At which stage of meiosis are bivalents formed?

A. Interphase

B. Prophase I

C. Prophase II

D. Metaphase II

B

This was a question that looked difficult because of the use of the word bivalent. It was not expected that candidates would necessarily know the term, but they were told that the diploid chromosome number of wild garlic is 14 and as there were 7 bivalents visible in the micrograph it was possible to deduce that they are pairs of homologous chromosomes. According to the programme, candidates are expected to know that homologous chromosomes pair up in the early stages of meiosis, so answer B (Prophase I) should have been chosen. The commonest incorrect answer was C (Prophase II). Perhaps the seven bivalents were identified by candidates as seven chromosomes, each of two chromatids. This possibility can be rejected because the paired structures in the micrograph are held together in at least two places, which must therefore be chiasmata rather than centromeres.