Something Other than 46 Chromosomes

Recently I had the pleasure of visiting Mrs. Wanda Hamilton's science classes at Bryson Middle School in Simpsonville. The day was spent reviewing basic genetic information and introducing students to human genetics and disorders.

The classes were thoughtful, inquisitive, and were the source of many good questions. It was great fun for me to be with these students.

I would like to share with you one of the questions from the class.

Mollie Hamilton and Sara Kelley wanted to know about individuals born with 47 and not the normal number of 46 chromosomes. Would these persons have certain characteristics or problems?

Most middle school students have been introduced to the process of meiosis as being the means by which reproductive cells or gametes are formed. Sometimes meiosis does not go smoothly, resulting in an abnormal separation of the sex chromosomes, the X and Y. Gametes are then produced that contain an unusual number of sex chromosomes. Offspring developing from these unusual gametes would either contain an extra chromosome or lack a sex chromosome.

Nondisjunction is a genetic term used to describe the failure of a pair of chromosomes to separate during meiosis. When nondisjunction occurs in the sex chromosomes of humans, the genetic make-up(genotype) and the physical appearance(phenotype) can be severely affected. Certain syndromes can be associated with abnormal numbers of sex chromosomes.

Persons with only one X and no Y chromosome(XO) have a total of 45 chromosomes and are typically short, underdeveloped, and sterile females. This condition is called Turner syndrome.

Females with an extra X chromosome (XXX) have a total of 47 chromosomes and may have limited fertility and slight intellectual impairment. An XXY combination with a total of 47 chromosomes produces males who are usually tall, sexually underdeveloped, and may have slight intellectual impairment. This condition is called Klinefelter syndrome. Nondisjunction may also occur with other human chromosomes. A condition known as trisomy occurs when an individual has 3 of the same chromosome in his cells.

In Trisomy 21 or Down syndrome, the individual has three number 21 chromosomes and is characterized by short stature, limited mental abilities, characteristic facial features, and probable heart defects. It has been found that mothers of age 35 and older have an increased chance of nondisjunction occurring and of having a child with Down syndrome.

Thank you, girls, for your question!