What Is Polysomy?

Maggie J. Hall

People with polysomy have an extra copy of a chromosome. The problem may arise during meiosis, the cell division that produces sperm and ova, when a pair of chromosomes fails to separate completely, resulting in cells with either more or fewer than the normal number of chromosomes. Translocation, or exchange of genetic material, between chromosomes can also be a cause. Down's syndrome is a widely known disorder, which occurs because of an extra chromosome. Kleinfelter's syndrome is another form of polysomy, in which males have an extra X sex chromosome, in addition to an X and Y.

Those born with trisomy 21 have a condition known as Down syndrome.
Those born with trisomy 21 have a condition known as Down syndrome.

Down's syndrome children have a third chromosome linked to the 21st pair, making the total number of chromosomes 47 rather than the normal 46. Health care professionals also refer to the syndrome as trisomy 21. Children born with this type of polysomy tend to have flat noses, small ears and mouths, and slanting eyes. They are often slow learners and suffer from various medical conditions. Studies suggest that at least half of the infants born with trisomy 21 have heart conditions that include defects in the wall, or septum, that divides the heart.

Polysomy may arise during cell division when a pair of chromosomes fails to separate completely.
Polysomy may arise during cell division when a pair of chromosomes fails to separate completely.

Male babies born with Kleinfelter's syndrome, have three sex chromosomes, XXY, instead of the normal pair of sex chromosomes, XY. Infants may or may not develop symptoms as a result of this abnormality but tend to have weaker muscles, which prevents them from sitting up, crawling, or walking as soon as other infants of similar age. Through puberty, these males grow taller, but leaner, less well-defined muscles than other males. Teenagers with Kleinfelter's may have less body and facial hair, develop broader hips, and be deficient in testosterone production. Testosterone replacement may alleviate difficulties during adulthood that include diabetes and osteoporosis .

Females may have a congenital abnormality known as triple X syndrome, in which three or more X sex chromosomes are present. Physical characteristics of the condition include long legs and torsos resulting in greater height. Girls with this form of polysomy often have learning problems and act in ways that are emotionally inappropriate for their age. Many XXX females, however, mature to adulthood with no social difficulties. The more X chromosomes a child has, the greater the likelihood that she will exhibit symptoms.

Researchers discovered that individuals who display polysomy on chromosomes 3, 17, and 31 may have a greater risk of developing certain types of bladder, breast, lung, and skin cancers. Women with polysomy 17, in particular, may have an increased number of human epidermal growth factor receptor 2 (HER-2) sites, which may increases the incidence of breast cancer. Cancers stemming from this congenital disorder respond well to anthracyclines as part of the chemotherapy regimen.

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