Dr. Joanne Norton
 Freshwater And Marine Aquarium magazine

Half-Black Combinations In Angelfish

Half-Black Combinations In Angelfish

The half-black angelfish is black on its tail and rear one-third of the body.  By combining the half-black pattern with other patterns, we can see what these combinations look like and also we can investigate the genetic relationships between the genes in angelfish.

For several years angelfish having half-black along with other patterns have been available from Singapore. Thus I obtained, through a wholesaler, black lace half-black (Fig. 1), zebra half-black (Fig. 2), zebra lace half-black (Fig. 3), and blushing half-black (Fig. 4). By appropriate crosses, I obtained smokey half-black (Fig. 5) and marble half-black (Fig. 6).

To obtain smokey half-blacks, I crossed a half-black female and smokey male. The offspring were some each of silver and smokey. Next I crossed one of these smokey (a male), which carried half-black, with a half-black female. Some of the smokey offspring from this cross also had the half-black pattern, looking like smokey except that it was solid black rather than the mottled pattern that is characteristic of smokey (Fig. 7).

Marble half-blacks were produced by first crossing a half-black female with a marble male, producing silvers and marbles. Crossing one of these marble offspring with a half-black produced some marble half-blacks.

Ghost half-blacks (Fig. 8), having one dose of the gene for stripeless, were obtained from a cross of blushing half-black (having two doses of the gene for stripeless) with a half-black. The ghost half-black never exhibits a vertical black  bar on the front of the body. This bar appears and disappears in a half-black, depending on mood changes in the fish.

I have never seen a gold half-black and have not tried to produce it. Also, gold half-blacks are not offered in wholesale lists that I have seen.  Evidence that the half-black pattern cannot develop in a gold was obtained by Bill Lutz. He crossed a half-black with gold, producing silver offspring, the expected result because both half-black (as I shall discuss later in this article) and gold are due to recessive genes. Then he crossed these F1 brother to sister to produce some fish homozygous for both gold and half-black. This genotype would be expected to occur because the genes for gold and half-black are not alleles, and inference that I made because gold acts as an allele of dark and marble (Norton, 1982), and half-black is not an allele of dark or marble  (explained later in this article). Some half-blacks and some golds were produced, but no gold half-blacks. It appears, then, that gold is epistatic to half-black, homozygous gold preventing expression of half-black. Homozygous gold also prevents development of zebra and smokey (Norton, 1982).

To understand the inheritance of half-black combinations, first we need to know how half-black is inherited. Earlier (Norton, 1985) I stated, "Therefore, I am concluding, tentatively until more ratios are obtained, that half-black is due to a single recessive gene." Now I can say, backed up by further evidence, that the half-black pattern is due to a single gene that is recessive to wild type.

Understanding of the inheritance of half-black was elusive for many years because environmental factors influence whether or not the pattern develops in a fish that is genetically a half-black (homozygous for half-black). Unlike the other angelfish pigment pattern genes (dark, marble, gold, stripeless, zebra, and smokey), all of which are expressed in stunted fish, the half-black pattern does not develop in stunted fish (Norton, 1985). Insufficient feeding or inadequate water changes can result in genetic half-blacks that do not develop the half-black pattern. Half-blacks must be raised under excellent conditions to develop the half-black pattern.

Give the fry heavy feedings of live newly hatched brine shrimp. I feed them twice a day every other day. On alternate days they are fed once a day. The abdomens of the fish should bulge after every feeding. On the days that I feed twice, I do not feed any more to fish that are still full from the morning feeding. Usually older fry can use two feedings per day.

Half-blacks should receive frequent large water changes in their first few months of life, until their patterns are established. From the time the fry are three weeks old, change 90% of their water two or three times a week. Later, frequency of water changes may need to be increased to as often as every day, depending on the tank size and, of course, on the number and size of the fish.

From the time that genetic half-blacks become free swimming, it takes over a month (in the ones that I have raised ) before the half-black pattern appears in any of them. In spawns from half-blacks, the first half-blacks became evident in the following number of days after the fry became free swimming: 30, 31, 34, 34, 36, 42, 43, 44, and 45. A partial half-black pattern, consisting of black only in part of the tail to black in the tail and extending slightly onto the body (a partial half-black pattern) is a frequent occurrence in fish that are starting to get the half-black pattern or in fish that are losing all or part of the pattern if environmental conditions become unfavorable for development and maintenance of the pattern.  Adult genetic half-blacks do not change even if environmental conditions become better or worse. If they did not develop the pattern they will not get it. If they have a full pattern or partial pattern, these remain.

Evidence that half-black is due to a recessive gene is that crosses of smokey x half-black and marble x half-black produced no offspring with the half-black pattern; but backcrosses to half-black did produce some fish with and some without the half-black pattern.

It is evident that the gene for half-black is not an allele of the other pigment pattern genes that are present in today's commercially produced angelfish types.

The cross that combined one dose of the gene for marble and one dose of the gene for half-black did not produce any marble half-blacks. It took a backcross to get marble half-blacks, which have two doses of the gene for half-black. Only two genes of a set of alleles occur in an individual. If marble and half-black were alleles, then we would not be able to get a fish with one dose of the gene for marble and two doses of the gene for half-black. Therefore, marble and half-black obviously are not alleles.

Because gold and dark act as alleles of marble, it is concluded that half-black is not an allele of gold or dark.

My cross of smokey x half-black did not produce any smokey half-blacks. It took a backcross to produce smokey half-black, which has one dose of the gene for smokey and two doses of the gene for half-black. My conclusion is that smokey is not an allele of half-black.

Blushing angelfish are homozygous for stripeless (Norton, 1971). If the genes for stripeless and half-black were alleles, then a blushing half-black would have one dose each of the genes for stripeless and half-black; then blushing half-blacks would produce three kinds of offspring: blushing, blushing half-black, and half-black. But this does not happen; blushing half-blacks breed true. I deduce that stripeless and half-black are not alleles.

Stripeless and zebra behave as alleles (Norton, 1982). Because stripeless is not an allele of half-black, I conclude that zebra also is not an allele of half-black.