Two chromosomal abnormalities − Reciprocal translocations and Robertsonian translocations − may be the reason behind a couple’s infertility. They involve the rearrangement of chromosome fragments.
Reciprocal translocations occur in approximately one out of 625 individuals. Also called non-Robertsonian translocations, they have been associated with male factor infertility. This type of chromosome abnormality has been more frequently found in men with azoospermia and in fewer cases in men with oligozoospermia.
Robertsonian translocations (centric fusions or whole-arm translocations) affect roughly one out of 1000 people born. They involve the rearrangement of two homologous or non-homologous pairs of chromosomes. When this type of chromosome translocation occurs, two chromosomes out of the 2 pairs of 23 chromosomes fuse in one, and the result is a karyotype with 45 chromosomes instead of 46.
Although balanced translocations (BT) do not involve obvious symptoms other than infertility, BT carriers are statistically more likely to create gametes (sperm or eggs) with an unbalanced translocation.
Chromosome Translocations and Genetic Disorders
When a child is born with an unbalanced translocation it means that he/she has inherited a chromosome with less or excessive genetic material from one BT-carrier parent. So despite the parent’s balanced translocation (an even exchange between two non-homologous chromosomes, with no genes lost or in excess), the offspring with the unbalanced translocation will present a genetic condition such as Patau Syndrome (trisomy 13) or Down’s Syndrome (trisomy 21) associated with a Robertsonian translocation.
Chromosome Translocations and Recurring Miscarriages
According to research, roughly one in two spontaneous abortions involves an unbalanced translocation.
PGD for Translocation Determination
Carriers of balanced reciprocal or Robertsonian translocations are usually unaware of their chromosomal abnormality. However, when there are recurrent miscarriages or when a child is born with a genetic disorder, the couple wishing to achieve a normal pregnancy may consult a genetic counsellor. Since a balanced translocation carrier runs an increased risk for an embryo with an unbalance chromosome translocation, the genetic scientist will explain the risks for a miscarriage and/or the child’s congenital defects (mental retardation etc), and inform the couple of their options.
Preimplantation Genetic Diagnosis (PGD) for translocation determination may improve pregnancy rates after failed IVF attempts, as well as live birth rates in cases of spontaneous abortions.
Dr Paraschos participated in the world's first PGD.