With regards to biology bivalent is a pair of homologous chromosomes that occurs during meiosis.
During meiosis, in the prophase stage there are five sub stages, in which during zygonema pair of homologous chromosomes undergoes synapsis process.
At the end of synapsis process, in the sub stage of pachynema, synapsed set of chromosomes that are already replicated earlier, contain four chromatids.
These four chromatids together called as a bivalent or a tetrad.
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Image: http://www.sci.sdsu.edu/classes/bio100/Lectures/Lect05/Image183.gif
