Definition for Trivalent
From Biology Forums Dictionary
Abnormal pairing of trisomic chromosomes in cell division.
In a trisomic organism, the segregation of chromosomes in meiosis is upset because the trisomic chromosome has two pairing partners instead of one. In some cells, the three chromosomes form a trivalent in which distinct parts of one chromosome are paired with homologous parts of each of the others. The result is that at the end of both meiotic divisions, one pair of gametes contains two copies of the trisomic chromosome, and the other pair of gametes contains only a single copy.