Genetics of the Domestic Cat a Lab Exercise

How Genetics of the Domestic Cat A Lab Exercise

______Roger E. Quackenbush

The fruit fly has long been the clas- sical laboratoryorganism for the study of genetics, even though many high schools find it difficultto study within the confines of the classroom setting and with the number of students involved. Using higher organisms to

study genetics is difficult since they Downloaded from http://online.ucpress.edu/abt/article-pdf/54/1/29/45557/4449392.pdf by guest on 02 October 2021 tend to have long life cycles, produce few offspring and are difficultto main- tain under normal laboratory condi- tions. An exception to this is the ubiq- uitous domestic house cat (Feliscatus). Because it is so common, is present in such large numbers and has many recognizablecharacteristics, the genet- ;~~~~~~~~ ics of the cat can be easily studied in Female cheetah in the MasaiMs the classroom. Many of its fur and color characteristicsfollow the simple rules of genetics which can be under- stood by 10th grade biology students. The domestic cat also exhibits many of the more complex genetic concepts such as epistasis and pleiotropy as studied by AP biology students and undergraduatecollege students. In trying to make the subject of genetics more personal and meaning- ful, I developed a simple lab on the Female cheetah in the Masai Mara of Kenya, summer of 1991. genetics of the domestic cat using pictures found in cat calendars and monthly cat magazines. Thus I have been able to accumulate a series of sive b allele producing a brown color. condition is caused by the presence of pictures that illustrate the major con- The gray/tan background, found in a light band of pigment ringing a black cepts of cat genetics. many wild animals such as rabbits and hair close to its tip. The dominant gray squirrels, is called agouti. This allele A produces the agouti condition. Common Characteristics Figure 1 shows a list of the more Figure 1. Common phenotypes and alleles of the domesticated cat. common phenotypes, with the appro- priate allele, for non-pedigree domes- WildTypes Mutants ticated cats. For a more complete list- ing, including descriptions, refer to Allele Phenotype Allele Phenotype Robinson (1977), or Wright and Wal- ters (1980). A --- Agouti a --- Non-agouti (black) A tabby with parallel black stripes B --- Blackpigment b --- Brown pigment against a gray/tan background is con- D --- Dense pigment d --- Dilute pigment sidered to be a wild type cat (Lloyd i --- Wild type pigmentation I --- Inhibits agouti (silver) 1986). The black color is produced by L --- Short hair 1 --- Long hair the dominant allele B, with the reces- m --- Long tail M --- Manx (stubby) tail o --- Non-orange (usually black) 0 --- Orange pd --- Normal number of toes Pd --- Polydactyly (extra toes) Roger E. Quackenbush teaches at s --- No white spots S --- White spotting (piebald) BethlehemCentral High School, 700 Del- T --- Mackerel striped tb Blotched striped aware Ave., Delmar,NY 12054. w --- Not all white W --- All white

CATGENETICS 29 The homozygous recessive condition, aa, is epistatic in that it masks or prevents the expression of the light colored band. A cat with the genotype aa would be pure black, although the presence of "ghost" bands may be seen which do not constitute a true striped condition (Wright & Walters 1980). The recessive allele, a, only acts on black or brown hair, thus orange cats (discussed below) always show alternating stripes of orange and yel- low regardless of the presence of A or a. The stripes of the tabby cat, either mackerel(parallel) or blotched (stripes of irregular width that tend to run together), are determined by the alleles T andtb respectively (Figures 2a,b).

