are bivalent & tetrad the same thing?
The terms bivalent & tetrad are used here with reference to the visibility difference of chromosomes at the various stage of cell division. Bivalent can also be called tetrad at pachytene stage. Understand by reading this.
During zygotene stage, when homologous chromosomes start to pair up they appear as bivalents. Means only two homologous chromosomes appear together, not their sister chromatids, although at this stage they have sister chromatids also. Each bivalent is formed of one paternal and one maternal chromosome.
But in the next stage called pachytene, further condensation of paired chromosomes occurs. Now, each chromosomes is visibly formed of two sister chromatids joined at centromere and is called as dyad. Each bivalent has two dyads and is called a tetrad.