Chromosomal Abnormalities

Chromosomal abnormalities can occur in any cell, and they can occur as an abnormal chromosome number or as an abnormality in the structure of the chromosome itself With Down syndrome, an individual has 47 chromosomes instead of the normal 46. With Turner's syndrome, the individual has 45 chromosomes.

A loss, a gain or a repositioning of chromosome material identifies a structural abnormality. Terms used to describe these various conditions include additions, duplications, deletions, inversions and translocations.

Causes of Chromosomal Abnormalities
A couple of possible causes of chromosomal abnormalities are advanced parental age and radiation exposure of mother, father or fetus, which affects gene regulation.
Advanced parental age usually refers to the mother's age, but some research has also implicated the father's age. Age is a factor for the mother because females are born with all the eggs they will ever have. They are exposed to radiation and other teratogens in the environment throughout their lives. The exposures have an additive effect-the more teratogens a woman is exposed to, the greater her chance of damaging the eggs.

Radiation exposure, such as X-rays, can damage genetic material and affect gene function and regulation. This is serious because genes control development of the fetus, its organs and growth. Three of the most common examples of chromosomal abnormalities are discussed below.

Down syndrome is the most common chromosomal abnormality; it occurs in about 1 in 600 births. An extra chromosome causes the syndrome. Those born with Down syndrome can live fairly long lives and may be moderately to severely mentally retarded. About half have congenital heart disease; particular physical abnormalities are common to the syndrome.

Trisomy 18 (Edwards' syndrome) is a severe chromosome abnormality occurring in approximately 1 in 6,000 births. An abnormal chromosome 18 causes the syndrome. Babies with trisomy 18 have multiple abnormalities of major organs; most die within the first year of life.
Trisomy 13 (Patau's syndrome) occurs about once in 5,000 births. An extra chromosome 13 causes the syndrome. Babies born with trisomy 13 may have multiple abnormalities. Prognosis for these infants is poor; most die by the age of 3.

Sex Chromosome Abnormalities
Abnormalities of the sex chromosomes are relatively common: 1 in every 500 births. It is believed this type of abnormality causes about 25% of all miscarriages. We discuss three of these syndromes below.

Turner's syndrome occurs about once in every 10,000 female births. It is the most common chromosome abnormality identified in miscarriages. Instead of having two X chromosomes, a baby girl born with Turner's syndrome has only one X chromosome, called 4SX. The mature girl with Turner's syndrome is short, sexually underdeveloped, with a webbed neck, very small ovaries, and often heart and kidney problems. The condition may not be identified until she reaches puberty.

Klinefelter's syndrome is found in approximately 1 in every 1,000 males. The most common characteristic is very tall stature when the boy reaches maturity. These boys are born with an extra sex chromosome, thus the 47XXY label. A boy is underdeveloped sexually and has small testes.

Fragile-X syndrome (also called X-linked] is one of the most common causes of mental retardation in males. Major physical characteristics include large ears, large hands and language-development delays.

Congenital Infections
Rubella was one of the first maternal infections researchers identified as the cause of fetal malformations. Other infections that can cause malformations include cytomegalovirus (CMV), toxoplasmosis, herpes simplex, parvovirus 19 and syphilis.

Congenital infections can cause many problems in a baby. These range from major malformations, such as heart defects, to newborn infection at birth.