Mutants may occur within an outbred stock, and with larger animals, transgenes will usually be developed in outbred stocks. Similarly, in rodents, outcrossing in order to obtain better breeding performance is quite common in the development of transgenic animals. In these situations, the breeder needs to consider how the stock is to be maintained in the future.
With large animals or species for which inbred strains are not available, there is little option but to maintain the mutation/transgene on the outbred background, taking account of the possible need to avoid genetic drift and inbreeding, which could reduce breeding performance. With mice and rats, a choice is possible. The stock can be maintained as an outbred stock, the colony could be inbred, or the mutation/transgene could be backcrossed to an existing inbred strain. Scientific considerations need to be taken into account. The expression of most mutants and transgenes is influenced by the genetic background. If this background is heterogeneous, the expression of the mutation will also be variable. Moreover, if the genetic background drifts because the colony could not be maintained so as to avoid inbreeding and genetic drift, or if outcrossing to another strain is used to boost breeding performance, then the expression of the mutation may vary over time. For this reason, most geneticists would recommend that the mutation/transgene be transferred to an inbred, or possibly hybrid, background by backcrossing, as described below in the development of a congenic strain.32 Once a congenic strain has been developed, its characteristics will be fixed, and it can be maintained in small numbers without problems of genetic drift.
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