1. Brittle endosperm starch (bt bt bt).
In the course of analyses of endosperm starch of non‑waxy back‑mutants from mutable‑waxy (wxm) stocks, the opportunity presented itself to determine the starch composition of homozygous brittle endosperm, as an approach to the question of brittle gene action. The fact that supersugary (su du Wx) kernels contain endosperm starch assaying 65% amylose (Cameron, Genetics 32: 459), suggested that the brittle phenotype might also be due to an abnormality in starch synthesis expressed in an off‑ratio of amylose to amylopectin.
The brittle phenotype is characterized by the shriveled and translucent appearance of dried kernels. Some starch is present; the starch grains are smaller than normal, and round rather than angular, indicating that they probably are not closely packed in the endosperm cells.
The granular starch of brittle endosperm was found to contain 27.8% amylose by the potentiometric titration method of Bates, French and Rundle (Jour. Amer. Chem. Soc. 65: 142). This value is well within the range reported for a large number of corn varieties and inbreds by many investigators, and may be considered "normal", i.e., unaffected by the recessive brittle allele. That the small amount of starch present does not result from some mechanical block to nutrients entering the endosperm from the plant, is shown by the occurrence of brittle endosperm mosaics.