Other examples of epistasis involve Downloaded from http://online.ucpress.edu/abt/article-pdf/54/1/29/45557/4449392.pdf by guest on 02 October 2021 the alleles W and 0 which mask the expression of any other allele for color that may be present and produce the colors white and orange (any reddish/ yellowish color) respectively. The W allele may also cause deafness, as well as eyes that are usually blue, orange or some odd color. This is an example of the type of inheritance called pleiotropy, where one gene can affect more than one characteristic. The 0 allele is found only on the X chromosome, and thus is sex-linked. The genotype 00 produces an orange female, while OY produces an orange male. The allele o causes normal pigmentation (usually black in non-pedigree cats), therefore the genotypes oo and oY produce a black female and male respectively. The genotype Oo produces the orange and black coloration found only in females called tor- toise shell. In a female, one of the X's randomly becomes inactive during early embryonic development. Thus a cell will end up with either the 0 or the o allele; upon further cell division patches of orange and black will be produced. This inheritance was worked out by British geneticist Mary d e Lyon and is called the Lyon or Mosaic Effect. If the female cat also has some Figure 2. Figure (a) shows parallel or mackerel stripes. Figure (b) illustrates white coloration, the resulting condi- irregularwidth or blotched stripes. Note that these stripes tend to meet and may tion is called calico. Since the male can produce whorls. Figure (c) shows a pure black cat with two small patches of inherit only a single 0 or o he must be white; the genotype would be aaSs. Figure (d) shows a normal right front paw either orange or black. with five toes. Note that the fifth toe is off to the side in the "thumb" position. In the wild type tabby cat the allele I Figure (e) shows a polydactylyright front paw with six toes. Note the presence of inhibits the agouti gray/tan hair color, not just an extra toe, but also an enlargementof the toe in the "thumb"position. but allows the black to appear. The agouti region thus becomes white, the so-called silver color in the tabby cat. The result is an exceptionallybeautiful front paws (Figure 2d) and four toes world. This condition is caused by the silver and black striped cat that is on their rearpaws. Polydactyly,or the dominant allele Pd, while the normal featured in many magazines and cal- presence of extra toes (Figure2e), ap- number of toes is recessive. The rear endars. The recessive allele, i, allows pears to be a mutant limited in its paws rarely have extra toes, and then wild-type pigmentation to be pro- distribution.Lloyd (1986)states that it only if the front paws exhibit polydac- duced. is a common mutant in the New En- tyly (Robinson 1977). Cats normallyhave five toes on their gland states, but rare elsewhere in the The alleles for white spotting (called

30 THEAMERICAN BIOLOGY TEACHER, VOLUME 54, NO. 1, JANUARY1992 Figure3. Studentlaboratory sheet and datasheet.

Biology Lab Name The Genetics of the Cat In this lab you will look at photographs of many different cats and determine the genotypes of the various characteristicsobserved. For each cat picture fill out a checklist for the characteristicsobserved. At the bottom of the checklist, fill in the genotypes that can be determined. For genotypes that cannot be determined, leave the spaces blank. For those characteristicsthat could be either homozygous dominant or hybrid, place a "2" for the second allele, for example B-. The "2"indicates that the allele could either be a dominant or a recessive.

Phenotype Cat 1 Cat 2 Cat 3 Cat 4 Cat 5 Agouti Black Brown Downloaded from http://online.ucpress.edu/abt/article-pdf/54/1/29/45557/4449392.pdf by guest on 02 October 2021 Dense Dilute Non-silver Silver Short hair Long hair Long tail Stubby tail Non-orange Orange Normal number of toes Polydactyly No white spots White spots (more than half) White spots (less than half) Mackerelstripe Blotched stripe Not all white All white l l l l _l

Genotypes:

Cat 1

Cat 2

Cat 3

Cat 4

Cat 5

piebald) demonstrate inheritance by much variation, in general, SS pro- two alleles, it is thought that perhaps incomplete dominance. White spotted duces cats with extensive spotting modifier genes or polygenic inheri- cats inherit the alleles independent of (more than half the body), Ss are cats tance may be involved (Robinson1977; any other alleles for color. Thus white mildly spotted (less than half the Wright & Walters 1980). spotting can be found in conjunction body) and cats with the genotype ss Two other mutants that may be en- with any other coloration.White spot- exhibit no white spotting. An other- countered are dilution and an absence ting is also independent of the pure wise black cat with just a small patch or extreme shortening of the tail called white condition caused by the allele of white hairs would be classed as Ss the Manx condition. The presence of W, and the two should not be con- (Figure 2c). Since not all of the spot- the recessive allele d causes the pig- fused with one another. While there is ting variationcan be explained by just ments black and orange to become

