Mendelian Inheritance of Chromosome Shape in Matthiola.

A variety of stock (Matthiola ilzcana R. Br.) known as Snowflake has been shown by FROST (1919) to produce frequent mutants of at least eight distinct kinds. These mutant forms are genetically inconstant, part of the progeny always being normals. These forms have been determined (FROST and MANN 1924, and papers in preparation) to be trisomic. Within, the variety Snowflake, all plants cytologically examined, whether normal or with one or more extra chromosomes, have had strikingly long first-metaphase chromosomes in their pollen mother cells. Miss ALLEN (1924) figured chromosomes for several pure single varieties of Matthiola, showing them to be much shorter than those found in Snowflake. The four pure single races used in the hybridization studies described below likewise all had very short chromosomes at and immediately before the first metaphase. The contrast in chromosome shape between Snowflake and these pure single varieties was very striking. These short-chromosome varieties were crossed with Snowflake trisomics. The F1 hybrids had short chromosomes, about like those of the pure single parents. It had seemed possible that the first-metaphase bivalents of these hybrids might each consist of a short and a long element, representing the respective parental types, but no suggestion of such a situation was observed. The short form was dominant in the group as a whole; each bivalent was composed of two similar members, whose source could not be determined. When Fz hybrids were examined, it was at once evident that genetic segregation for chromosome shape had oc-