White spotting: When there’s more than two alleles

So far, we have discussed genes which have only two alleles. However, that is not always the case: there can be more than two alleles for a given gene. One example is the MITF gene, which is the major gene that controls white spotting in dogs. This protein is required for the migration and survival of melanocytes into the skin during development. If it is not functional, it impairs the ability of the skin to make pigment, thus “covering up” the effect of other color genes. There are thought to be at least four alleles that can contribute (Karlsson, 2007). Depending on which alleles are present in a dog, the amount of white can vary from none (a solid-colored dog) to mostly white (Table 2 and Figure 24).

Table 2: Combinations of different alleles for MITF result in different amounts of white present in the coat.

Alleles Amount of white
SS None (solid colored)
Ssi Small amounts of white possible on chin, chest, feet, and tail tip
Ssp Pied markings where the coat is more than 50% colored, with white on the face, chest, feet, collar, underbelly, and tail tip
sisp Approximately even amounts of color and white
sise More than 50% white with irregular splashes of color
sese Mostly white with only minimal areas of color, perhaps on one or both ears, an eye patch, or a spot near the tail
12.varyingwhite

Figure 24: These dogs have different combinations of alleles of the MITF gene. The first dog probably has the genotype “SS”; the dog in the center is likely “Ssp“; the dog on the right is likely “sese“. (Credits: Funny black dog by X posid from Publicdomainpictures. A black and white dog by Petr Kratochvil from Free stock photos. White dog with black ears by RetyiRetyi from Pixabay.)

Human Connection – Blood Type

Human blood type was discussed in the previous section. You may remember that there are three alleles for the ABO gene: A, B, and O. A and B are codominant, meaning that if both alleles are present, both will be seen in the phenotype. A person with type AB blood has one A allele and one B allele.

O is recessive to A and B. A person with the genotype AO will have Type A blood. A person with the genotype BO will have type B blood. Type O blood results from two O alleles.

13.ABO_Blood_type

Photo credit: Kalaiarasy, from Wikipedia.

References

Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.

Karlsson EK, Baranowska I, Wade CM, Salmon Hillbertz NH, Zody MC, Anderson N, Biagi TM, Patterson N, Pielberg GR, Kulbokas EJ 3rd, Comstock KE, Keller ET, Mesirov JP, von Euler H, Kämpe O, Hedhammar A, Lander ES, Andersson G, Andersson L, Lindblad-Toh K. 2007. Efficient mapping of mendelian traits in dogs through genome-wide association. Nat Genet. 39(11):1321-8.

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