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6.3: Example of Multiple Genes Affecting One Character

6.3.1 Cat Fur Genetics

Most aspects of the fur phenotypes of common cats can be explained by the action of just a few genes (Table 6-2). Other genes, not described here, may further modify these traits and account for the phenotypes seen in tabby cats and in more exotic breeds, such as Siamese. 

For example, the X-linked Orange gene has two allelic forms. The OO allele produces orange fur, while the OB alleles produce non-orange (often black) fur. Note however, that because of X-chromosome inactivation the result is mosaicism in expression. In OO / OB female heterozygotes patches of black and orange are seen, which produces the tortoise shell pattern (Figure 6-13 A,B). This is a rare example of co-dominance since the phenotype of both alleles can be seen. Note that the cat in part A has short fur compared to the cat in part B; recessive alleles at an independent locus (L/l) produce long (ll) rather than short (L_) fur. 

Figure 6.13: Representatives of various fur phenotypes in cats: Tortoise shell (A,B) pigmentation in cats with short (A) and long (B) fur; black (C) and grey (D) cats that differ in genotype at the dilute locus. The pure white pattern (E) is distinct from piebald spotting (F). (flickr-cwbuecheler–CC:AN, flickr-=-.0=-CC:AND, flickr-Leonardo Zanchi CC:AS, flickr-malfet_ CC:AN, flickr- valentinastorti-CC:AN, flickr- Denni Schnapp-CC:AS)

Alleles of the dilute gene affect the intensity of pigmentation, regardless of whether that pigmentation is due to black or orange pigment. Part C shows a black cat with at least one dominant allele of dilute (D_), in contrast to the cat in D, which is grey rather than black, because it has the dd genotype.

Epistasis is demonstrated by an allele of only one of the genes in Table 6.2. One dominant allele of white masking (W) prevents normal development of melanocytes (pigment producing cells). Therefore, cats with genotype (W_) will have entirely white fur regardless of the genotype at the Orange or dilute loci (part E). Although this locus produces a white colour, W_ is not the same as albinism, which is a much rarer phenotype caused by mutations in other genes. Albino cats can be distinguished by having red eyes, while W_ cats have eyes that are not red.

Piebald spotting is the occurrence of patches of white fur. These patches vary in size due to many reasons, including genotype. Homozygous cats with genotype ss do not have any patches of white, while cats of genotype Ss and SS do have patches of white, and the homozygotes tend to have a larger proportion of white fur than heterozygotes (part F). The combination of piebald spotting and tortoise shell patterning produce a calico cat, which has separate patches of orange, black, and white fur.

Table 6.2: Summary of simplified cat fur phenotypes and genotypes.
Trait Phenotype Genotype Comments
fur length short LL or Ll L is completely dominant
  long ll
all white fur (non-albino) 100% white fur WW or Ww If the cat has red eyes it is albino, not W_. W is epistatic to all other fur color genes; if cat is W_, can’t infer genotypes for any other fur color genes.
  <100% white fur ww
piebald spotting > 50% white patches (but not 100%) SS S is incompletely dominant and shows variable expressivity
  < 50% white patches Ss
  no white patches ss
orange fur all orange fur XOXO or XOY O is X-linked
  tortoise shell variegation XOXB
  no orange fur (often black) XBXB or XBY
dilute pigmentation pigmentation is intense Dd or dd D is completely dominant
  pigmentation is dilute (e.g. gray rather than black; cream rather than orange; light brown rather than brown) dd
tabby tabby pattern AA or Aa This is a simplification of the tabby phenotype, which involves multiple genes
  solid coloration aa
sex female XX  
  male XY

References

  1. Adapted from Christensen (2000) Genetics 155:999-1004)

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