When One and One Makes Three - Chromosomal Abnormalities

Thanks to medical research, couples are now able to determine if there are genetic problems with their unborn babies. Chromosomal abnormalities affect one in 150 babies in the US alone. With the modern methods of determining possible abnormalities, parents can be better prepared for the baby's needs. The effects of chromosomal abnormalities vary:

· Many babies born with chromosomal abnormalities have mental and/or physical deficiencies

· Early pregnancy miscarriage is often the result of chromosomal abnormalities

· Stillbirth can be the result of chromosomal abnormalities

A Little Biology Lesson

The human body contains between 20,000 and 25,000 genes that are contained in chromosomes within cells. These genes are the deciding factors in determining hair color, eye color and growth and development of the body. Normally, a person has 23 pairs of chromosomes or 46 units. The pairs are comprised of one chromosome from the mother and one from the father, beginning at conception with the egg and sperm cell contain 23 chromosomes each.

Sometimes an information error occurs when the egg or sperm cell develops that can cause an abnormality in the chromosomes of the baby. There are no specific reasons for these errors, they just are. Nobody did anything wrong. The egg or sperm cell may divide in such a way as to create an excess or deficiency in the number of chromosomes. When the cell with the wrong amount of chromosomes joins with a normal cell, the result is a chromosomal abnormality in the baby.

Trisomy and Common Structural Abnormalities

One of the more common types of chromosomal abnormalities is called a trisomy, which means there are three copies of a certain chromosome instead of just two. Often, when an embryo has chromosomal abnormalities there is a spontaneous abortion - a miscarriage. Many times the miscarriage occurs before the woman even realizes she is pregnant. It is estimated that at least 50 percent of first-trimester pregnancy losses are due to chromosomal abnormalities in the embryo.

Another type of chromosomal abnormality alters the structure of one or more chromosomes. This happens before fertilization. Those with structural abnormalities usually have the correct number of chromosomes but small pieces of the chromosome may be rearranged into one of the following patterns:

· deleted

· duplicated

· inverted

· misplaced

· exchanged with part of another chromosome

Sometimes these structural chromosomal abnormalities have no effect on an individual and at other times they result in birth defects or miscarriage.

The Mosaic of Chromosomes

Cell division errors may happen shortly after fertilization takes place resulting in a condition called mosaicism, which means the individual has cells with genetic compositions that differ from one another. Turner Syndrome is one condition that arises from this type of abnormality where some of the cells are missing an X chromosome while other cells have the chromosome. The severity of the impact of this situation varies with the number of abnormal cells that are present.

There are several chromosomal abnormalities that are more common than others. They include the following:

· Down syndrome, the most common with one in 800 babies affected. Individuals have varying degrees of intellectual disability, characteristic facial features; often have heart defects and some other problems.

· Trisomies 13 and 18 are more severe than Down syndrome with one in 16,000 babies affected with trisomy 13 (Patau syndrome) and one in 5,000 affected with trisomy 18 (Edwards syndrome). The babies have severe mental retardation and physical birth defects and seldom live past their first birthday.


Variations in Sex Chromosomes

Sex chromosome abnormalities appear when the normal combination of XX for girls and XY for boys is altered. When this happens the results can be:

· infertility

· growth abnormalities

· behavioral and learning problems

The good news is that most people with sex chromosome abnormalities live relatively normal lives. The commonly known syndromes for sex chromosomal abnormalities include:

· Turner syndrome, which causes girls to have one complete X chromosome and either a missing chromosome or only a part of the other X chromosome. Girls with Turner syndrome are short, may have health problems, and are generally of average intelligence (some with learning difficulties).

· Triple X affects one in 1,000 girls who have an extra X chromosome. These girls are tall and do not present with any physical or mental difficulties. Some may have learning challenges, but overall they are normal and fertile. The only way the parents may learn their baby girl has a chromosomal abnormality is if they have prenatal testing.

· Klinefelter syndrome affects about one in 500 to 1,000 boys with extra X chromosomes along with their Y chromosome. Affected boys produce too little testosterone and are infertile. They are usually of average intelligence but may have learning difficulties.

· XYY is when a boy is born with extra Y chromosomes. Those affected with XYY are usually taller than average but are otherwise normal and fertile. They may sometimes have learning, behavioral and speech problems. As with girls with triple X, the only way the parents will know is with prenatal testing.

The Ones Nobody Knows About

There are several other types of chromosomal abnormalities that may not even be visible under a high-powered microscope. Some of them are so rare that they only affect a small handful of babies, which means that health care professionals cannot effectively predict a child's health or development in the long run. Some of the abnormalities may have no effect if there is no genetic substance missing from the chromosome. The uncommon abnormalities are:

· deletions, where a small part of the chromosome is missing

· microdeletions, an even smaller part is missing

· translocations, where a part of a chromosome is attached to another chromosome

· inversions, a piece of a chromosome is cut out and flipped over then reinserted

· duplications, a piece of the chromosome is duplicated creating excess genetic material

· ring chromosome, where the ends of the chromosome are deleted and the new ends join together to make a circle

The occurrence of chromosomal abnormalities tends to increase as the age of the mother increases. You can learn more about the chances of chromosomal abnormalities in babies in our article in this section.

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