CATGENETICS 31 diluted, resulting in the hair colors mate, and the computerwill randomly dents lived on a farm that had barn gray and cream, respectively. The determine the offspring. The program cats, and after a litter of kittens was Manx (stubby) tail phenotype, caused uses graphics to show the characteris- born, she brought to class a descrip- by the dominant allele M, is lethal in tics of the parents along with each of tion of the mother and all the kittens. the homozygous dominant genotype. the possible offspring. The user can The class worked backwardsto deter- It is rarely encountered in the New easily go back and forth between mine the mother's recessive alleles World (Blumenberg 1986), and there- screen-printedphenotypes and the di- and what the fathermight have looked fore its true distributionin the United agrams. Although the programworks like. Unfortunately, the student could States and Canada has yet to be deter- adequately on a monochrome moni- not locate the actual father after its mined. tor, a color monitor adds much to the brief, but fruitful, visit to the barn. The reader is referred to Lloyd ease in interpretingthe diagrams. In- (1986) and Todd (1977) for further in- cluded is an introductionto the genet- formation on the genetics of the do- ics of the cat, as well as many sug- References mesticated cat, the origin and distribu- gested crosses. The programdoes not tion of the cat and some excellent print out the genotypes of the off- Anderson, M.M. & Jenkins, S.H. (1979). photographs and paintings of various spring, but I find that this forces stu- Gene frequenciesin the domestic cats of phenotypes. dents to think about the genotypes Reno, Nevada: Confirmationof a recent hypothesis. TheJournal of Heredity,70(4), and how to write them out. This pro- 267-269. Lab Procedure gram is complemented by the use of Blumenberg, B. (1986). Historical popula- actual magazine or calendar pictures, tion genetics of Felis catus in Humboldt Downloaded from http://online.ucpress.edu/abt/article-pdf/54/1/29/45557/4449392.pdf by guest on 02 October 2021 The students are given a procedure providing students a solid foundation County, California. Genetica,68(1), 81- sheet as shown in Figure3, along with in introductorygenetics. 86. the list of phenotypes and alleles as Many studies have been done on Chu, K. (1986).Mutant allele frequenciesin the distributionof the various mutant domestic cats of Taiwan. The Journalof presented in Figure 1. Each character- Heredity,77(4), 277-278. istic is discussed with the class and a alleles throughout the world: Reno, Nevada (Anderson & Jenkins 1979); Dreux, P. (1967). Gene frequencies in the couple of sample genotypes are done. cat population of Paris. The Journalof For example, a cat with parallel black Taiwan(Chu 1986);Paris (Dreux 1967); Heredity,58(2), 89-92. stripes against a gray/tanbackground, Cyprus (Robinson 1972); New York Kinnear,J. (1986).CATLAB (2nd ed.). Iowa five toes on each front paw and four City (Todd 1966);Chicago (Todd 1969); City, IA: Conduit. toes on each rear paw, has long hair and Iceland (Todd et al. 1975). As a Lloyd, A.T. (1986). Pussy cat, pussy cat, and a long tail, and a single white spot class project, students could do a sur- where have you been? NaturalHistory, on its chest, would have the genotype vey on the cats found in the school 95(7), 46-53. district, calculate the frequencies of Robinson, R. (1972).Mutant gene frequen- A-B-D-iillmmoopdpdSsT-ww(the - in- cies in cats of Cyprus. Theoreticaland dicates that the allele could either be the mutants named in this paper and compare the results with those found AppliedGenetics, 42(7), 293-296. dominant or recessive). Robinson, R. (1977). Geneticsfor cat breeders in the above mentioned papers. Not (2nd ed.). New York:Pergamon Press. only would the students get a taste of Todd, N. (1966). Gene frequencies in the Other Applications what real scientific research was like, cat population of New York City. The they might also find some new infor- Journalof Heredity,57(5), 185-187. An interesting alternative is to give mation that could be published. Todd, N. (1969). Cat gene frequencies in the students the genotype of a cat and My students have been enthusiastic Chicago and other populations of the have them match it to one of several about this activity and eager to learn United States. The Journalof Heredity, available pictures. practicalgenetics about an animal that 60(5), 273-277. The genetics of the cat as outlined in is so close to many of them. After Todd, N. (1977). Cats and commerce. this can be ScientificAmerican, 237(5), 100-107. paper applied to CATLAB doing the lab in class many of the Todd, N., Fagen, R. & Fagen, K. (1975). by Judith Kinnear (1986), an excellent students delight in either bringing in Gene frequenciesin Icelandiccats. Hered- program available for the Apple II pictures of their cat or describingtheir ity, 35(2), 172-183. computer. Students can select the phe- cat to the rest of the class, with the Wright,M. & Walters,S. (Eds.). (1980).The notypes of a male and a female to genotype worked out. One of my stu- bookof thecat. New York:Summit Books.

32 THEAMERICAN BIOLOGY TEACHER, VOLUME 54, NO. 1, JANUARY1992