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Backcross vs Testcross difference: Research on mitochondrial genes
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[QUOTE="BillTre, post: 6222356, member: 581757"] [B]Overview:[/B] Its sounds like you understand the difference between the nuclear genome and the multiple copies of the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome in cells. Many different slightly different versions of the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome can be inherited from the mother. The father does not contribute to their offsprings's collection of [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes. The nuclear genome (C57BL/6J)is well defined can be genetically manipulated using standard Mendelian crossing methods. Since you are starting with a 57BL/6J background and all the crosses are made on the 57BL/6J background. It is a reasonable to assume that there is no difference between the different 57BL/6J backgrounds (unless otherwise stated in the paper). Therefore making crosses between the 57BL/6J background mice should have no effect on the genetic background of the mice. On the other hand, there maybe some variability in the 57BL/6J genetic backgorund they are using (this can happen for a variety of reasons). Backcrossing to a male ancestor (whch could be prolonged through 12 generations by using frozen sperm) would be a way of reducing the 57BL/6J background variability. Mitochondrial genome manipulations can only done by controlling the female side of the crosses. Multiple [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL] (and their genomes) are inherited from the mother as a subset of the different versions of [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes present in her germ cells. This can be a diverse set of genomes with genetic differences. To make lines of mice with identical [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes, they used 12 generations of backcrosses. [B]Conplastic[/B] strains would (presumably) be strains of mice with identical [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes). Once a female mouse is generated with only one kind of [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome, then it could be expanded into a line of identical mice that could be tested vs. other lines of mice. [B] Test crosses[/B] are crosses set-up to detect a genetic condition that is not easily determined in an animal. An example would be to determine if some animal were carrying a recessive mutation as a heterozygote. This is most easily done by crossing the animal to a known carrier for the recessive gene in question. If the first animal is a carrier, then the cross should have ~1/4 of the offspring as showing the recessive trait (however, any recessives would be taken as a positive result). This is an analytic method. [B]Backcrosses[/B] are crosses between offspring and parent (usually). They could be used analytically but usually for producing some particular genetic result (therefore a production method). In this case, they are want to reduce the number of different kinds of [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes in a line to one. Back crossing a parent (or between siblings) is a common method for generating a line of mice, all with the same nuclear genome. 10-12 generations of crosses is often considered enough to generate isogenic lines. If they were backcrossing to a single male, then the 57BL/6J nuclear genome they carry would approach the 57BL/6J genetic background which the single male carried. Different female carried [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes with different genetic varients could be crossed in while maintaining 57BL/6J background. Crossing to females of the 57BL/6J would preserve most of the nuclear genetic background. Crossing back to the same original male would then tend to "washout" nuclear genes from the female over teh following generations. WRT the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome, since each new offspring might (by chance) not inherit all of the variability of the parent, the offspring could be sorted through (presumably using molecular genetic techniques) to find those offspring carrying the least variability in their [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes. By repeatedly finding and breeding mice carrying fewer variants of the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes in each generation, they could eventually generate mice with a single version of the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome. These individual mice could then be used to start a line for their experiments (or tests). If this were repeated several times in parallel, then several lines differing only in the kinds of mutations in the [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genome will be generated. Each different line being conplastic. This might have already been done since they mantion [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l donor lines. However, since [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genes are reproduced more times than the genes of their host cells and have a higher mutation rate, any previously generated lines could redevelop new mutations and therefore variability in their [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes. I'm guessing that backcrosses were used in this aspect of the line generation to eliminate residual variation in the 57BL/6J genetic background while a collection of lines with different [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes on the 57BL/6J background. Sibling crosses would all inherit closely related [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes since they would all have the same mother, but if there were variability in the 57BL/6J background they had available to use. Since they mention [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l donor strains, they may already have strains of mice with well defined [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l genomes which they used to cross into the lines they wanted to make. If your interpretation: is correct, then they used the backcrosses only to initially generate the lines, after which they were further inbred using sibling crosses. The backcross would be to add [URL="https://www.physicsforums.com/insights/when-did-mitochondria-evolve/"]mitochondria[/URL]l variants, while the further breeding would be to make the lines more 57BL/6J isogenic. [B][/B] [/QUOTE]
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