Unbalanced Translocation

Definition - What does Unbalanced Translocation mean?

An unbalanced translocation is an abnormality in the structure of one or few chromosomes in which each cell contains an additional piece of one chromosome and/or a missing piece of another chromosome. Exchange of chromosome segments between chromosomes can occur during cell division and does not usually cause a problem as long as the total number of genes and chromosomes in the cell remain constant. During fertilization, if there is a net loss of a segment or the addition of an extra segment of DNA in the fetus, it can lead to a genetic imbalance that can result in fetal abnormalities or miscarriage.

FertilitySmarts explains Unbalanced Translocation

Strands of DNA, the molecule that carries genetic information in the cell, are coiled around proteins to form structures that are known as chromosomes. There are 46 chromosomes in each cell of the human body that are arranged as 23 pairs in the nucleus of the cell. Cells inherit one set of these 23 chromosomes from the sperm and the other set from the egg during fertilization.

A gene is a segment of the long DNA strand, that carries a particular bundle of genetic information. It is estimated that each cell contains about 20 000 genes, which are divided among the 46 chromosomes. From the point of fertilization to growth and development into a grown adult, it is vital to have the correct number of chromosomes and genes in every cell.

There are abnormalities that can happen in the cells, primarily during cell division, that can affect the structure and number of these chromosomes. During formation of the germ cells (sperm and eggs) or at the very early development of the embryo, a part of these chromosomes can break and get reattached on a different chromosome. This is known as chromosomal translocation. If Chromosomal translocation occurs after fertilization, it usually does not cause a problem in the same person because the total number of chromosomes and genes are intact in the cell, although a segment of the chromosome and along with it some genes may be rearranged and lying on a different chromosome. This is called balanced translocation.

When a person with a balanced translocation reproduces, since germ cells inherit only half of the chromosomes in the cell, it is possible for an egg or a sperm to contain a chromosome that has extra or missing genes. This is what is known as unbalanced translocation. The fetus produced by fertilization of such a germ cell will either lack important genetic material, or carry additional genes that are not required for the function of the cell. This can result in fetal abnormalities or miscarriage.

If the unbalanced translocation is due to the presence of a balanced translocation in one of the parents, there is a chance of repeated miscarriages and fetal abnormalities. If a fetus is detected to have an unbalanced translocation, genetic testing on both male and female partners can help to evaluate the cause of the translocation.

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