Robertsonian translocation and translocation Down syndrome
A Robertsonian translocation fuses the long arms of two acrocentric chromosomes (tiny short arms), so a parent who is usually healthy can have a child with trisomy 21 (three copies).
Remember
A balanced Robertsonian translocation, in which no important DNA is gained or lost, leaves the person carrying it usually healthy, but the fused chromosome can pass into an egg or sperm together with a normal chromosome 21, and if that gamete is fertilised the child has trisomy 21.
Memory aid
Robertson fuses the long arms and loses the short ones
Think of the name: it reminds you that the long arms fuse and the short arms are lost.
A chromosome has a short arm, called p, and a long arm, called q. The chromosomes numbered 13, 14, 15, 21 and 22 are acrocentric, which means that their short arms are very small. In a Robertsonian translocation (a translocation is a rearrangement in which chromosome material is exchanged or joined between chromosomes), two of these chromosomes break close to the centromere (the pinch point where the two arms meet), lose their tiny short arms, and the long arms fuse into a single large chromosome.
The person carrying it, the carrier, is usually healthy. The lost short arms carry little essential genetic material, and no important DNA is gained or lost, so the translocation is called balanced. A carrier of a fused 14 and 21 chromosome has 45 chromosomes in each cell, because two have become one.
The problem arises in the next generation. When a carrier makes eggs or sperm, the fused chromosome can pass into one gamete together with a normal chromosome 21. If that gamete is fertilised, the embryo has three copies of chromosome 21 material, called trisomy 21, which causes Down syndrome. This is translocation Down syndrome. The child still has 46 chromosomes, because the fused chromosome counts as one. Ordinary trisomy 21 from nondisjunction (an error in which a pair of chromosomes fails to separate) gives 47.
After a child with translocation Down syndrome, karyotyping of the parents (a test that photographs the chromosomes) is offered, because the chance of another affected pregnancy is greater if a parent is a carrier than after a single nondisjunction event.
Key facts
- Chromosomes involved
- the acrocentric chromosomes 13, 14, 15, 21 and 22.
- The change
- the short (p) arms are lost and the long (q) arms fuse into one large chromosome.
- Carrier
- usually healthy, with 45 chromosomes in each cell when a 14 and a 21 have fused.
- Child with translocation Down syndrome
- 46 chromosomes but three copies of chromosome 21 material.
- Ordinary trisomy
- 47 chromosomes in each cell, from nondisjunction.
- Reported risk
- for female carriers of a 14;21 translocation, about 10% of having a liveborn child (one born alive) with trisomy 21.
Exam traps
- An inversion, in which a segment is reversed within one chromosome, changes gene order but not the number of copies, so it does not lead to an extra chromosome 21.
- Nondisjunction in a body cell after fertilisation gives mosaicism (a mixture of normal and trisomic cells) and is not passed on by a healthy parent, so it does not explain a balanced carrier who has an affected child.
- A reciprocal translocation swaps segments between two chromosomes. It does not require acrocentric chromosomes and is not defined by the long arms fusing after the short arms are lost, so it is not a Robertsonian translocation.
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