CHROMOSOME
ABNORMALITIES
Chromosome abnormalities are detected with increasing frequency in birth defects. The most common type is a change in
total chromosome
number. In general, reduction of the total number of autosomes is incompatible with life. An infant with an extra chromosome involving virtually any autosome pair also has a limited life span, with multiple physical abnormalities and mental retardation.
The most common chromosomal
abnormality is Down
syndrome, or mongolism, which involves the chromosome designated as number 21. This defect is termed Trisomy 21 because all cells in the infant's
body carry an extra number 21 chromosome. This condition occurs in 1 out of 800 liveborn infants. Maternal age is a factor in the frequency of Down syndrome; for example, 1 out of 300 mothers in their 30s, and 1 out of 40 mothers over 40 produce such infants. All human chromosome abnormalities can be detected in the developing fetus either through chorionic villus sampling, which can be done as early as the 9th week of gestation, or through amniocentesis, possible as early as the 14th to 16th week.
In addition to other abnormalities, aberrations of the sex
chromosomes may result in impaired or absent fertility. For example, one abnormality is known as 45X, or Turner syndrome Turner syndrome. Individuals with this condition, who are classified as female, most commonly are short in stature and show impaired development of female genitalia and of such secondary sex characteristics as the breasts and the distribution of body hair; they are also sterile. Another abnormality, called Klinefelter syndrome, occurs when the individual has 47 chromosomes in the cells of the body, with an XXY sex-chromosome composition. The individual is tall, male in appearance, has sparse hair distribution over the body, and is sterile.
Regardless of other sex-chromosome anomalies, when the Y chromosome is present, the physical appearance is nearly always male. As the total number of sex chromosomes present increases, greater prevalence of associated birth defects and mental retardation results.
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