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You are watching: Part c - the effect of a third gene on fur color

Griffiths AJF, miller JH, Suzuki DT, et al. An development to genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.


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Studies of coat color in mammals expose beautifully how different genes cooperate inthe determination of one character. The mouse is a an excellent mammal for genetic studiesbecause that is tiny and for this reason easy to preserve in the laboratory and also because itsreproductive cycle is short. The is the best-studied mammal in regard come the geneticdetermination of cloak color. The hereditary determination that coat shade in othermammals very closely parallels the of mice and, for this reason, the computer mouse acts as amodel system. Us shall look at at instances from other mammals together we proceed. In ~ leastfive major genes communicate to determine the coat shade of mice: the gene areA, B, C, D,and S.


A gene

This gene determines the distribution of colours in the hair. The wild-typeallele A produce a phenotype referred to as agouti. Agouti is anoverall grayish color with a brindled, or “salt and pepper,” appearance. The is acommon shade of mammals in nature. The effect is brought about by a band of yellow onthe otherwise dark hair shaft. In the nonagouti phenotype (determined by theallele a), the yellow tape is absent, so over there is hard darkpigment throughout (Figure 4-18).


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Figure 4-18

Individual hair from an agouti mouse and a black color mouse. The yellowband on every hair offers the agouti pattern its brindledappearance.


The lethal allele AY, debated in one earliersection, is one more allele the this gene; it renders the whole shaft yellow. Stillanother allele is at, which outcomes in a “black andtan” effect, a yellow belly through dark pigmentation elsewhere. For simplicity, weshall not include these two alleles in the following discussion.


B gene

This gene identify the color of pigment. There space two significant alleles:B coding for black color pigment and b forbrown. The allele B provides the normal agouti color incombination with A however gives solid black witha/a. The genotypeA/– ; b/b provides astreaked brown shade called cinnamon, anda/a ; b/bgives solid brown.

The adhering to cross illustrates the inheritance pattern of the Aand B genes:

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The breeding of domestic horses appears to have removed the Aallele the determines the agouti phenotype, although details wild family members ofthe steed do have this allele. The shade that us have referred to as brown in mouse iscalled chestnut in horses, and this phenotype also is recessive come black.


C gene

The wild-type allele C permits color expression, and also the allelec stays clear of color expression. Thec/c structure is epistatic to the othercolor genes. The c/c animals, doing not have coatpigment, are referred to as albinos. Common in plenty of mammalian species, albinos have alsobeen reported among birds, snakes, fish (Figure4-19), and also humans (Chapter1). Another allele that the C gene is thech (Himalayan) allele. This allele can beconsidered a heat-sensitive variation of the c allele. Just atthe colder body four times is ch useful andable to make pigment. In warm parts the the body, that behaves as with the albinoallele c. A Himalayan computer mouse is displayed in figure 4-20, which also shows the activity of the sameallele in rabbits and also in cats, whereby it to produce the Siamese phenotype.



Figure 4-19

Albinism in reptiles and also birds. In each case, the phenotype isproduced through a recessive allele the determines an inability toproduce the dark colors melanin in skin cell. (The regular alleledetermines the ability to synthesize melanin.) (a) In thisrattlesnake (more...)



Figure 4-20

Temperature-sensitive alleles the the C gene resultin similar phenotypes in several different mammals. This allelesresult in really much decreased or no synthesis of the dark pigmentmelanin in the skin covering warmer parts of the body. At lowertemperatures, (more...)


The c/c structure produces the standardrecessive epistasis amendment ratio, as watched in the complying with cross (in whichboth parents space a/a):

D gene

The D gene controls the intensity of pigment specified by the other coat-colorgenes. The genotypes D/D andD/d permit complete expression of color inmice, yet d/d “dilutes” the color, making itlook “milky.” The effect is due to an uneven circulation of colors in the hairshaft. Dilute agouti, dilute cinnamon, dilute brown, and dilute black coats areall possible. This is an additional example of a full gene. In the followingcross, we assume the both parents area/a ; C/C:

In horses, the D allele shows incomplete dominance. Figure 4-21 shows exactly how dilution affect theappearance of chestnut and bay horses. The milky effect of D isoften watched in residential cats.


Figure 4-21

S gene

The S gene controls the distribution of coat pigment throughoutthe body. In effect, the controls the presence or lack of spots. The genotypeS/– outcomes in no spots, ands/s produce a spotting sample calledpiebald in both mice and also horses. This pattern can be superimposed on any kind of of thecoat colors thought about so far—with the exemption of albino.

Let united state summarize the foregoing conversation of coat color in mice. The typical coatappearance in wild mouse is created by a facility set of interacting genesdetermining pigment type, pigment distribution in the individual hairs, pigmentdistribution top top the animal’s body, and the presence or lack of pigment. Suchinteractions room deduced from crosses in which 2 or an ext of the interactinggenes room heterozygous because that alleles that modify the regular coat shade andpattern. Number 4-22 illustrates part ofthe pigment trends in mice. Connecting genes such as those in mice determinemost characters in any organism.


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Different kinds of amendment dihybrid ratios point to different ways in whichgenes can interact with each other to identify phenotype.

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