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  • Dr Norton Article: Chromosomes, Locus, Genes and Alleles

    Dr. Norton's Articles - Part 3
    Reprinted with permission from:
    Dr. Joanne Norton
    Freshwater And Marine Aquarium magazine
    Angelfish Genetics
    Photos and Text by Dr. Joanne Norton
    FAMA: July 1982, Vol. 5, #7


    Part Three
    In Parts 1 and 2, I discussed the inheritance of mutant genes that affect the color of aquarium-bred angelfish. In this article I shall add information on the location of these genes. This additional knowledge is essential in order to know which gene combinations are possible and also to enable you to predict correctly the outcome of crosses involving more than one mutant gene.

    The angelfish's chromosomes (containing the genetic material) are in pairs, one chromosome of each pair having come from each parent. Each gene occurs at a specific location, called locus (plural, loci), on the chromosome. Variations of a gene are called alleles. Only one allele occurs at a locus; another allele of that gene can occur at the same locus on another chromosome.

    Tests for Allelism
    A marble female (having two doses of marble), mated to a black lace male (one dose of dark), produced 23 marble and 22 black (one dose of marble and one dose of dark). This type of black angelfish, called marble lace (Norton, 1971), has a marbled pattern on the body that can be seen by shining a flashlight on the fish. A true black (two doses of dark) has faint vertical body stripes that can be seen by using a flashlight.


    Black Angel. (1 dose of dark and 1 dose of marble.)
    When a wild-type (silver) female was mated to a marble lace male, the offspring were 48 black lace and 46 marble. Since there were no wild-type offspring, I concluded that the genes for dark (D) and marble (Dm) act as alleles. In other words, these genes are at the same locus.

    A wild-type female and zebra lace male (one dose each of dark and zebra) produced 9 wild-type, 6 black lace, 4 zebra, 5 zebra lace. It is concluded that the genes for dark (D) and zebra (Sze) are not alleles since the above cross produced some wild-type.


    Zebra Lace. (1 dose of Zebra and 1 dose of dark.)
    A new gold female and true black male (D/D) produced 100% black, which were not as deeply pigmented as the true black. This type of black (one dose of new gold and one dose of dark) has a slight bronze cast on the body and evident vertical bars. Crossing two of these blacks resulted in an F2 consisting of 62 new gold and 200 black, no black lace or wild-type. Also a new gold female and black male (whose parents were new gold and true black) gave 214 black and 207 new gold offspring. Thus the gene for dark (D) and new gold (dng) behave as alleles.


    Black Angel. (1 dose of dark and 1 dose of new gold.)
    Black lace were obtained by mating true black and Hong Kong gold. These black lace, which carried the recessive gene for Hong Kong gold (see Part 2), produced an F2 of 29 wild-type, 49 black lace, 12 black, 4 Hong Kong gold. It is concluded that the genes for dark (D) and Hong Kong gold (hg) are not alleles since some wild-type offspring were produced.

    Numerous blushing marble were obtained from a blushing crossed with a fish that had one dose each of stripeless and marble (having come from a blushing x marble cross). Since a blushing marble would have to have one gene for marble and two genes for stripeless, it is deduced that the genes for stripeless (S) and marble (Dm) are not alleles.

    A smokey female, crossed with a male having one dose of marble, produced 79 wild-type, 157 marble, 86 smokey. Some of the marbles were suspected of having one dose of smokey because they exhibited one or more of these smokey features: (1) black mouth, (2) black-tipped dorsal fin, (3) gray in the white streaks in the outer part of the caudal fin.

    Two such males were tested for smokey by mating them to wild-type females. This produced: male #1: 126 wild-type, 269 marble, 123 smokey; male #2: 27 wild-type, 47 marble, 21 smokey. Not only was smokey present in the tested marble males but the genes for marble (Dm) and smokey (Sm) did not act as alleles since the crosses produced some wild-type offspring.

