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Inherited Disorders and Birth Defects

Balanced Translocation



If a normal woman with 45 chromosomes has a child with downs syndrome, this child has 46 chromosomes. How is this so?


What you describe is what happens when a parent has a balanced chromosomal translocation but passes an unbalanced translocation on to the baby.

Every cell in the body has 46 chromosomes, except eggs and sperm that have 23. Chromosomes are the structures that carry genes that tell our body how to form and how to function. When cells divide to make new cells, half the chromosomes go to one cell and the other half go to the other cell. When eggs and sperm are made they should only have 23 chromosomes each, so that when they unite at conception, the embryo gets half their chromosomes from their mother and half from their father and has the correct number of chromosomes again - 46.

Sometimes, two chromosomes will stick together and look like they are just 1 chromosome. However, the right amount of chromosome material is there and is functioning correctly. The case you are asking about – the woman “looks like” she has only 45 chromosomes, because 2 of her chromosomes are stuck together – and in the situation of Down syndrome, one of those chromosomes would be a number 21 chromosome.

When this woman becomes pregnant, she can pass on an egg that has 1 normal chromosome 21 and the chromosome that has her 2nd 21 chromosome stuck to it. When this egg is fertilized by a sperm, the pregnancy gets the mom’s normal 21, the dad’s normal 21 and the chromosome with the extra 21 chromosome attached to it. In this case, the baby “looks like” it has only 46 chromosomes, but actually has an extra chromosome 21 (the one that is stuck to another chromosome). And because the baby has an extra 21 chromosome – the baby has Down syndrome.

Importantly, if this means that a person with a balanced translocation involving a chromosome 21 has an increased chance of having more than one child with Down syndrome. It is important that these people have genetic counseling to talk about these chances and options available to them

Related Resources:

Genetic Counseling Resources

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Response by:

Anne   Matthews, RN, PhD Anne Matthews, RN, PhD
Associate Professor of Genetics
School of Medicine
Case Western Reserve University