An Introduction to the Genetics of the
Domestic Cat, Felis catus
Although there is not complete agreement on the origin of the domestic cat, Felis catus, it is believed that
domestication occurred in Egypt some 3500 years ago. The most likely wild African candidate for the ancestral
cat is the African wild cat (Felis libyca). However, the European wild cat (Felis silvestris) may also have
contributed to the genetic composition of the domestic cat by hybridizing with the African wild cat.
Each group of students will receive a manila envelope with four cat pictures one of which will be a tortoise shell
or a calico.
What is expected of you in this lab.
You will use the cat pictures provided to gain a greater understanding of basic genetical principles.
1.
2.
You are to determine the phenotype for each locus that you can see in each photograph.
You are to determine the genotype for each locus of the cat that you were able to assign a phenotype in
number 1.
a. If a dominant autosomal trait is being expressed in the picture, write the genotype as the capital
letter of the dominant allele followed by an underline (underscore).
3. Imagine you a cat breeder and must use your knowledge of the cat to produce broods with specific
phenotypes. For each cat, determine a monohybrid cross that would produced any specific trait. Do this
for at least three traits per cat. You must come up with a cross that would have produced the tortoise
shell or calico and remember that this will be a dihybrid cross of one autosomal (the pigment gene) locus
and a X sex-linked locus, Dominant Orange.
Please be a responsible cat breeder and make certain that none of the kittens produced in any of crosses will be
crippled or still born (dead at birth).
IMPORTANT CONCEPTS AND FACTS ABOUT CAT GENETICS
Phenotype vs. Genotype
A trait is some aspect of an organism that can be described or measured. The phenotype is the observed state
of the trait. Phenotype is a product of the interaction between genotype and environment. First, we will
consider the effects of genotype.
Cats have 19 pairs of chromosomes. One member of each pair comes from the Queen (mother) and the other
the Tom (father). Along the chromosomes are genes. Genes provide the information necessary to produce a cat.
All the genes in all the chromosomes comprise the genome, which is the complete cat blueprint. Each gene is
also called a locus (plural: loci), indicating that it has a physical location on the chromosome. Thus, each cat has
two alleles of each gene; one inherited from each parent. The genotype is the actual genetic makeup of the
individual at any given locus and is determined by the identities of those two alleles present in that locus.
Alleles are alternate forms of a gene (locus) that are different phenotypic expressions of that locus.
For example, the genotype of a longhaired cat at the longhaired locus, which controls fur length, is ll. This
genotype is homozygous (both alleles are the same).
Dominance
LONGHAIRED
It is possible to determine the longhaired genotype of a shorthaired cat simply by observation. This is because
the shorthaired allele, L, is dominant to the longhaired allele, l, which means that in a heterozygote (Ll, i.e.,
each allele is different) the effect of the shorthaired allele over-rides, or dominates the effect of the recessive
longhaired allele. Dominant alleles are symbolized by capital letters and recessive alleles with lower case
letters.
It is possible to infer the genotype of a shorthaired cat by observing the phenotypes of its offspring from a
mating with a longhaired cat, such a mating is called a test cross.
Question 1: How would you determine the genotype of a shorthaired cat?
Other genes with dominant alleles are the agouti gene, which controls color expression along the length of each
shaft of hair, and the eumelanin and dilute genes, which also influence coat color.
AGOUTI
Some cats have hairs in which there is more than one color distributed along the hair shaft. Banded hairs of this
type are termed agouti, and they produce a ticked, or agouti, coat. Agouti is the typical fur color found in many
wild animals such as mice squirrels and rabbits, and is assumed to be of importance in their ability to blend into
the background. Agouti is determined by the dominant agouti allele, A. In contrast, hairs on non-agouti cats
are unbanded, producing a solidly colored coat. Such a cat is homozygous for the non-agouti allele (aa) at the
agouti locus (Fig. 1).
Question 2: How would you determine the genotype of an agouti cat?
Figure 1.
More than two alleles
The colors in hair, skin, and eyes are the result of deposition of a pigment called melanin as microscopic
granules of melanin in the hair shafts. The shape, size, and arrangement of these granules affect coat color.
There are two different kinds of melanin: eumelanin and phaeomelanin. Eumelanin granules are thought to be
spherical in shape and absorb almost all light, giving black pigmentation. Phaeomelanin granules are thought to
be elongated like footballs, and reflect light in the red-orange-yellow range.
