By Nanna Andersen | Published: 19.08.12 | Edited: 02.02.24 | Oversat til dansk:
Breeding rats and mice involves different breeding methods, each with its advantages and challenges. Here's a simple breakdown of three methods. Inbreeding Inbreeding means mating closely related animals from the same family, most often brother/sister, but also mother/son and father/daughter. Inbreeding aims to enhance the desired genetic traits by increasing the likelihood of offspring inheriting identical alleles from both parents, this can be variety, temperament, health and conformation. While inbreeding can intensify desirable traits, it can also magnifies the expression of detrimental recessive alleles, potentially leading to increased health issues and reduced genetic diversity, which is calcified as inbreeding depression. Careful monitoring and selection are essential to avoid the negative consequences of excessive inbreeding. Linebreeding Linebreeding is a more controlled form of inbreeding, where breeders aim to maintain a connection to a particular ancestor while minimizing the risks associated with excessive inbreeding. By selecting offspring that share common ancestors but are not as closely related as those produced through inbreeding, breeders can maintain a certain genetic lineage while reducing the potential for health problems. Linebreeding requires a deep understanding of the rats genealogy and careful consideration to ensure genetic diversity. Outcrossing Outcrossing involves mating individuals from unrelated or distantly related lines. This method introduces fresh genetic material into a breeding program, potentially enhancing overall health, vigor, and adaptability. Outcrossing can be particularly useful when addressing specific health issues or traits that have become problematic due to excessive inbreeding. However, it can also dilute desired traits and take several generations to restabilize the traits in the line. A quick summary Each breeding method carries its own set of advantages and challenges. Inbreeding and linebreeding focus on intensifying specific traits but require careful management to prevent negative consequences. Outcrossing introduce genetic diversity, potentially leading to improved health and vigor, but also demand thorough planning to achieve consistent and desired traits in offspring. The choice of breeding method depends on the goals of the breeder, and the intended outcomes for the next generation. A summary of: "Behavioral Aspects of Animal Domestication" by Edward O. Price. The Quarterly Review of Biolog, 59(1), 1-32 Inbreeding, which is particularly unavoidable within small, closed captive populations, can lead to a decline in variability in a genetic level. This can result in a reduction of vigor or fitness, known as "inbreeding depression," as recessive deleterious genes are expressed due to the lack of dominant or epistatic alleles. Domestic animals have shown inbreeding depression in traits like hatchability, clutch size, milk yield, and growth rate. Moreover, juvenile mortality increases in captive animal populations due to inbreeding depression. Although inbreeding eventually leads to a loss of variability, this effect is less problematic in populations maintained by humans, as they tend to protect variants that might not naturally survive. This preservation of variants helps maintain desired traits despite the potential drawbacks of reduced genetic diversity caused by inbreeding. A summary of: "Inbreeding, eugenics, and Helen Dean King (1869-1955)" by Marilyn Bailey Ogilvie. Journal of the History of Biology (2007) 40:467-507 Throughout history, scientists have conducted breeding experiments with rats to understand the effects of inbreeding, linebreeding, and outcrossing on their genetics and physical traits. One notable researcher in this area was Dr. Helen Dean King, who conducted meticulous breeding experiments with albino and Norway rats. Dr. King's inbreeding experiments demonstrated that, with proper selection, inbreeding did not necessarily harm the offspring's growth and vitality. Over the course of 22 generations, Dr. King collected data on approximately 12,000 inbred rats and found that inbred rats remained larger than outcrossed rats, even after 28 generations of brother-sister matings challenging the belief that inbreeding always harms growth and health. Overall, Dr. King's groundbreaking research on diverse rat strains provided valuable insights into the effects of inbreeding, linebreeding, and outcrossing. Her work challenged conventional beliefs and emphasized the importance of genetic selection and environmental factors in determining rat traits and health. A summary of: "Pathologic Characterization of Brown Norway, Brown Norway X Fischer 344, and Fischer 344 x Brown Norway Rats With Relation to Age" by Ruth D. Lipman, Clarence E. Chrisp, DeWitt G. Hazzard, and Roderick T. Bronson. Journal of Gerontology: BIOLOGICAL SCIENCES 1996. Vol. 51 A. No. l". B54-B59 This paper provides insights into three rat lines: Brown Norway (BN), Fischer 344 (F344), and their resulting hybrids. The researchers established a linebred colony, including two strains of Brown Norway (BN) rats: BN/SsN and BN/Rij which were both linebred. Linebreeding was used for all the BN rats. Inbreeding was used for all the F344 rats. To increase genetic diversity, the researchers produced hybrid lines by breeding the inbred F344 females with males from the following lines:
Each breeding approach has its advantages and challenges. Linebreeding enhanced genetic homozygosity and specific traits but limited genetic diversity over time. Inbreeding achieved genetic homozygosity faster, but greatly reduced genetic diversity and led to inbreeding depression. Outcrossing introduced variability and novel traits but complicated the certainty of the genetics and increased the risk of introducing unwanted genes. A summary of: "Inbreeding and outbreeding impact on developmental stability of laboratory rat Rattus norvegicus" by Borisov, V. I., et al. Inbreeding and Outbreeding Impact on Developmental Stability of Laboratory Rat Ratus Norvegicus. 1997. The study found that the homozygous or heterozygous state of a single gene locus does not commonly affect developmental stability. Instead, the overall genotype characteristics play a more significant role, meaning the genotype as a whole will more often than not be responsible for defects. An increase in genotype homozygosity, as seen in inbreeding, leads to a decrease in developmental stability known as inbreeding depression. Conversely, an increase in heterozygosity, as seen in linebreeding or outcrossing, results in improved developmental stability. Crosses between closely related inbred strains remove inbreeding depression, while crossing different strains can disrupt genetic coadaptation and decrease developmental stability. Developmental stability depends on the overall genotype, especially genetic coadaptation, i.e genes that support each other to help the organism function properly. In the world of genetics, coadaptation helps organisms adapt and survive in their environment. Interestingly, both inbreeding and outbreeding can disrupt genetic coadaptation, affecting the rat's developmental stability, proving that neither breeding method is superior. Understanding these genetic influences can help breeders make informed decisions to maintain or improve the developmental stability of their rats. My conclusion based on the summaries Linebreeding and outcrossing can lead to improved developmental stability, while inbreeding may result in decreased stability due to inbreeding depression or disruption of genetic coadaptation after extensive inbreeding. Whether you use all methods or only one your preferences may be cultural or contextual. Using all in a mindful and strategic way could be the ideal way to create a better line while keeping desired traits alive in the fancy.
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