Sectoring was induced with x-rays or ultraviolet in a diploid yeast strain heterozygous for seven genes located on one chromosome arm. The frequencies of sectoring of different genes were approximately linearly related to their distance from the eentromere. If two or more adjacent genes sectored, the event could be explained by mitotic crossing over. Sectoring of single genes, however, was mostly nonreciprocal and resembled a conversion-type event. Approximately 80 % of the sectored colonies could be explained single mitotic crossovers in one of the intergenic regions.
The genotype of mitotically dividing diploid cells is generally assumed to be maintained with relatively high precision. However, phenotypie expression of recessive genes, formerly in heterozygous state, does occur at a low frequency in some organisms. STE~ (1936) observed concomitant expression of recessive linked traits in Drosophila in the form of "twin spots" and concluded that mitotic crossing over was causally associated with this event. Subsequently, mitotic segregation has been observed in various fungi: in Aspergillus nidulans by PO~TECOaVO, ROPE~, HE~MO~S, MAcDonALD, and BUFTO~ (1953), in Aspergillus niger by Po~TECO~VO, t~oPE~, and ForBEs (1953), in Penicillium chrysogenum by PONTECO~VO and SER~O~TI (1954), in Saceharomyces cerevisiae by JAMES (1955), and Ro~AN (1956), and in U~.tilago maygis by ~c: [OLLIDAY (1961).