BLACK
The black gene has three alleles that control the density of eumelanin granules in the hair shaft. The black allele,
B, is dominant and produces a black (actually super-dark brown) coat. This black color is evident in all-black
cats, the black stripes of a tabby cat, and the dark ear-tips, feet and tails (the points) of seal point Siamese cats.
The dark-brown allele, b, reduces black to a dark brown, chocolate color. This allele is less common and
occurs almost exclusively in pedigreed cats such as Siamese and Burmese. It is believed that the b allele
migrated into Europe from Asia via importation of Siamese cats during the 1800s. The light-brown allele, bl,
further reduces melanin density to produce a medium brown coat, called cinnamon. It is not too likely that you
will find cats homozygous for these recessive alleles in Seattle’s general cat population. The dominance
hierarchy of these alleles is B > b > bl (Figure 2).
DILUTE
The dilute locus has two alleles that affect the distribution of pigment granules in the fur. The dominant dense
allele, D, produces dense color (full expression of the alleles present at the pigment locus, no clumping of
pigment granules), whereas the recessive dilute allele (d) causes clumping of pigment granules in the hair shaft,
leaving large areas between the clumps containing no granules. These open areas cause color dilution (Figure
2).
Figure 2. Effect of black (eumelanin), orange (phaeomelanin), and dilute genes on cat coat colors.
TAIL LENGTH (MANX CATS)
The length of a cat’s tail is controlled by a locus named for the dominant mutant form of the gene: Manx. Cats
heterozygotes for Manx, Mm, have a short tail that is the result of a shortened spine. Cats expressing the Manx
allele also have a shortened life span and you will never see a cat that is homozygous, MM. The MM genotype
is lethal and the kittens die in utero and are still born. Cats that are homozygous for the recessive wild type
allele, mm, have normal (long) length tails.
Question 2: What cross would a reputable cat breeder perform to be assured of Manx kittens in every
litter without any still births? Is it possible to produce a pure breeding strain of Manx cats?
EPISTASIS: Gene-Gene Interaction
ORANGE
The orange gene has two alleles: non- orange and orange. The non-orange allele, o, is recessive and allows
full expression of the black (pigment) locus. The dominant orange allele, O, however, influences expression of
the black and agouti loci because it substitutes the production of phaeomelanin for eumelanin. It masks the
effect of the black gene by converting a black or brown coat to orange. The ability of one gene to mask the
effect of another gene is called epistasis. Further, all orange cats are tabbies because the orange allele is
epistatic to the non-agouti (solid coat) phenotype normally produced by aa at the agouti locus. This masking
occurs because the orange band of phaeomelanin granules in the hair shaft is not visible against the yellowish
background of the hair without melanin granules.
Sex-linked Traits
The other interesting characteristic of the orange gene is that it is carried on the X chromosome, which makes it
sex-linked. In male cats, this locus can normally produce only two phenotypes, black or orange, whereas in
females it can produce three phenotypes: black, orange, and tortoiseshell. This is because males are normally
XY (heterogametic), and therefore have only one X-chromosome. Thus, if a male carries the orange allele at
all, he will be orange (XOY). Females are XX, meaning they have two X-chromosomes (homogametic). If both
chromosomes carry the orange allele, then the cat will be orange. However, if she is heterozygous (XOXo), her
coat will be a patchwork of orange and black patches, called tortoiseshell. This pattern reveals an interesting
phenomenon. To ensure that the amount of gene product in female cells is equal to that found in male cells,
early in female embryonic development, one X-chromosome is inactivated in each cell. Thus, in a female that is
heterozygous at the orange locus, some cells produce phaeomelanin (the active X-chromosome contains the XO
allele) and others do not (the active X-chromosome contains the Xo allele). Which X-chromosome is inactivated
is an entirely random decision. Further, this decision is passed on to all cells in any cell line, making the female
a mosaic of cell patches with one or the other X-chromosomes actively producing its gene products.
Question 3: Develop a table describing the possible genotypes and their phenotypes that could result from
a cross between a tortoiseshell and a black cat. Assume both cats are homozygous BB.
Complex Traits
Complex traits are controlled by more than one gene. Of course, cat color is controlled by multiple genes
(polygenic inheritance). However, even within this complex trait, categories can be distinguished which allow
us to ascertain genotype at several loci. Some component traits are themselves complex, probably because they
are determined in part by a locus with major effect, but also in part by loci with small effects. These loci with
small effects are often called minor loci or modifier genes.
TABBY
The tabby gene causes banded (ticked) hairs to alternate with stripes, blotches, or spots of solidly colored hairs,
creating the striping pattern in cat coats. There are two common striping patterns (Figure 3): mackerel (parallel
stripes) or classic (characterized by thick stripes or whorls that create a blotched or bulls-eye pattern). The
parallel stripe is produced by the dominant tabby allele, T. It is probably the ancestral striping pattern, which is
seen in the African wild cat (Felis libyca) and the European wild cat (Felis silvestris). The recessive blotched
allele, tb, produces the classic pattern, and probably arose much later by mutation. A third allele in the series,
Abyssinian (Ta) produces the Abyssinian phenotype that has faint striping only on the face or tail and has a dark
stripe down the center of the back. The hierarchy of dominance for the three alleles is Ta > T > tb. However, Ta
allele shows some degree of incomplete dominance since either TaT or Tatb heterozygotes may show faint
striping on the legs and tail.
Question 4: What effect might a striped (vs. solid) coat have on the fitness of a wild cat?
Figure 3. Striping patterns determined by the tabby gene.
All cats have one of the above tabby patterns - even solid black cats. However, because the agouti gene is
epistatic to the tabby gene, striping is not phenotypically expressed in non-agouti cats. This is because the
recessive aa agouti genotype blocks the production of banded hairs, thereby masking the expression of the
tabby phenotype. It is nevertheless sometimes possible to see “ghost” striping on black cats, especially kittens,
when viewed in the proper light. It is also important to note that because the XO allele at the orange locus is
epistatic to the agouti gene, tabby striping is not masked on solid orange cats, or on orange patches on
tortoiseshell cats. That is, XO is epistatic to the epistatis of aa on the tabby gene.
The genetic mechanism of expression of the spotted tabby is not completely understood. There are probably
two different mechanisms. The first appears to be a modification of the mackerel tabby, in which the narrow
bars are broken up into small dark patches. This effect is thought to be the result of several modifier genes
acting on the major tabby gene. There is another rarer spotted phenotype called the Ocicat (because of its
resemblance to the wild ocelot) which has larger, more distinct round spots. This phenotype may be due to a
separate major gene. However, because it is so rare, for the purposes of this project we will consider any
spotted tabby to have the mackerel tabby phenotype, with modifications.
PLEIOTROPY
When a gene affects more than one trait, it is termed pleiotropic. Most coat color genes have pleiotropic
effects on eye color. However, two genes also have an interesting pleiotropic effect on hearing ability.
DOMINANT WHITE
The dominant white allele, W, overrides all other genes for pigmentation, and produces a white coat and blue
eyes. Thus, the dominant white allele is epistatic to all other coat color genes. Of course, the other genes for
color and pattern are present, but their effects are completely hidden because the dominant white mutation
blocks production of specialized melanin-producing cells called melanocytes.
Interestingly, the cochlea of the ear contains a band of melanocytes that regulate ion balance. Normally, hearing
occurs via electrical signals stimulated by vibration of the hair cells in the cochlea. Transmission of these
signals to the brain requires constant regulation of ion balance. When ion balance is not maintained, signal
transmission to the brain degenerates within a few days after birth, producing irreversible deafness. Thus, the
dominant white locus exhibits plieotropy, affecting both coat color and hearing.
PIEBALD SPOTTING
White spotted, or piebald, cats are very common. Spotting may occur with any coat color, and is mainly due to
the effect of one gene with two co-dominant alleles. The spotted allele, S, is phenotypically expressed as white
spots, while the s allele is expressed as no white spots or even any white hairs on the cat. Hence, the
homozygote, ss, has no white spots. The heterozygote, Ss, has restricted areas of white spotting; usually the
feet, nose, chest, and belly. Finally, the SS homozygote has white regions covering more than half the body. In
the latter case, people usually consider the dark patches to be spots. However, in reality, it is the larger white
area that is the spot, and in fact, a “spotted” (SS) cat may even be completely white!
The degree of spotting varies tremendously, but spotting patterns usually follow a regular progression. Cats
with the least spotting have small spots on the breast and belly. Increased spotting seems to progress to cover
the entire belly, the neck, chin and front feet. Finally, cats with the most spotting have spots up the sides, over
the back and onto the head. The tail seems to be the last area to have white spots. Since cats vary continuously
in the extent of spotting, and the pattern of spotting is not completely random, it is unlikely that only one gene
determines the degree of spotting. There is evidence of at least one other gene that has a weak spotting effect.
It can cause a very small white spot on the throat, breast, or on the belly near the hind legs. In fact, several
genes probably modify the action of the major spotting gene to produce the continuum of spotting patterns seen
in cat populations. Since the action of these weak modifier genes is not well understood, we will score only the
major spotting gene in our cat survey.
