Biology as Poetry: Genetics

Bacteriophage Ecology Group

True Breeding

What happens when two homozygotes for same allele are crossed.

True breeding is often considered from the perspective of only one or a few loci, that is, unless that two individuals that are being crossed are clonally related (i.e., genetically identical) and are homozygous across all loci (which would be an unlikely situation to say the least!). Thus, a population of organisms that are true breeding will display among their offspring only a particular variant (trait) of whatever character (or characters) is under focus.

Note that whether or not matings give rise to true breeding can be obscured by recessive alleles. As a consequence, it often is important to assess not just the next generation (F1 generation) but the progeny of these progeny as well (F2 generation). Indeed, in practice, and if going on phenotype alone, it typically will be necessary to follow an inbred population – that is, one that is exclusively interbreeding – for some time before one can declare that a population is true breeding for a specific character. In this modern age, however, one is just as likely to explore this issue by means of direct genotype determination, e.g., such as through DNA sequencing.

Note that determination of true breeding can be trivial for recessive traits. This is because dominant alleles that are also found at the locus in question will always be seen (assuming 100% penetrance). Therefore, it is particularly for dominant alelles that some work must be put in to declare that a population as true breeding, and in practice that work can consist simply of following a population through a number of generations as well as using test crosses to assess the genotypes of individuals members of the population.

In Mendelian genetics, true breeding populations hold a special place. That is, these are the individuals for which genotype at specific locus is known, particularly the homozygous dominant genotype. Such true breeding individuals are typically used to initiate experiments monohybrid or dihybrid crosses as well as the various genetic experiments that are performed to measure the degre of genetic linkage between loci.

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