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oxidation of tyrosine to DOPA- the first step in melanin synthesis. The dominant C allele is the wild type, expressed as functional tyrosinase enzyme. The recessive c allele is a loss of function mutation expressed as a nonfunctional protein. The homozygous cc genotype results in an inability to produce melanin and results in albino phenotype. Hence the cc genotype is epistatic to the other color-related genes, since a lack of pigment precludes any variation in pigmentation. A third allele possible for this locus, c', is expressed as a temperature sensitive version of tyrosinase. Cats homozygous cc, such as Siamese as Burmese breeds, have lightly pigmented fur on their bodies close to their core, but darker pigmented fur on the cooler tips of their snout, ears, and extremities. O (orange) gene. Two major classes of melanins are produced in most melanocytes: the black/brow eumelanin and the red/orange phaeomelanin. The dominant wild type O results in predominantly black eumelanin. The recessive o allele causes almost entirely phaeomelanin production. The O gene is sex-linked on the X chromosome. Hence homozygous 00 female cats and hemizygous OY male cats produce black fur, but homozygous oo female cats and hemizygous oY male cats produce orange fur. Because of X-inactivation, heterozygous Oo female cats demonstrate a pattern called tortoiseshell with mosaic of patches of melanocytes producing either black or orange fur. B (browning) gene. The B gene expresses the tyrosinase-related-protein-1 (TRP-1), another protein in the metabolic pathway for variation in the structure of eumelanin. The dominant wild type B results in the usual black eumelanin. The recessive b allele results in a dark brown ("chocolate") form and anothe recessive b' allele cause a light brown ("cinnamon") phenotype. D (dense pigment) gene. Sometimes called a diluter gene, D gene codes for melanophilin, a factor needed to transfer melanosomes from the melanocytes to the keratinocytes in the hair follicles. Recessit mutant forms result in reduced transfer and thus more muted colors. With the homozygous dd at this locus, black becomes gray, chocolate becomes lilac, cinnamon becomes fawn, and orange becomes cream colored. S (piebald-spotting) gene. During embryogenesis, the precursor melanocytes must migrate from their site of origin to the basement layer of the epidermis. The S gene is somehow related to this migration. The dominant Sallele results in patches of melanocyte-free epidermis and thus spots with white fur. This gene has incomplete dominance. Homozygous SS cats have large white patches such as
white blaze across the face or the tuxedo or bib pattern on the chest ("piebald"). Heterozygous Ss cats have fewer, smaller white patches. Wild type homozygous recessive ss cats lack any white spots. Mc (tabby) gene. In cats, when the embryonic melanocytes are migrating to the epidermis, they accumulate along the edges of the developing somites. This results in relatively darker horizontal stripes along the cat's body and face. With the dominant wild type Mc allele, the stripes are thin ("mackerel- striped") and often broken into rows of thin spots or bars. The recessive mc allele results in broader dark bands or whorls. T (unstriped) gene. Epistatic to the Mc gene, the dominant TA allele results in an unstriped coloration. The wild type recessive tallele allows the expression of the Mc phenotype. W (white-masking) gene. Another epistatic gene, the dominant Wallele triggers apoptosis in the embryonic melanocyte precursor cells before their migration. Hence the fur is almost all white despite the genotype of the other color-related genes. Since these cats can produce melanin, they are not considered true albinos. Some embryonic melanocytes often survive and may migrate to form a few isolated islands of colored fur amidst the white background. The color of the spots depends upon the C, O, B & D loci. Cats must be homozygous ww for the normal melanocyte migration and the uniform pigmentation phenotype to be expressed.
b. What was the phenotype of this presumed ancestral cat? 2. Describe the phenotype of the cats with these genotypes (If allele not specified, assume wild type.): a. CC ss OO BB dd McMc tt ww b. cc ss OY bb T^t C. Cc SS Oo BB T^T^ d. CC Ss oY bb dd tt 3. Describe the genotype of cats with the following phenotypes (Describe only the genes that are not homozygous for the wild type allele.): a. A grey cat with small white patches. b. A white cat with orange spots. c. A pale colored cat with brown tips to its tail, ears, paws, and snout. d. A cream-colored cat with darker cream color forming broad bands and whorls along its body.
Inheritance of a single gene: 4. A "chocolate" cat mates with a cat heterozygous at the gene for TRP-1: a. What is the probability of their kitten having brown coat color? b. What is the probability of their kitten having black coat color? c. What is the probability of their brown kitten being homozygous for this gene? d. What is the probability of their black kitten being homozygous for this gene? 5. A pure-bred uniform-color cat (homozygous for the T* gene) mates with a mackerel-striped cat: a. What is the probability of their kitten having uniform coloration? b. What is the probability of their kitten having mackerel-striped coloration? c. If two of these kittens grow up and mate, what is the probability of their second-generation (F₂) kitten having uniform coloration? d. What is the probability of their second-generation (F₂) kitten being mackerel-striped? e. What is the probability of their F₂ uniform-color kitten being homozygous for this gene? f. What is the probability of their F₂ mackerel-striped kitten being homozygous for this gene? 6. A uniformly colored black cat mates with a black cat having small white patches: a. What are the possible phenotypes of their kittens? b. For each of these phenotypes, what is their probability and genotype? 7. A tortoiseshell cat mates with an orange cat: a. What is the probability of their kitten having all-orange fur? b. What is the probability of their kitten having tortoiseshell coloration? c. What is the probability of their kitten having all-black fur? d. What is the probability that their black kitten will be male? e. What is the probability that their orange kitten will be male? f. What is the probability that their tortoiseshell kitten will be male? Inheritance of multiple genes: 8. A pure-bred grey cat mates with a chocolate cat: a. What are the possible phenotypes of their kittens? b. For each of these phenotypes, what is their probability and genotype? 9. An all-black cat gives birth to a mostly white kitten with black spots: a. If the only three male cats around were all-brown, all-black, and white with brown spots, which one sired this kitten? b. If this white kitten with black spots matures and mates with an all-brown cat, what are the possible phenotypes of their kittens? 10. A cat with broad pigmented bands and whorls mates with a cat heterozygous for the tabby gene. Their first kitten is albino. a. What is the probability that their next kitten will also be albino? b. What is the probability that their next kitten will be mackerel-striped?