''Let me explain why. Assume we have a set of genes (a) found on one chromosome, and a set of genes (b) found on another. Everyone has two copies of each set, so in a normal diploid cell, we have (a) (a) (b) (b). In meiosis, the cellular mechanisms segregate the chromosomes in an orderly way, so each gamete gets one set (a) and one set (b), each gamete looks like this: (a) (b).
In an individual with a Robertsonian fusion, though, each diploid cell looks like this: (a) (b) (a:b). They have three chromosomes instead of four, even if they do have the proper doses of (a) and (b). Now when meiosis occurs, the cell has to sort 3 chromosomes into two cells, and there are multiple ways this can happen:
(a) (b) a normal gamete : normal
(a:b) a gamete carrying the fusion, but with the normal complement of genes: normal
(a) (a:b) a gamete with an extra (a)—lethal
(a) a gamete with an no (b)—lethal
(b) (a:b) a gamete with an extra (b)—lethal
(b) a gamete with a no (a)—lethal
As you can see, several of the combinations produce viable gametes, and this individual can have healthy children with no detectable problems, although half of them will carry the Robertsonian fusion. The other gametes have serious problems, and will typically lead to very early miscarriages, especially if they involve a large chromosome, like chromosome 2. They will have more problems conceiving, but their children will be normal.''
