How could the chromosome number change?

During the process of meiosis (gamete production) the chromosomes can recombine (swap material) with their homologous partners. Each cell in your body contains 23 pairs of chromosomes, one of each from your mother and one from your father. During egg or sperm production those matching pairs swap material to produce the unique new chromosomes which will be passed on to the next generation. So, for example, chromosome 14 originating from your father would normally only swap regions with the chromosome 14 originating from your mother.

Sometimes this recombination goes wrong and instead of a chromosome swapping genetic material with its partner, it tries to swap material with another chromosome e.g. 14 with 21. Because the chromosomes don't match they cannot make an equal swap of material so the exchange ends up creating non-viable chromosomes. However if they join at their tips it can result in a single large chromosome which is made up of the genetic material from both of the original chromosomes.

It turns out that this particular arrangement is viable and if the fused chromosome ends up in a gamete responsible for the next generation then the child ends up with the correct amount of genetic material but with some DNA contained in one chromosome instead of two (Ref 1).

Robertsonian translocation

Two pairs of chromosomes, each chromosome beside its homologous partner Each chromosome duplicates itself, resulting in two joined chromatids A chromosome picks the wrong chromosome to recombine with The mauve and orange chromosomes remain joined when the cell splits The cell divides again, separating the chromatids Each resulting gametes contains the correct amount of genetic material, but one now contains a fused chromosome (the top one with two centromeres)

The fused chromosome can result in a perfectly healthy individual who has 45 instead of 46 chromosomes but who still has a full complement of DNA. The fused chromosome which is made up of the two original chromosomes now joined together, takes on the functions of both of them. There are about one in a thousand individuals in the population with fused chromosomes, the most common fusions being between chromosomes 14 and 21 (10%) and chromosomes 13 and 14 (75%) (Ref 1).

Chromosomes 13,14 and 21 are acrocentric chromosomes, which means they have a long arm containing the majority of the DNA and a short arm which contains very little DNA. When two chromosomes fuse, both of their centromeres end up in the fused chromosome and genetic material from their arms can be lost (the fragment is shown in the diagram above). Losing sections like this would normally lead to a non-viable gamete. However because there are few genes in the short arms of chromosomes 13, 14 and 21, and those genes that are there are repeated elsewhere, some material from the short arms can be lost without problem (Ref 2).

Note also that the fused chromosome may end up with two centromeres. This will be a problem if they are far apart but if they are close together, they can act as if there was only one centromere present. This is discussed in more detail on the next page.