Surely carriers would be infertile?

At first glance it looks as if individuals with a fused chromosome (and thus a different chromosome count) should be infertile. The chances of another mutation producing the same fused chromosome twice in the same population would be very low. However if both partners have to have the same number of chromosomes how could a 45 chromosome person ever have children?

On the other hand, if the same chromosome number is not needed then how can the chromosomes possibly line up with its homologous partner to form the gametes for the next generation? It would seem that one of the chromosomes must be partnerless if there is an odd number of chromosomes present.

It turns out that the chromosomes can still line up and recombine but because the fused chromosome is now playing the part of two chromosomes it needs to pair up with both the partnering chromosomes. This is known as a trivalent (Ref 1) and it allows the chromosomes to recombine their genetic material as normal.

Fused chromosome recombination during meiosis
(the separate chromatids are not shown here)
  1. A fused chromosome and a pair of matching partner chromosomes
  2. The two individual chromosomes line up with the matching locations on the fused chromosome
  3. Recombination happens as normal
  4. The cell divides, separating the fused chromosome from its two partners
  5. Gametes are produced containing either the fused chromosome or the two individual chromosomes

The fact that it is possible for fused chromosomes to produce viable gametes and offspring does not mean that it is problem free though, especially if there is only one fused chromosome present and thus an odd chromosome count of 45. Some of the potential problems are as follows:

Duplicate centromeres
Centromeres are the central structure (shown as small circles on the chromosomes in the above diagram) and they are used as attachment points for structures called spindle fibres which pull the chromosomes to opposite side of the cell before it divides. Robertsonian translocation can result in a chromosome with two centromeres which can behave as a single centromere, if they are close enough together. However if they are too far apart the spindles fibres may pull it in two directions at once and break the chromosome during cell division (Ref 2).

Difficulty assigning chromosomes to gametes
When two chromosomes have finished recombining, the spindle fibres attach to each of the homologues and pull them apart so each gamete ends up with one of each pair.

A trivalent formation has three chromosomes instead of two which means that for viable gametes to be produced then the fused chromosome has to be pulled to one side of the cell and the two individual chromosomes must be pulled to the opposite side.

If this goes wrong and one of the individual chromosomes is pulled to the same side as the fused chromosome then one gamete will have too much genetic material and another will be missing that material.

Meiosis error

Normally this leads to a non-viable foetus but in the case where the fused chromosome is a fusion of chromosomes 21 and 14 it can result in a Down's syndrome child. This happens if the fused chromosome (containing the material from both chromosomes 14 and 21) is pulled into the same gamete as the normal chromosome 21. Any resulting foetus will have three copies of the genetic material from chromosome 21 once the normal chromsome 21 from the other parent is added. This is know as as trisomy 21 and results in Down's syndrome.

This is not a major cause of Down's syndrome but around 3% of cases have a parent with a fused chromosome (Ref 3) and it is estimated 5-10% of births to possessors of 14/21 fused chromosomes will have trisomy 21 (Ref 4).

Interestingly the problem with the wrong number of chromosomes ending up in the gametes only occurs because one chromosome is paired with two partners. If both parents have a fused chromosome then chromosomes can pair up as normal, no trivalent is formed and the problem goes away. That means that 44 chromosome humans would not suffer from an increased incidence of trisomy.

Missing genetic material
When the chromosomes fuse, a small amount of material is lost from the short arms of the chromosomes. If the sequences don't code for anything important or if they are duplicated in other parts of the genome then this may be fine, but if there is anything vital in the lost section then it may cause problems.
Additional side note: There is evidence that chromosomes which have part of their sequence inverted solve the problem of not being able to line up with their partners by simply not taking part in recombination (Ref 5). This lead to a barrier to gene flow between the chromosomes with and without the inversion. This may be one of the mechanisms of speciation but it would not cause fertility problems.
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References:
  • Ref 1 : Abnormalities in chromosome structure,University of South Dakota, accessed 31/07/09
  • Ref 2 : Hartl and Jones, Essential Genetics, P167
  • Ref 3 : Hartl and Jones, Essential Genetics, P189
  • Ref 4 : David Rowell, MOLECULAR-EVOLUTION/bionet.molbio.evolution Archives, accessed 07/08/09
  • Ref 5 : Navarro and Barton, Chromosomal speciation and molecular divergence, Science 300, P321-324