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Inherited Disorders and Birth Defects

Balanced Translocation of Chromosomes 1 & 10

12/22/2008

Question:

I was diagnosed with a balanced translocation of chromosomes 1 and 10. My wife and I are using IVF and PGD. We were told that only around 20% of the embryos produced would be normal or balanced. We were under the impression that the percentage would be higher. Is 20% accurate or does it seem too low?

Answer:

The chance of having a sperm or egg that has the unbalanced translocation vs. the normal or balanced rearrangement depends on how the chromosomes line up, divide and segregate - that is how the chromosomes divide into two daughter cells. Segregation of chromosomes can occur in multiple ways. Of the multiple ways that balanced translocated chromosomes can line up and divide, 2 are most common - about 50% have what is called an alternate 2:2 segregation and 50% have an adjacent 2:2 segregation.

Of these two types of segregation, 6 different types of gametes (sperm or egg) can be produced. 4 of the 6 gametes produced will be unbalanced (this is a result of adjacent 2:2 segregation), 1 of 6 is balanced (like you) and 1 of 6 is normal (these last 2 are a result of alternate 2:2 segregation).

However, depending on the chromosomes involved in the translocation, the size of the translocation and location on the chromosome - the theoretical 50-50 chance of segregating so that that gametes with a normal or balanced translocation will be produced - can vary considerably. The 20% number you were given is most likely in the ballpark.

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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