By Nanna M. Andersen | Published: 14.12.20 | Edited: 02.07.23 The first linkage group we will examine is the linkage between the c, r, and p alleles, as described by Dunn and Robinson (Dunn 1920, Robinson 1960). This linkage has been extensively studied, as these alleles are among the oldest known in rats. The recombination percentage between c and r is 1.8±0.54, indicating that they are closely linked on the chromosome and are more likely to crossover together as Cr or cR. Similarly, the recombination percentage between c and p is 21.1±2.92, and 18.3±0.38 for r and p. This suggests that p has a slightly higher chance of recombining separately with the other alleles as "cc pp" or "rr pp" (Dunn 1920). For convenience, c is placed at 0 to indicate the distance between the r and p alleles. These findings align with Castle's observations (Castle 1960), and when we merge the two results, we obtain the following distances, which indicate that r is closer to c than initially shown by Dunn. The close linkage between c and r implies that they are much less likely to crossover independently as the recessive alleles (cc rr), although not impossible, the chances are extremely low. So why don't we see more rats with the genotypes "pp rr," "cc pp," or "cc rr"? It is speculated that these particular genotypes, especially "pp rr," result in a very light Champagne or PEW (Pink-Eyed White) phenotype. Since these colors do not stand out as unique, it is difficult to spot these genotypes in a litter, and they are often mistaken for PEW or Champagne rats. Now, let's shift our focus to three other linked loci that exhibit more obvious crossovers: Agouti (A), Pearl (Pe), and Fawn (f) - it is important to note that Fawn should not be confused with the fancy's Fawn (A* rr). Fawn is believed to be a reoccurrence of the Mink (mm) gene based on the description. The recombination percentages between A and Pe are only 0.73±0.42 (Robinson 1994), as shown on the picture below, this indicates that it is rare for A and Pe to crossover together but despite their close linkage, we do observe Cinnamon Pearl rats in the fancy. On the other hand, the recombination percentages between Pe and f are approximately 44.66±0.14 (Robinson 1960). This suggests that f is more likely to undergo independent crossovers from A and Pe, meaning A or Pe is far more likely to crossover with the recessive f allele. References
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Februar 2024
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