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An Introduction to Genetic Analysis

7th

Anthony J F Griffiths,1 Jeffrey H Miller,2 David T Suzuki,1 Richard C Lewontin,3 and William M Gelbart3

1University of British Columbia

2University of California, Los Angeles

3Harvard University

W. H. Freeman and Company0-7167-3520-22000

geneticsmolecular biology

Chapter 4: Gene interaction in coat color of mammals

Studies of coat color in mammals reveal beautifully how different genes cooperate in the determination of one character. The mouse is a good mammal for genetic studies because it is small and thus easy to maintain in the laboratory and because its reproductive cycle is short. It is the best-studied mammal in regard to the genetic determination of coat color. The genetic determination of coat color in other mammals closely parallels that of mice and, for this reason, the mouse acts as a model system. We shall look at examples from other mammals as we proceed. At least five major genes interact to determine the coat color of mice: the genes are A, B, C, D, and S.

A gene

Figure 4-18
Individual hairs from an agouti mouse and a black (more...)

An external file that holds a picture, illustration, etc., usually as some form of binary object. The name of referred object is ch4f18.jpg.

Figure 4-18

Individual hairs from an agouti mouse and a black mouse. The yellow band on each hair gives the agouti pattern its brindled appearance.

This gene determines the distribution of pigment in the hair. The wild-type allele A produces a phenotype called agouti. Agouti is an overall grayish color with a brindled, or “salt and pepper,” appearance. It is a common color of mammals in nature. The effect is caused by a band of yellow on the otherwise dark hair shaft. In the nonagouti phenotype (determined by the allele a), the yellow band is absent, so there is solid dark pigment throughout (Figure 4-18

The lethal allele A^Y, discussed in an earlier section, is another allele of this gene; it makes the entire shaft yellow. Still another allele is a^t, which results in a “black and tan” effect, a yellow belly with dark pigmentation elsewhere. For simplicity, we shall not include these two alleles in the following discussion.

B gene

This gene determines the color of pigment. There are two major alleles: B coding for black pigment and b for brown. The allele B gives the normal agouti color in combination with A but gives solid black with a/a. The genotype A/– ; b/b gives a streaked brown color called cinnamon, and a/a ; b/b gives solid brown.

The following cross illustrates the inheritance pattern of the A and B genes:

The breeding of domestic horses seems to have eliminated the A allele that determines the agouti phenotype, although certain wild relatives of the horse do have this allele. The color that we have called brown in mice is called chestnut in horses, and this phenotype also is recessive to black.

C gene

Figure 4-19
Albinism in reptiles and birds. In each case, the (more...)

An external file that holds a picture, illustration, etc., usually as some form of binary object. The name of referred object is permission.jpg.

Figure 4-19

Albinism in reptiles and birds. In each case, the phenotype is produced by a recessive allele that determines an inability to produce the dark pigment melanin in skin cell. (The normal allele determines the ability to synthesize melanin.) (a) In this rattlesnake species, the normal dark coloration is due entirely to melanin, so the albino allele results in a completely unpigmented appearance. (b) In this penguin species, melanin normally makes dorsal feathers black, but the reddish orange colors in the head feathers and beak are due to another pigment chemically unrelated to melanin. The recessive albino allele results in no melanin, but the reddish parts are unaffected and retain their normal coloration. (Part a from K. H. Switak/NHPA; part b from A.N.T./NHPA.)

Figure 4-20
Temperature-sensitive alleles of the C gene result (more...)

Figure 4-20

Temperature-sensitive alleles of the C gene result in similar phenotypes in several different mammals. These alleles result in very much reduced or no synthesis of the dark pigment melanin in the skin covering warmer parts of the body. At lower temperatures, such as those found at the body extremities, melanin is synthesized, producing darker snout, ears, tail, and feet. (a) Himalayan mouse. (b) Siamese cat. (c) Himalayan rabbits, which are often sold as pets. All three are of genotype c^h/c^h. (Part a from Anthony Griffiths; part b from Walter Chandoha; part c from Dan McCoy/Rainbow.)

The wild-type allele C permits color expression, and the allele c prevents color expression. The c/c constitution is epistatic to the other color genes. The c/c animals, lacking coat pigment, are called albinos. Common in many mammalian species, albinos have also been reported among birds, snakes, fish (Figure 4-19

), and humans (). Another allele of the C gene is the c^h (Himalayan) allele. This allele can be considered a heat-sensitive version of the c allele. Only at the colder body extremities is c^h functional and able to make pigment. In warm parts of the body, it behaves just like the albino allele c. A Himalayan mouse is shown in Figure 4-20

, which also shows the action of the same allele in rabbits and in cats, where it produces the Siamese phenotype.

The c/c constitution produces the standard recessive epistasis modified ratio, as seen in the following cross (in which both parents are a/a):

D gene

The D gene controls the intensity of pigment specified by the other coat-color genes. The genotypes D/D and D/d permit full expression of color in mice, but d/d “dilutes” the color, making it look “milky.” The effect is due to an uneven distribution of pigment in the hair shaft. Dilute agouti, dilute cinnamon, dilute brown, and dilute black coats are all possible. This is another example of a modifier gene. In the following cross, we assume that both parents are a/a ; C/C:

Figure 4-21
The modifying effect of the dilution allele on basic (more...)

An external file that holds a picture, illustration, etc., usually as some form of binary object. The name of referred object is ch4f21.jpg.

Figure 4-21

The modifying effect of the dilution allele on basic chestnut and bay genotypes in horses. Note the incomplete dominance shown by D. (From J. W. Evans et al., The Horse. Copyright © 1977 by W. H. Freeman and Company.)

In horses, the D allele shows incomplete dominance. Figure 4-21

shows how dilution affects the appearance of chestnut and bay horses. The milky effect of D is often seen in domestic cats.

S gene

The S gene controls the distribution of coat pigment throughout the body. In effect, it controls the presence or absence of spots. The genotype S/– results in no spots, and s/s produces a spotting pattern called piebald in both mice and horses. This pattern can be superimposed on any of the coat colors considered so far—with the exception of albino.

Figure 4-22
Some coat phenotypes in mice.

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Figure 4-22

Some coat phenotypes in mice.

Let us summarize the foregoing discussion of coat color in mice. The normal coat appearance in wild mice is produced by a complex set of interacting genes determining pigment type, pigment distribution in the individual hairs, pigment distribution on the animal’s body, and the presence or absence of pigment. Such interactions are deduced from crosses in which two or more of the interacting genes are heterozygous for alleles that modify the normal coat color and pattern. Figure 4-22

illustrates some of the pigment patterns in mice. Interacting genes such as those in mice determine most characters in any organism.

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Different kinds of modified dihybrid ratios point to different ways in which genes can interact with each other to determine phenotype.

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