Angelfish Genetics

Article Index

This section will give you a basic overview of genetics and how it applies to angelfish. Much of the information on angelfish genetics contained here, has come from Dr. Joanne Norton's research on angelfish genetics and from the practical knowledge obtained by Steve Rybicki, with the hundreds of crosses he has made over the last 35+ years. Dr. Norton's information on angelfish genetics was first published in FAMA Magazines articles starting in the early 80's. A majority were published by the end of 1992. Dr. Norton's angelfish genetics research was later published in a couple books, one put out by Tetra in 1992 called Aquariology. Steve personally confirmed all these crosses in his hatchery before 1990 with the exception of Albino and Streaked, which came out later.

The angelfish gene names used here are those that were originally used by Dr. Norton or the person first researching the new angelfish gene mutation (up until now, no one else has done any documented research that has demonstrated the inheritance characteristics of any gene mutations in angelfish). Many different common angelfish names are used, so it is important to also attach universally used angelfish gene names for the alleles that are proven in any particular angelfish strain.

Did you Know... That angelfish have more chromosomes than humans? It's true, angelfish (Pterophyllum scalare) have one more pair of homologous chromosomes than do humans. While human somatic cells (body cells) contain 23 homologous pairs for a total of 46 chromosomes, angelfish have 24 pairs of homologous chromosomes in their somatic cells giving them a total of 48 chromosomes.

In genetics, there are specific terms used to refer to the number of chromosomes in a cell. Gametic cells (gametes, or sperm and egg) contain half of an organism's genetic information, so that when combined, two gametes will produce one zygote with the proper number of chromosomes. The haploid number of an organism refers to the number of chromosomes contained in each of its gametes. With angelfish, 24 is the haploid number and is represented by the letter n. After fertilization of an angelfish egg, the resulting zygote contains a full set of chromosomes and is known as a diploid cell. The diploid number of an organism refers to the number of chromosomes contained in its somatic cells. The diploid number, for angelfish 48, is represented by the symbol 2n.

Genetic Terms

Understanding the terms in this section can help you better track the results of your angelfish crosses and to predict the expected results of future angelfish pairings. Gene: Genes contain the genetic material by which hereditary characteristics are transmitted. Each gene occurs at a specific location (called locus) on each chromosome, whose existence can be confirmed by allelic variants (mutations). Chromosomes occur in pairs, one coming from each parent. Therefore, each gene has an opposing gene on the other chromosome.

Allele: Alleles are the mutational variations of a gene that can occur at each locus. Each is distinguished from other alleles by the effect it has on phenotype. There may be several alleles possible at any given location, though no more than two can occur at the same time in any individual.

Locus: The place on the chromosome where a particular gene resides. This location is like an address and is what is used in gene mapping.

Dose: A term commonly used to designate the allele donated by a parent. For example, a Double-Dose Black means that each angelfish parent transmitted one allele of Dark to the offspring.

Hybrid: An angelfish trait that was derived from two different alleles. The angelfish hybrid will not breed true, however, a percentage of the hybrid's offspring will be true breeding.

Mutation: The process of mutation is the source of new alleles. It is an abrupt and permanent change in the genetic material. Minor mutations are relatively common and somewhat insignificant. They are either slight or have no desirable quality. Since mutations are random in the genetic code and the proper code is very specific, useful mutations of a major proportion are rather unlikely. Truly valuable or significant mutations are quite rare. In angelfish, there have been no more than 2 to 3 dozen major mutations that were perpetuated in the last 50 years. The gradual change in the looks of a fish, over many generations, is also caused by mutations. The result is simply the accumulation of less significant, minor mutations.

Homozygous: A trait determined by two identical alleles at the same locus, resulting in a true breeding strain for that trait. i.e. a true breeding Double Dose Black is homozygous for Dark.

Heterozygous: A trait determined by two different alleles at the same locus, resulting in a hybrid angelfish that will not breed true for that trait. i.e. a single dose of Dark and a single dose of its allele, Gold, will produce 25% homozygous Blacks, 25% Gold and 50% single dose Blacks that have a single dose of Gold, thus the single dose Black is heterozygous for Dark and Gold.

Recessive: When different alleles occur at the same locus and one of the alleles is not expressed in the presence of the other, the non-expressed trait is considered recessive.

Dominant: When different alleles occur at the same locus and one of the alleles is not expressed in the presence of the other, the expressed trait is considered dominant.

Intermediate Inheritance: When different alleles occur at the same locus and there is a blending of traits, each trait is an incomplete-dominant (or Co-dominant). In Black Lace Angelfish is there is a blending of traits, therefore Dark and Silver (wild type) act as incomplete dominants to each other.

Phenotype: The physical appearance of a trait. To be considered a distinct phenotype the trait should be uniquely identifiable.

Genotype: The written expression of the genetic makeup of the trait in question. With angelfish we use one or two letter notation to represent this. There are two notations for each gene location. For example, Gm/g represents two possible alleles that could be found at this location - Gold Marble (Gm) and Gold (g).

Penetrance: Expressed as a percentage. It is the frequency with which a given genotype expresses its mutant phenotype to at least some degree. A good example is Halfblack angelfish, which is influenced by environment. 100% penetrance occurs under ideal tank conditions with some lines. If at least a partial pattern develops on all the angelfish, then 100% penetrance has occurred. Some Halfblack angelfish lines appear to contain modifiers that inhibit the expression of Halfblack. In these lines, penetrance will be less than 100%, even under ideal conditions.

