Various authors have described the B chromosome in pachytene of meiosis. These include McClintock (1933), Ward (1973a) and Pryor et al. (1980). The descriptions vary greatly, depending on the degree of condensation of the bivalent. The distal heterochromatic blocks have been seen as divided into from three to seven subunits. Ward's description probably represents the most commonly seen morphology. He divides the heterochromatin into 4 blocks, with the third most distal being larger than the others. Beckett (1991) labeled these heterochromatic blocks as DH1 to DH4, with DH4 being the most distal. Ward also identified the distal euchromatic tip of the B as being distinct from H4. Fig. 1 shows a pair of Bs in pachytene, while Fig. 2 is a diagram of the pair. Evidence for existence of a small B short arm is genetic, as discussed later.

Randolph (1941) did a comprehensive study of maize B chromosome inheritance. He produced three tables full of data on the progeny of crosses between plants with different B chromosome numbers. Despite Randolph's work, the mode of inheritance of Bs was not clearly understood until Roman (1947, 1948) developed translocations between the B and various A chromosomes. The great advantage of the translocations is that they attach genetic markers to the B, allowing classification of crosses with phenotypic markers rather than by cytological examination. In a typical translocation, two chromosomes are produced, the B-A chromosome and the reciprocal A-B. The B-A chromosome, with the B centromere, shows the same behavior as the standard B chromosome. Roman showed that transmission of the B-A chromosomes through the female was normal but transmission through the male was not. He described an accumulation mechanism, which involves nondisjunction of the B-A (or B) at the second pollen mitosis, producing one sperm with two B-A chromosomes and another with zero B-As. Subsequently, the sperm with B-A chromosomes has an advantage in fertilization of the egg.


Accumulation Mechanism Chromosome Type Pollen Mitosis Double Bridge Centric Heterochromatin 
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© Springer Science + Business Media, LLC 2009

Authors and Affiliations

  1. 1.Department of Biological SciencesUniversity of IowaIowa City

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