The histidine locus is complex in Neurospora crassa. Many multiple alleles are known, such as 245, CD-16, 430, and 261, which lack the ability to synthesize the amino acid histidine. Such strains will consequently be unable to grow on minimal medium. Provide an interpretation for the following observations involving these histidine strains of Neurospora.

a) When heterokaryons are produced (single “cells” containing nuclei from two different strains), the combination of strains 245 and 261 always produces growth on minimal medium, whereas the combination of strains CD-16 and 245 never produces growth on minimal medium.

b) When strain 430 is crossed with strain 261, rare ascospores are produced, which one can grow on minimal medium.

Separate and clearly label your answers for parts a-b in the box below.