iGen3 16-18

Robertsonian Translocations

     Robertsonian Translocations result in  a conservation of arm number with a change in chromosome number (counted as centromeres). A Robertsonian fusion combines the long arms of two telocentric chromosomes: the tiny short arms are lost, and the result is a reduced number of chromosomes. The karyotype 'loses' two telocentric chromosomes and 'gains' a single meta- or acrocentric chromosome. The reciprocal event [not shown], a Robertsonian fission, occurs when a single meta- or acrocentric chromosome breaks at the centromere, such that the two arms segregrate independently as two telocentrics.

    Note that the gene content is largely conserved. During meiosis, the arms of the metacentric chromosome will typically pair with those of the two telocentric chromosomes. So long as the three centromeres segregate properly [two telocentrics one way, the biarmed chromosome the other], two balanced gametes will result. If in the next generation two translocation gametes unite, the resutant offspring will inherit the translocation in a stable manner and will be fully fertile. For this reason, Robertsonian Transolcation are an important mechanism in evolutionary speciation.

Figure ©2010 PJ Russell, iGenetics 3rd ed.; all text material ©2014 by Steven M. Carr