Dr. Joanne Norton
 Freshwater And Marine Aquarium magazine

Understanding inheritance helps the angelfish breeder explain seemingly surprising results. Twenty years ago there were only two types of marble angelfish, lightly-marbled and heavily-marbled. Later, after the appearance of gold and gold marble angelfish, additional types of marbles emerged. Understanding the inheritance of these marble types can enable the angelfish breeder to explain seemingly surprising results from certain crosses. For example, a cross of "silver marble" x gold produces no offspring like either parent. Instead, you get gold marbles and silvers. Another advantage of understanding the genetics of marble angelfish is that you can predict and control the types of angelfish that you produce.

Marble angelfish were introduced by Ash (1969). The marble pattern is due to a dominant gene that produces a more extensive black-pigmented pattern in a homozygous fish, one having two doses of the gene, than in a heterozygous fish, having one dose of the gene for marble. (Norton, 1971, 1982a) I refer to these angelfish as "original marbles." The genes for marble and gold behave as alleles (Norton, 1982a, b), genes that occur at the same location on a chromosome. A chromosome can have one of a set of alleles, not more.

The other chromosome of that pair also can have one of the same set of alleles. One of these chromosomes came from one parent and the other chromosome of that pair came from the other parent.

A marble pattern that is more intensely pigmented than the pattern of heterozygous original marble appears in a fish that results from a cross of an original marble with a gold. This type of marble angelfish has the gene for marble on one chromosome and the gene for gold on the other chromosome of that pair (Norton, 1982b). The chromosome carrying marble came from the marble parent and the chromosome carrying gold came from the gold parent.

Gold marble angelfish appeared on the market later, after original marbles and the type just mentioned that came from a gold x marble cross. Gold marbles have black markings, in contrast to the mixed black and gray markings of heterozygous original marbles. Gold marbles that are heterozygous for marble have less extensive black markings than in gold marbles that are homozygous for marble (Norton 1988).

Fig. 1: Marble type No. 1. Homozygous original marble. It has marble on both chromosomes.

Fig. 2: Marble type No. 2. Heterozygous original marble, one chromosome: marble, other chromosome: wild-type.

Fig. 3: Marble type No. 3. This marble (from original marble x gold) has marble on one chromosome and gold on the other.

Fig. 4: Marble type No. 4. Gold marble that is homozygous for marble. Both chromosomes have gold marble.

Fig. 5: Marble type No. 5. Gold marble that is heterozygous for marble. One chromosome has gold; the other chromosome has gold marble.

Fig. 6: Marble type No. 6. Silver marbles, about nickel body size. One chromosome: wild-type, other chromosome: gold marble.

Fig. 7: Marble type No. 7. This fish came from a cross of the homozygous original marble female (Fig. 1) x a gold marble male that was homozygous for marble. It has marble on one chromosome, gold marble on the other chromosome.

Fig. 8: Adult silver marble undisturbed.

Fig. 9 Adult silver marble, the same fish as in
Fig. 8, when disturbed, causing the stripes to fade.

Whether a gold marble angelfish has a new gene for marble or whether it has the original marble gene plus, closely linked to it, the gene for gold, is not known. In either situation, the gold marble's gene for marble is on a chromosome of the same pair that is the location of the original marble gene and the gene for gold . Thus special marble angelfish (obtained from a cross of gold marble x silver), when crossed with gold, produced gold marbles and silvers (Norton, 1988).

Because I do not know whether a gold marble angelfish has a new gene for marble or closely-linked original marble plus gold (resulting from a crossover between marble and gold if they are close but not alleles), I shall us the symbol GM to denote the marble-carrying chromosome that is present in a gold marble angelfish.

There are four kinds of chromosomes that can occur in marble and gold marble angelfish, although an individual fish has only two of these chromosomes, one from each parent. I shall label these four chromosomes as follows:

• W: wild-type, having no gene for marble or gold

• M: having the original marble gene

• G: having the gene for gold

• GM: the chromosome that is present in a gold marble fish. Making all the possible combinations of these chromosomes results in seven genetically different marble angelfish:

• Type 1. M/M. Homozygous original marble (Fig. 1)

• Type 2. M/W. Heterozygous original marble (Fig. 2)

• Type 3. M/G. Intensely-pigmented marble (Fig. 3) from a cross of original marble x gold

• Type 4. GM/GM. Gold marble, homozygous for marble (Fig. 4)

• Type 5. GM/G. Gold marble, heterozygous for marble (Fig. 5)

• Type 6. GM/W. Silver marble (Fig. 6), obtained from a cross of gold marble x silver (wild-type)

• Type 7. M/GM. Deeply-pigmented marble (Fig. 7), obtained from a cross of original marble x gold marble (both homozygous for marble)

Fig. 10: A lightly marked stripeless gold marble or stripeless silver marble (see text)

Fig. 11: Left side of a stripeless gold marble or stripeless silver marble (see text).

Fig. 12: Right side of the fish in Fig. 11. I refer to the GM/W as "silver marble" because the fish has the marble pattern in addition to vertical bars like the stripes in a silver angelfish. Disturbing this fish does not affect its marble pattern but causes the vertical bars to fade temporarily. The fish in Fig. 8, when disturbed, quickly faded (Fig. 9). Juvenile silver marbles (Fig. 6) look like silvers with some extra markings that are gray, not black. As the silver marble matures, its marble pattern becomes darker.

A blushing angelfish has two doses of the gene called stripeless (Norton, 1971, 1982a). All of the offspring from a cross of a blushing female with a silver marble male were, therefore, heterozygous for stripeless. Those offspring having the marble pattern had no vertical bars even though they had the chromosomes that occur in a silver marble. Thus one dose of the gene for stripeless prevents expression of vertical bars in a silver marble. This "stripeless silver marble" resembles a heterozygous original marble.

From a silver female (carrying gold) crossed with a blushing gold marble male, some of the marble offspring were very lightly marbled like the fish in Fig. 10. These are either stripeless gold marble (if they received gold from the female parent) or stripeless silver marble (if they did not inherit gold). Several fish were marbled only on one side of the body (Figs. 11, 12).

It is possible to ascertain the genotype by the appearance of some individuals, such as a silver marble (having vertical stripes in addition to marbling) or a homozygous marble (mostly black, with only a little white). In other instances, it is possible to figure out the genotype of a fish if the genotypes of its parents are known. For example, all of the marble offspring of a gold marble x gold cross are gold marbles that are heterozygous for marble. If a marble angelfish is not one that is distinctive in phenotype from all other marbles, and if its parentage is not known, it is necessary to do test crossing to find out the genotype. For example, types 3 and 4 have indistinguishable phenotypes, as far as I know. Yet they produce different results when crossed with a gold. Number 3 x gold produces 50% deeply pigmented marbles like itself and 50% golds. But number 4 x gold produces 100% gold marbles.

If you know the genotypes of the parents, you can predict the types of offspring expected from a marble cross, using symbols for the seven types of marble angelfish and W/W for wild-type (silver). For example, crossing number 3 with number 7 can be diagrammed as follows: