A-B Translocation in Maize

In some genetic stocks of maize there exists supernumery chromosomes called 'B' chromosomes. The 'A' chromosomes are members of the basic set. B chromosomes are abnormal and may be due to some defect in normal A chromosomes. Normal maize has a basic set of X=10 chromosomes. The B chromosomes are in addition to the 2X=20 diploid number of chromosomes.

See - Roman, H. 1947. Mitotic nondisjunction in the case of interchanges involving the B-type chromosome in maize. Geneteics 32:391-409.

Roman, H., and A.J. Ullstrup. 1951. The use of A-B translocations to locate genes in maize. Argon. J. 43:450-454.

B-type chromosomes "do not have specific genetic effects and do not impair the viability of the plant even when present in relatively large numbers. The B-type chromosome is essentially orthodox in its behavior in meiosis and mitosis, except for the division of the generative nucleus in the microspore. In this division, the chromosome frequently fails to disjoin, with the result that one of the sperm nuclei receives both chromatids. Thus, if nondisjunction occurs, a microspore with one B-type will develop into a pollen grain in which one sperm has two B-types and the other has none." pg 450. Agron J. 43:450-454.

Translocations between A and B types have been induced by X-ray irradiation. These A-B translocations have an A-type centromere with part of a B chromosome or a B-type centromere with part of an A chromosome attached. A-B translocations can be used to map genes to a specific chromosome. The chromosome which bears the B centromere is the non-disjunctional chromosome.

B^A is the result of an A-B translocation and this chromosome has the B centromere.

A^B is the result of an A-B translocation and this chromosome has the A centromere.