The spotted allele, S, hampers the migration of melanocytes during embryonic development. White spots are
areas lacking melanocytes. Thus, within these patches a spotted cat will exhibit the same pleiotropy as caused
by the dominant-white gene. For example, if an eye is within a spot, it will be blue. Thus, spotted cats may be
blue-eyed or odd-eyed. Further, if the spot encompasses an ear, the cat will be deaf in that ear. Since a spot
often covers the eye when it covers the ear, an odd-eyed cat will frequently be deaf on the blue-eyed side.
The spotting gene has in interesting effect on the patterning in tortoiseshell (XOXo) cats. With the ss genotype,
there is no white spotting and the orange and black are intermingled, usually without large patches of either
orange or black. With either Ss or SS genotypes, there are white spots and the orange and black occur as
distinct patches. These cats are called calico cats (or, sometimes, tortoiseshell and white). This pattern seems
to be due to the effect of the spotting allele on embryonic melanocyte migration.
Special note about white cats
White cats are produced in three distinct ways. It can be dominant white (W-), albino (cc), or spotted (S-) with
one big white spot all over its body. It is therefore important to observe white cats carefully and take notes
about their eye-color.
The dominant white allele, W, and the homozygous spotted genotype, SS, can both produce a translucent allwhite coat with either orange or medium blue eyes. However, the spotted white cat often has a tuft of color
somewhere on its body. If the cat has even one colored hair on its body, it is spotted rather than dominant white
The albino gene controls the amount of pigment produced in the melanocytes. A cat homozygous for albino,
cc, at the albino locus is a true albino, in that its melanocytes cannot produce any pigment. It has a translucent
white coat and pink eyes. A caca genotype at the albino locus produces a white cat with pale blue (almost grey)
eyes. Both these alleles are very rare. Deafness in white cats is associated with white spotting (S), and with
dominant white (W), but not with albino white (cc or caca).
Question 5: If you mated two white cats, would you be confident of always obtaining white progeny?
Explain.
_____________________________________________________________________________
Table 2. Effects of agouti and tabby on the black, dilute and orange gene effects. Note that here, an orange cat
is described as “red”, and an Abysinnian is called a “ticked tabby”. The agouti and tabby genes also influence
expression of the albino gene, but their effects have not been tabulated here.
CAT GENETICS
TRAITS/ALLELES/PHENOTYPES
WILD TYPE
ALLELE
B
A
T
MUTANT
ALLELE
Pigment locus. Completely
dominant over both recessive
alleles. Black coat.
b1
Agouti. Striped hairs
a
Striped (Mackeral) tabby pattern,
stripes similar to those of a tiger.
Recessive to TA (Abyssinian/ticked
pattern.
Completely dominant over the tb
(Blotched Pattern)
b
Ta(B)
tb
D
Full expression of pigments in
hairs.
d
w
Full expression of pigment gene.
W
s
Full expression of pigment genes in
homozygote.
S
Xo
Full expression of pigment genes in
homozygous females and
hemizygous males
XO
m
Normal length(long) tail.
M
fd
Pricked (normal) ears.
Fd
pd
Normal number of toes on paws.
Pd
L
Short hair. Completely dominant.
l
Dark brown (chocolate) coat. Completely
dominant over b1 allele.
Light brown (cinnamon). Recessive to both of the
other alleles.
Non-agouti. No striping, solid pigmentation of
hairs.
Abyssinian (ticked) tabby pattern. Completely
dominant over both other tabby alleles.
Classic (blotched) Tabby.
Dilute. Intermediate expression of pigments in
hairs of coat. Epistatic to pigment gene.
Black becomes blue (gray)
Chocolate becomes lilac
Cinnamon becomes fawn
Red (orange) becomes cream
Dominant white. Cats typically have blue eyes
and are prone to be deaf.
Pie-bald white. White spotting in heterozygote
and homozygote. Amount (degree) of spotting
variable.
Sex-linked red (orange). Masks all other colors:
expressed as red in males and homozygous
females and tortoise shell (black and red patches)
in heterozygous females.
Manx (short) tail. Results from shortening of the
spine. In many cases cats have short life span.
Homozygote is lethal and still borne (dies in
utero). Completely dominant.
Fold ears. Cartilage in ears folds forward over
pate of head. Crippling in homozygotes.
Completely dominant.
Polydactyly. More than the normal numbers of
toes on fore or hind paws or both. Completely
dominant.
Long hair.