Download Lab # 6

Laboratory 5.
Cat Genetics*
Genetics is both more quantitative and more analytical than other areas of biology. The
mathematical descriptions of Mendelian inheritance and population genetics are sometimes
daunting and students frequently have powerful misconceptions about population genetics. Good
examples to illustrate principles of incomplete dominance, epistasis and sex-linkage abound, yet
these examples are often from organisms outside the student’s experience or interest (fruit flies,
snapdragons, peas, roosters, etc.) Common misconceptions by students about population genetics
are that dominant alleles will take over a population, that the most abundant phenotype in a
population represents the dominant trait, and that deleterious alleles will be eliminated quickly.
In this exercise we will try to find examples of Mendelian principles by observing the phenotypes
and determining the genotypes of the domestic cat, an organism with which most of us are quite
familiar
You will be asked to determine the genotype at 7 unlinked loci of at least five cats. You will find
a form on which to fill out this information. To fill out the form you will have to grasp such
concepts as dominance, incomplete dominance, temperature-sensitive mutations, epistatic
interactions, sex linkage and variable expressivity. In the form you will write down information
on the sex of the cat, length of hair, color, agouti (stripedness), white spotting and temperature
sensitivity. The genotypes of some traits can be determined by directly looking at the cat (i.e. if it
is a gray cat is must be homozygous recessive) while with dominant traits you can determine that
the cat carries one dominant gene but you cannot determine the trait carried on the other gene
without knowing the parentage and pedigree. In this situation you may leave the second gene
blank or put the possible alleles. In other words, if you want to score a black cat (black being
dominant), you know that one of the alleles is B (black) and the other could be B or b (brown). In
either case, the cat would be black phenotypically.
You should be able to find 5 cats and score their genotypes. You can go to the humane society,
pet shelters (PetSmart has a small shelter) or you can score your own cat(s) or those of friends or
relatives. While you are doing this, think of the variations in color, coat length, coat pattern and
body shape which occur within domestic cats. Some of this variation is “man-made” in that
breeders have bred Siamese cats to be extremely thin with wedge-shaped heads and Persians to be
rounder (cobbier) with large heads and small almost non-existent noses. If you look at pictures of
Siamese cats or Persian cats from 100 years ago their shape is quite similar with none of the
extreme body types that we see in cats exhibited in cat shows today.
The following pages contain information on specific genes associated with coat color and pattern
in the domestic cat. These include traits such as long hair, tabby pattern and Siamese coloring.
There are many other traits which we will not explore, such as eye color, body shape, polydactyly
(extra toes). Explore the genetic variation observable in our feline friends.
* Adapted from: Christensen, Alan C. , Genetics Education “Cats as an Aid to Teaching Genetics” , Genetics 155:9991004,2000.
Color genes and Diluting genes:
The four major colors found in cats are the following: Black, brown (chocolate), blue/gray and
lilac (light pink-gray). Orange is a special case and will be discussed later. The two color genes
are black and brown with black being dominant to brown. Black is signified as B, brown as b. So
a homozygous black cat would be BB, a heterozygous black cat would be Bb and a homozygous
brown cat as bb. Thus, the mating of a homozygous black cat with a homozygous brown cat
would result in all kittens being phenotypically black, but the genotype would be Bb. (See Fig1)
In the two examples above, black and brown, the pigment granules of the hair are densely packed.
There is another gene that determines the density of the pigment granules. This is called the
diluting gene. The dominant trait is for the pigment granules to be densely packed. This is
denoted by D. The mutant gene, which causes pigments to be less densely packed (dilute) is
recessive and is denoted d. Thus, for diluted pigment granules to be expressed, both genes must be
dd. If a cat whose genotype is BB or Bb (indicating black) has the gene denoting diluted pigment
color (dd), then this cat would appear Blue/Gray. The black pigment granules have been “diluted”
so that, instead of appearing black, the cat appears gray. If a cat has the gene bb(indicating brown)
but also has the gene for diluted pigment granules (dd) then this cat would appear, not brown, but
a pink/gray (lilac). Since the gene for diluted pigment granules is recessive, then both genes must
express this trait. If a black cat (BB or Bb) was heterozygous for the diluting gene (Dd), then this
cat would be phenotypically black. Thus, the four major colors are denoted by the color genes,
black and brown and the diluting genes – which convert black to blue/gray and brown to lilac.
