Genetics Terms Starting With B

Backcross is a mating between a hybrid offspring and one of its parental genotypes, used to determine the genotype of the hybrid parent.

Backcrossing is a foundational technique in classical genetics for revealing whether an individual is homozygous or heterozygous at a given locus. In plant and animal breeding, repeated backcrossing transfers a desired trait from one strain into the genetic background of another; each generation of backcrossing recovers approximately 50 percent more of the recurrent parent’s genome, so breeders typically require 6 to 8 generations to achieve more than 99 percent recovery. The proportion of offspring showing the recessive phenotype in the first backcross generation directly reveals the genotype of the hybrid parent.

Balanced Polymorphism is the stable maintenance of two or more alleles in a population at frequencies higher than can be explained by mutation alone.

Balancing selection maintains genetic variation through mechanisms such as heterozygote advantage, frequency-dependent selection, and temporal or spatial variation in selective pressures. The classic example is sickle cell trait, where heterozygotes carrying one normal and one sickle allele survive malaria-endemic environments at higher rates than either homozygote. In sub-Saharan African populations where malaria transmission is intense, the sickle cell allele reaches frequencies of 10 to 20 percent, far above what mutation alone could sustain.

Balanced polymorphisms represent reservoirs of genetic diversity that persist across many generations because selection actively opposes the loss of any single allele.

Base Pair is a pair of complementary nitrogenous bases, such as adenine and thymine in DNA, bonded across opposite strands of the double helix.

In DNA, adenine bonds with thymine through two hydrogen bonds, while guanine bonds with cytosine through three hydrogen bonds; these complementary pairing rules hold the two strands of the double helix together. During DNA replication, DNA polymerase reads the template strand and adds nucleotides following base pairing rules, achieving error rates below one mistake per billion bases due to proofreading mechanisms. The human genome contains approximately 3.2 billion base pairs, and accurate pairing of each one ensures faithful copying of genetic instructions during cell division.

Bivalent is a paired structure consisting of two homologous chromosomes held together by points of crossing over during prophase I of meiosis.

During prophase I of meiosis, each chromosome pairs with its homologous partner through a process called synapsis, forming a bivalent that contains four chromatids in total. Crossing over occurs within the bivalent at sites called chiasmata, where non-sister chromatids exchange segments and generate new combinations of alleles. Human cells form 23 bivalents during meiosis I, one for each homologous pair, and each bivalent typically displays one to three chiasmata depending on chromosome length.

The bivalent disassembles as meiosis I progresses, with homologous chromosomes pulled to opposite poles during anaphase I.

Bottleneck Effect is a reduction in genetic diversity that occurs when a population is drastically reduced in size by a random catastrophic event.

When a population passes through a bottleneck, rare alleles are often lost entirely and surviving alleles may be present at frequencies very different from the original population. The reduced genetic diversity can limit a population’s ability to adapt to future environmental changes. Recovery of diversity after a bottleneck depends on mutation, gene flow, and the time elapsed since the event.