translocation occurs when fragments from non-homoloous
chromosomes recombine in such a way that a single centromere
is retained. In the example, a break in the p arm of
the shorter chromosome allows a large acentric fragment from
the q arm of the longer chromosome to recombine
with it. The reciprocal product would combine the the small
p arm of the longer chromosome with a small acentric
Robertsonian fusions combine
the long arms of two telocentric chromosomes: the
tiny short arms are lost, and the result is a conservation
of arm number with a reduction in chromosome number
(counted from centromeres). That is, the karyotype 'loses'
two telocentric chromosomes and 'gains' a single metacentric
chromosome. [A Robertsonian fission occurs when a
metacentric chromosome breaks at the centromere, such that
the two arms segregrate independently as telocentrics].
Note that the gene
content is largely conserved. The translocated metacentric
chromosome may pair with the two separate telocentric
chromosomes, and if the three centromeres separate properly
two balanced gametes will result. If two translocation
gametes unite, the offspring will inherit the translocation
in a stable manner.