    A male having one dose each of marble, zebra and smokey was mated to a wild-type female, producing 37 wild-type, 136 marble (including marble with zebra and/or smokey), 34 zebra, and 59 smokey (including zebra-smokey). Zebra-smokey looks like zebra until the stripes disappear as the smokey pattern develops. Since there were wild-type offspring, it is concluded that the genes for marble (Dm) and zebra (Sze) are not alleles.

    A marble female was mated to a new gold male, producing 241 wild-type and 217 marble. One of the marble F1 males, mated to a new gold female, produced 105 marble and 111 new gold. Since no wild-type offspring were produced, it is concluded that the genes for marble (Dm) and new gold (dng) act as alleles.


    1 dose of marble and 1 dose of new gold.
    Since (1) the genes for dark and marble behave as alleles, and (2) the genes for dark and Hong Kong gold do not act as alleles, it follows that the genes for marble (Dm) and Hong Kong gold (hg) are not alleles.

    A smokey male was tested and found to have one dose of stripeless since, when mated to a wild-type female, he produced wild-type, smokey and stripeless offspring. Since there were some wild-type offspring, it is concluded that the genes for smokey (Sm) and stripeless (S) are not alleles.

    A female with one dose of zebra, mated to a blushing male, produced all stripeless offspring, some of which had more black blotches on the body than I had ever seen on fish with one dose of stripeless but without zebra. When the ones with increased splotching matured, their dorsal fins had an number of light-colored horizontal bars (compared with wild-type), a zebra characteristic. One of these that was suspected of having one dose each of zebra and stripeless was tested by mating him to a wild-type female. From the results, 188 stripeless and 220 zebra, it is concluded that the tested fish did have one dose each of stripeless (S) and zebra (Sze) and that these genes behave as alleles.

    A blushing female, mated to a Hong Kong gold male, produced 100% stripeless, non-gold offspring. The F2 included some each of blushing, Hong Kong gold blushing, stripeless, Hong Kong gold and wild-type. It is concluded that the genes for stripeless (S) and Hong Kong gold (hg) are not alleles, since wild-type appeared in the F2 and since Hong Kong gold blushing would have to have two doses each of Hong Kong gold and stripeless.

    Since (1) stripeless and zebra act as alleles, then it follows that the genes for smokey (Sm) and zebra (Sze) are not alleles.

    A smokey female x new gold male produced smokey and wild-type offspring. One of the F1 smokey males was mated to a new gold female, giving 91 wild-type, 80 smokey, and 156 new gold. This fits the expected ratio (1 wild-type, 1 smokey: 2 new gold), if the smokey pattern is not expressed in a fish having two doses of new gold. Therefore, half of the 156 new golds were assumed to carry the gene for smokey. Further evidence that new gold, in double dose, blocks development of the smokey pattern was obtained by mating smokey parents, both of which came from a smokey female and new gold male. These produced 71 smokey, 33 chocolate (double dose smokey, see Part 1), 42 new gold, 32 wild-type. These numbers fit the expected ratio (6 smokey: 3 chocolate: 4 new gold: 3 wild-type), if the smokey pattern is not expressed in the presence of double-dose new gold. Since each of the spawns (from smokey male carrying new gold x new gold female, and from smokey parents each having one dose of new gold) produced some wild-type offspring, it is concluded that the genes for smokey (Sm) and new gold (dng) are not alleles.

    Since (1) zebra and stripeless behave as alleles and (2) stripeless and Hong Kong gold are not alleles, it is deduced that the genes for zebra (Sze) and Hong Kong gold (hg) are not alleles.



    Hong Kong gold - hg/hg
    smokey - Sm/sm+
    chocolate - Sm/Sm
    stripeless (ghost) - S/s+
    blushing - S/S
    zebra - Sze/Sze or Sze/s+
    zebra, one dose stripeless - Sze/S
    true black - D/D
    black lace - D/d+
    marble lace (black) - D/Dm
    new gold-dark (black) - D/dng
    light marble - Dm/d+
    dark marble - Dm/Dm
    marble with jet black pattern - Dm/Dng
    new gold - dng/dng
    zebra lace - Sze/- D/d+

    Literature Cited
    Norton, Joanne, Angelfish-breeding and genetics. The Aquarium 6 (No. 10): 34-41. 1971