This high-risk group includes couples where one partner is a carrier of a chromosomal rearrangement that predisposes to the production of chromosomally unbalanced gametes, as described in Chapter 2. The most common types are Robertsonian and reciprocal translocations. Robertsonian translocations between chromosomes 13 and 14 and 13 or 14 and 21 occur repeatedly and are common in the population (1 in 1000-2000), whereas with the exception of the t(llq;22q), each reciprocal translocation is unique to the family. Usually couples where one individual is a translocation carrier can achieve normal pregnancies, with the help of prenatal diagnosis if necessary. Those that present for preimplantation diagnosis are a select group at the extreme end of the normal range, who have suffered repeated spontaneous or induced abortions as a result of a series of abnormal conceptions. Table 12.1 illustrates this by showing the reproductive histories of four couples referred to the UCL Centre for PGD. In a total of 22 pregnancies there was only 1 normal child, with 13 miscarriages, 7 terminations of pregnancy (TOPs) and 1 chromosomally abnormal child. In some societies, where TOP for genetic abnormality is not an option, PGD may be the only option open to couples at a particularly high risk of repeated chromosomally unbalanced conceptions. Other chromosomal rearrangements that predispose the carrier to produce abnormal gametes are pericentric inversions, insertions and ring chromosomes; the latter two are likely to be rare (see Chapter 2).
One particular set of couples requiring assisted conception that are also at high risk of having a chromosomally abnormal conception are those requiring intracytoplasmic sperm injection (ICSI) because of a sperm problem (reviewed in Kim et al., 1998). Oligospermic males have about a 4% risk of carrying a chromosome rearrangement (Scholtes et al., 1998) and should be routinely karyotyped. Once an abnormality is detected these couples can then be offered PGD to counteract the elevated risk of chromosome imbalance at conception.
Strategies which can be used to detect chromosome imbalance in the preimplantation embryo resulting from parental chromosome rearrangement are now considered for each high-risk group, illustrated by details of PGD cases carried out to date.
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