One of the homeotic mutants of Bombyx mori, named EDs, has been assumed on the basis of genetical studies to have attachment or translocation between the 6th and the 7th chromosomal linkage groups. This mutant has been kept in the cross EDs/ + : +/q × +/+ : q/q. The present cytogenetical observation revealed that the EDs + males segregating in this cross had 27 or 28 chromosomes at metaphase I. The individuals with 27 chromosomes were divided into two types. (1) with a ring-shaped assemblage apparently consisting of four chromosomes, and (2) with a chain-shaped one again apparently consisting of four chromosomes. Also the individuals with 28 chromosomes were divided into two types; (1) with apparently normal configuration as to whole chromosomes, and (2) with a chain-shaped assemblage and a ball-like chromosome, the former apparently consisting of three chromosomes. At metaphase II of the EDs+males there were five types of chromosomal constitution, n = 26, 27, 28, 29, and 30, indicating that numerical meiotic non-disjunction had occurred. We concluded that EDs involves a chromosomal aberration due to a reciprocal translocation between the 6th and the 7th chromosomal linkage groups.
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