Two new B-10 translocations involved in the control of nondisjunction of the B chromosome in maize.

A B-A translocation, TB-10(18), has been established involving breakpoints in the proximal region of the long arm of chromosome 10 and the minute short arm of the maize B chromosome. TB-10(18) differs in its nondisjunctional behavior at the second microspore division from TB-10(19), which has a breakpoint in the same region of 10 but in the heterochromatic region of the long arm of B, in the following ways: (1) Nondisjunction of the B(10) chromosome of the TB-10(18) translocation occurs in the absence of the reciprocal element (10(B)), albeit at low frequency. (2) Presence of 10(B) increases the frequency of B(10) nondisjunction but not to the level found for TB-10(19) and certain other translocations. (3) The frequency of B(10) nondisjunction varies among closely related sublines both when 10(B) is present and when it is absent. It is inferred that the B(10) of TB-10(18) carries all the components of B necessary for nondisjunction but that expression is weak in the absence of 10(B), suggesting the existence in the B chromosome short arm of a factor influencing efficient nondisjunction.