have small testes and slight breast development. They also may have minor problems with learning and are usually infertile.

Another chromosomal disorder that affects the sex chromosomes is Turner syndrome, which affects 1 in 2,500 females. In this disorder, a female has one functioning X chromosome instead of two. Females with this condition are typically short, with a thick, webbed neck. They may have mild problems with learning, and they usually are infertile because they lack normal ovaries.

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C

Multifactorial Disorders

Multifactorial disorders are caused by several genes as well as the influence of a person’s environment, such as diet or lifestyle. An example of a multifactorial disorder is a category of birth defects called neural tube defects. In a neural tube defect, a fetus’s neural tube—the structure that develops into the spinal cord and brain—is damaged. The two most common types of neural tube defects are anencephaly and spina bifida. Anencephaly is a fatal condition in which a baby is born with only a partial brain or no brain at all. About 1,000 to 2,000 babies with anencephaly are born each year in the United States. Spina bifida results when a neural tube defect causes an opening in the spine. In the United States, about one infant in every 2,000 live births is born with spina bifida. These infants need surgery to close the opening in the spine, and they may develop problems with walking or with bowel or bladder control. Geneticists believe that certain genes may play a role in damage to the neural tube, but the mother’s diet during pregnancy also plays a role. A woman’s risk of giving birth to an infant with a neural tube defect significantly decreases if she consumes adequate amounts of folic acid, a vitamin in the B complex, during the first three months of pregnancy and one month before conception.

Some common diseases that run in families but do not display an obvious pattern of inheritance are also thought to be multifactorial. Two examples are coronary heart disease and diabetes mellitus. In both cases, genes may cause a person to be predisposed to develop the disease, but lifestyle choices can help to prevent the disease from developing or from worsening after it occurs.

D

Mitochondrial Disorders

All cells contain tiny self-replicating structures called mitochondria located outside of the nucleus. Each mitochondrion contains about ten single copies of small, circular chromosomes. These chromosomes contain a type of DNA that is different from the DNA found in chromosomes inside the cell nucleus. Both sperm and eggs contain mitochondria, but at fertilization, a sperm contributes only the genetic material in its nucleus to the new life form. All of a person’s mitochondria are genetic descendents of those mitochondria that were present in the egg before fertilization. Therefore, mitochondrial disorders are transmitted solely through the mother. Both males and females can be affected, but an affected male will not pass on the disorder to his children.

Conditions involving mitochondrial inheritance are rare, and they have been recognized only since the 1980s. One example is Leber’s hereditary optic neuropathy, a vision disorder characterized by shrinking of the optic nerve, which transmits visual images from the eye to the brain.

IV

Genetic Screening

Clinical geneticists and other health professionals use several screening tests and procedures to determine whether a person has a genetic disorder or is at risk of having a child with a disorder. These tests may be performed at various times in a person’s life. Some genetic screening tests are routinely performed on newborns. Among adults, genetic screening is always voluntary.

A

Preimplantation Diagnosis

A test that can be performed at the earliest possible stage of life is called preimplantation diagnosis. This test is used in conjunction with in vitro fertilization, a procedure for uniting an egg and sperm in a laboratory rather than in a woman’s body.

Before an embryo created using in vitro fertilization is surgically implanted into the mother’s uterus, physicians remove a cell from the developing embryo and analyze its DNA to learn if abnormalities associated with a specific genetic disorder are present. Although in vitro fertilization is a standard medical practice, removing cells from a developing embryo and preimplantation diagnosis are considered experimental procedures.

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