Has subterminal centromere and extremely short second arm in mitotic metaphase. Constitution confirmed by analysis of detachments with C(1)RA, all of which appear to result from exchange between the interstitial heterochromatin of the compound and the bb+-bearing short arm of y+YL (Sandler, 1954, D. I. S. 28: 153-54).
KL.(bb+) ac+ - pch+. pch+ - ac+ KL bb+.