The second pollen mitosis is normal, so the sperm come to carry the unreduced B number. As this happens through
both male and female tracks, the build-up of B chromosomes in the population is inevitable. Selection for high seed set eliminates Bs in modern cultivars as they adversely affect fertility.
The mechanism of nondisjunction at pollen mitosis shows sticking sites on either side of the centromere of the B,
probably in the pericentromeric domains, and it is this region of delayed separation that holds the B-chromatids together longer than the As. Furthermore, it is known that there is a genetic element in the distal part of the long arm of the rye B that produces a trans-acting
product (protein), which is essential for the sticking action of the sensitive receptors on either side of the centromere region. This is known because, in Bs that are deficient for the distal part of the long
arm of the B, nondisjunction fails unless there is another B in the same cell which carries this distal segment. Sequence organization of the distal end of the long arm of the B demonstrates two B-specific
sequences that have a complex organization, but it has not been possible thus far to identify genes which could code for proteins to interact with the pericentric receptors. The rest of the rye B has repetitive
DNA similar to that of the A chromosomes. It seems to be composed from chromosome 3R and 7R, while chromosome 7R carries a big piece of chrosome 5RL, considering the evolutionary synteny. In addition, it also has a highly conserved structure and, at the cytological level, it has a similar form in all the many different populations where it
occurs. It seems that it is a highly optimized selfish element for the function for which it has evolved – its own replication.
The Giemsa banding-positive heterochromatin subterminal domain of rye standard Bs undergoes
decondensation during interphase. Contrary to the heterochromatic regions of A chromosomes, this domain is simultaneously marked by trimethylated H3K4 and H3K27 histons. Both types of high-copy-repeat
families of the subterminal domain are transcriptionally active (395).