Download Complex gene interactions in coat color

Complex gene interactions in coat color
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The analysis of coat color in mammals is a beautiful example of how
different genes cooperate in the determination of overall coat
appearance. 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 with 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 our discussion proceeds. At least five major genes
interact to determine the coat color of mice: the genes are A, B, C, D,
and S
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
The A gene
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This gene determines the distribution of pigment in the hair. The wildtype 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. The
lethal allele AY, discussed in an earlier
section, is another allele of this gene; it
makes the entire shaft yellow. Still another
allele at results in a "black-and-tan" effect,
a yellow belly with dark pigmentation
elsewhere.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
The B gene
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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.
In horses, the breeding of domestic lines 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 we have called
brown in mice is called chestnut in horses, and this phenotype also is
recessive to black.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
The C gene
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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
are of course albinos. Albinos are common in many mammalian species and have
also been reported among birds, snakes, and fish.
In most cases, the gene codes for the melanin-producing enzyme tyrosinase. In
rabbits an allele of this gene, the ch (Himalayan) allele, determines that pigment will
be deposited only at the body extremities. In mice the same allele also produces the
phenotype called Himalayan, and in cats the same allele produces the phenotype
called Siamese.
The allele ch can be considered a version of the c allele with heat-sensitive
expression. It is only at the colder body extremities that ch is functional and can
make pigment. In warm parts of the body it is expressed just like the albino allele c.
This allele shows clearly how the expression of an allele depends on the
environment.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
The D gene
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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 all are possible. A gene with such an effect is called a modifier gene.
In horses, the D allele shows
incomplete dominance. The
figure shows how dilution
affects the appearance of
chestnut and bay horses. Cases
of dilution in the coats of house
cats also are commonly seen.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
The S gene
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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.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
Summary of coat color genetics in mice
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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.
Interacting genes such as those in
mice determine most characters in
any organism.
Some of the pigment patterns in
mice.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
Modifier genes
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Modifier gene action can be based on many different molecular mechanisms. One
case involves regulatory genes that bind to the upstream region of the gene near the
promoter and affect the level of transcription. Positive regulators increase ("upregulate") transcription rates, and negative regulators decrease ("down-regulate")
transcription rates.
As an example, consider the regulation of a gene G. G is the normal allele coding
for active protein, whereas g is a null allele (caused by a base-pair substitution) that
codes for inactive protein. At an unlinked locus, R codes for a regulatory protein that
causes high levels of transcription at the G locus, whereas r yields protein that
allows only a basal level. If a dihybrid G/g ; R/r is selfed, a 9:3:4 ratio of protein
activity is produced, as follows:
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini
Penetrance and Expressivity
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Penetrance is defined as the percentage of individuals with a given genotype who
exhibit the phenotype associated with that genotype. For example, an organism may
have a particular genotype but may not express the corresponding phenotype
because of modifiers, epistatic genes, or suppressors in the rest of the genome or
because of a modifying effect of the environment. Alternatively, absence of a gene
function may intrinsically have very subtle effects that are difficult to measure in a
laboratory situation.
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Another term for describing the range of phenotypic
expression is called expressivity. Expressivity
measures the extent to which a given genotype is
expressed at the phenotypic level. Different degrees
of expression in different individuals may be due to
variation of the allelic constitution of the rest of the
genome or to environmental factors. This figure
illustrates the distinction between penetrance and
expressivity.
Genetica per Scienze Naturali
a.a. 04-05 prof S. Presciuttini