Biotic factor
Other living organisms that are a factor of an organism’s environment, and form the biotic environment, affecting the organism in many ways.

Biotic stress
'Stress resulting from living organisms which can harm plants, such as viruses, fungi, bacteria, parasitic weeds and harmful insects.  cf abiotic stress

Biotoxin
A naturally produced toxic compound which shows pronounced biological activity and presumably has some adaptive significance to the organism which produces it.

Bivalent
A pair of synapsed associated homologous chromosomes (one of maternal origin; the other of paternal origin) that have each undergone duplication. Each duplicated chromosome comprises two chromatids. Thus a bivalent comprises four chromatids

B-lactamase
An ampicillin resistance gene. See selectable marker.

BLAST
A set of programs, used to perform fast similarity searches. Nucleotide sequences can be compared with nucleotide sequences in a database using BLASTN, for example. Complex statistics are applied to judge the significance of each match. Reported sequences may be homologous to, or related to the query sequence. The BLAST program is used to search a protein database for a match against a query protein sequence. There are several other flavours of BLAST.

BLAST2
A newer release of BLAST. Allows for insertions or deletions in the sequences being aligned.  Gapped alignments may be more biologically significant.
 
blastocyst (also blastocist)
A mammalian embryo (fertilized ovum) in the early stages of development, approximately up to the time of implantation. It consists of a hollow ball of cells