Genomic DNA was prepared from a selection of two,
three, and four viable spore tetrads, and the genotype of the
histone H3-H4 loci were scored by Southern blot analysis.
Tetrads with two viable spores were preferentially chosen
since only two DNA preparations were needed to score an
event. The results are presented in Table II. All of the scored
tetrads with four viable spores were parental ditype for the
copy-I and copy-II loci; that is, two spore colonies had the
copy-I wild-type gene set, and two had the copy-II wild-type
gene set. All of the three spore tetrads were tetratype for the
histone genes, and the missing spore would have carried a
deletion of both the copy-I and the copy-II loci. Finally, all
of the two spore tetrads scored were nonparental genotypes
and each of the two surviving spore colonies carried both of
the wild-type H3-H4 loci. In all cases, genetic segregants
that would have contained deletions of both sets of histone
H3 and H4 genes failed to grow. Therefore, from these experiments
we conclude that haploid cells must have at least
one of the histone H3-H4 gene sets, but that they need not
have both sets.