Examples of these four-color types are on the website and in pictures presented in the lab. Draw a
Punnet square for a cross between a cat heterozygous for black and heterozygous for the diluting
gene and another cat heterozygous for black and heterozygous for the diluting gene.
Sex-Linked Orange:
The color genes and diluting genes mentioned above occur on autosomes (non-sex chromosomes).
The gene for orange color in a cat is carried on the X chromosome. This gene is denoted O. The
phenotypes produced by the O and o genes are as follows: OO (ginger or orange female); Oo
(tortoiseshell female); oo (non-orange female- color determined by color genes mentioned above);
oY (normal colored male-color determined by the four color types mentioned above); OY (orange
male). Males are either orange or normal, almost never tortoiseshell, because the tortoiseshell
color is the heterozygote and requires two X chromosomes. The heterozygote Oo appears as
tortoiseshell and are “mosaics” of orange and non-orange fur due to a phenomenon called Xinactivation- where one of the X chromosomes is inactivated, allowing the color-coded on the
DNA of the other chromosome to appear. Figure 2 shows crosses between tortoiseshell females
and either an orange or a normal colored male. Interestingly, if a dilution gene (dd) is present,
then the orange appears as a cream color and a tortoiseshell (normally black and orange) would
appear blue and cream colored.
What would the genotype look like for a blue cream tortoiseshell?
Dominant White:
An all white cat with non-pink eyes (i.e. it’s not an albino) is denoted as W_ . Dominant white is
an example of epistasis, as it “overpowers” any of the other color-related genes and they are not
expressed in the presence of the dominant W. Dominant white kittens often have one or more
small pigmented spots on the forehead that usually fade by adulthood.
Temperature-Sensitive Genes:
You have probably seen Siamese cats, which have light-colored bodies and darker colored areas
(called points) on the face, ears, legs and tail. The color difference is due to a temperature
sensitive gene (cs) which causes the pigment to be produced only below a certain temperature.
Thus, the pigment is produced in the “colder” parts of the body- the face, ears, legs and tail. The
Siamese gene is recessive. The “wild type” gene being C and the Siamese gene cs. Thus a cat
who has the genotype Ccs would derive its color from the other color genes mentioned above.
There would be no temperature sensitivity in the formation of pigment granules. What would
happen to a Siamese cat who liked to stay outdoors even during the winter?
Tabby and Agouti:
Any discussion of color in cats should include the tabby. Everyone is familiar with the tabby cat!
In fact, tabby is a complex color. It is made up of two components, the agouti or ticked fur (the
hair is not one solid color but varies) and a background solid color. The agouti gene is dominant,
so a solid black cat would have two recessive (aa) genes, while a cat who was AA or Aa would
express the agouti gene. Draw a Punnet square for a cross between two heterozygous tabbies
(Aa X Aa).
Long hair:
We often see beautiful cats with long hair. Some of these long-haired breeds include the Persian
or the Maine Coon Cat, although long hair can be observed in many non-breed cats as well. Long
hair is recessive, and is indicated by ll. Short hair is dominant and is denoted by Ll or LL.
Piebald Spotting:
Piebald spotting is a good example of incomplete dominance and variable expressivity. Simplified, ss cats
have no white fur, Ss cats generally have < 50% white fur, and SS cats generally have > 50% white fur.
(Cats who have 100% white fur are scored as dominant white ). The S allele is incompletely dominant, but
variably expressed, so there is a more-or-less continuous gradient of white pigmentation in populations.
When you grade cats for your study using a 50% guideline works well.
Draw a genotype for a long-haired brown tabby with < 50% white spotting.
Comments on location (specify if the cat was not seen in Lincoln). or comments on
Genotypes not listed in the table (i.e. Manx, silver, the various purebreds, fancy breeds,
Cheshire, etc.)