Expressivity: The degree or range in which a phenotype for a given trait is expressed. Once again, Halfblack is a good example. This mutation can exhibit a wide range of expressivity. Halfblack angelfish patterns are seldom complete on every fish in a spawn. Various degrees of partial patterns are somewhat normal. This range can be due to incomplete penetrance and well as adverse environmental conditions.

punnett square

Punnett Square: This is particularly useful when first learning about angelfish genetics and how to identify the outcome of a monohybrid cross. It is recommended that you use this until you are very comfortable with how alleles are donated by each angelfish parent. Place the alleles of one parent across the top and the alleles of the other parent on the side. Then bring each allele down or across to the box in its row or column. Each box represents the possible outcome of each cross. This represents the genotype of each angelfish. The phenotype (how it looks) is determined by the dominant and recessive characteristics of each allele in its respective box. In the example below, each box represents 25% of the total spawn. Therefore, 25% of the spawn will be true breeding Double Dark Black angelfish (DD), 50% will be hybrid Black angelfish (Dg or gD) and 25% will be true breeding Gold angelfish (gg).

Branch Diagram: When dealing with multiple traits, it is much less complex to consider the genetic interaction of these traits separately, and then combine the results using a branch diagram.

Test Cross: There is a simple method used to distinguish the genotype of of an angelfish with an unknown genetic inheritance. The angelfish of unknown genotype is crossed to a homozygous recessive individual. For example, you have a dark angelfish that shows streaking in the fins and you want to determine its genotype.

Test cross: Cross this dark angelfish into a homozygous recessive gold angelfish (gg) that is a proven to have no other hidden mutations. Let's say the resulting (F1) shows 50% gold (gg) and 50% dark angelfish. Of the dark angelfish, 50% are streaked, as the one parent. The other dark angelfish appear as normal blacks.

Analysis: All of the F1's received a Gold allele from the Gold parent. The 50% gold fry means that the dark parent also contained the allele for Gold. The dark F1's without streaking, proves that a Dark allele (D) was donated by the dark adult angelfish. This means the dark parent has two identified alleles at this point - (D and g). Since Dark and Gold are alleles, we now know that the gene that causes streaking is not an allele of Dark. The dark angelfish parent could not have 3 alleles on one locus. Because the original gold did not contain the allele that causes streaking (came from dark parents that showed no streaking), we know that the streaking in the F1's came from the dark parent. 50% streaked dark F1's is consistent with a dominant allele that causes streaking and shows that the original dark parent was heterozygous for this trait. The original angelfish parent is proven to be heterozygous for Dark, Gold and Streaked (D/g-St/+).

Variety: Any of a group of widely separated variants within a single interbreeding population. As used within the hobby, varieties of angelfish include the different color types, i.e., Black, Gold, Marble, etc..

Strain: The descendants of a common group within a variety. I may be working on a strain of Koi angelfish that exhibits bright orange on most of the body and a yellow eye. Another breeder may have a different strain of Koi angelfish that exhibits yellow on the body and a red eye. They are both the same variety, but a different strain of that variety.

Line: The descendants of a common ancestor within a strain. For example, I decide to set up four separate lines within my strain of orange bodied, yellow eyed Koi angelfish. Each line is kept separate and periodically out-crossed to another of the four lines to create a new line. The use of lines helps to keep the immune system strong, while maintaining the same strain within the same variety.

Inbreeding: A breeding technique to fix traits known to a particular line of angelfish. This involves the mating of closely related angelfish, i.e. brother to sister, father to daughter. A strain's immune system is deteriorated using this method. With this technique you can also more easily identify recessive traits, both good and bad.

Line-breeding: A breeding technique for developing the positive attributes of a strain. This involves the mating of angelfish that are related, i.e. brother to grandmother, father to cousin, etc.. Separate lines are kept that are crossed to each other every 3rd or 4th generation. This is a more forgiving system then inbreeding, but care must be taken, because the immune system is still compromised, though to a lesser degree.

Out-crossing: A breeding technique of using unrelated parents. It can strengthen the immune system and it can be used to bring in desirable traits the current line does not have. Out-crossing can make it difficult to "fix" a characteristic and can also introduce hidden undesirable recessive traits to the line.

F1 Generation: F1 is the symbol given to the first filial generation of progeny from a cross of non-siblings. This cross is represented by (P1 x P2 = F1). When representing a cross, you use parent (P) to indicate the original breeders. You can only get to an F2 by breeding siblings (F1 x F1 = F2). If you cross back to a grandparent, the resulting fry are also F1's, because it would be represented the same way as the first example (P1 xP2 = F1).

The reason you want to track it in this manner, is so you will know whenever a different gene pool has been added to the line. When trying to fix certain traits, rather than strict inbreeding, you may want to start different lines that will be inbred and then crossed to each other every 3rd or 4th generation. You would then probably want to change to a different labeling scheme. I would suggest lowercase letters to represent each line, to avoid confusion with the generation numbers. Adult angelfish would be labeled as follows: Line (a) = (aP), line (b) = (bP), line (c) = (cP), etc. This way, you can track this modified form of inbreeding. (aP1 x aP1 = aF2), (aP3 x bP3 = aF1). Let's say you linebreed for another 10 generations and then decide to bring in other unrelated stock and cross it into Line 1. You will want to label the new line so you know it's unrelated. (aP3 x eP1(new) = aF1), or if you decide to keep line (a) and create a new line (e), it would be (aP3 x eP1 = eF1).

Angelfish Genetics part 2

© 2006 Angels Plus