Elementary Genetics

Elementary Genetics


Elementary Genetics For the Discus Breeder
By: Julia A. Mann, Esq.
Haughton, Louisiana USA


A basic understanding and knowledge of genetics is helpful to the Discus breeder who wishes to produce new or improved color forms or to strengthen desirable characteristics such as red eyes or body conformation which a breeding line may already possess. This article is intended as a brief overview of basic genetic principals with regards to inheritance and an introduction to the terms used in the study of genetics. A basic familiarity with the terminology is necessary for understanding the principals involved in inheritance and selective breeding.



Allele - Gene variations which can occur at each location (locus) of the chromosome. There is one gene variation (allele) at each location (locus). A different gene variation (allele) can occur at the same location (locus) on the second chromosome of the pair.

Chromosome - A DNA -containing linear body of the cell nuclei of plants and animals, responsible for the determination and transmission of hereditary characteristics. Chromosomes occur in pairs - each parent contributes ½ of the pair to the offspring.

Dominant - When different genetic variations (alleles) occur at the same location (locus) on the chromosome and one of the variations (alleles) is not expressed when the other is present, the trait which is expressed is considered dominant.

Dose - Commonly used to designate the gene variation (allele) transmitted by a parent. A "double dose" of a characteristic, such as eye color, means each parent passed one gene of the color on to the offspring.

Genes - Genes contain the genetic material which allows characteristics to be transmitted to the next generation.

Homozynous - A trait which is determined by two identical gene variations (alleles) at the same location (locus) on the chromosome. This results in a true strain which consistently produces that trait.

Heterozynous - A trait which is determined by two different gene variations (alleles) at the same location (locus) on the chromosome. This results in a hybrid which does not consistently pass that trait to offspring, although a percentage of the offspring will have that trait.

Hybrid - When used in the context of genetics it refers to an offspring resulting from a cross of two genetically varied parents, i.e., two different alleles. A hybrid will not breed true for all characteristics. The breeder can expect that a percentage of the hybrid Discus’ offspring to be true breeding Discus. 

Intermediate Inheritance - When different genetic variations (alleles) occur at the same location on the chromosome (locus) and the traits expressed by the variations are blended. Each trait is an incomplete dominant. This may result in the appearance that two traits blend such as when a cross between a red eyed Discus and a yellow eyed Discus results in a Discus with orange eyes. In actuality, the genes do not "blend", but the presence of each set gives that appearance.

Locus - The location where each gene occurs on each chromosome.

Mutation - An abrupt permanent change in the genetic material of a Discus. The mutation is a sudden change in the genetic material as opposed to a variation over generations of gradual change. A mutation rarely breeds true, but can be a quite valuable individual used for breeding if the mutation is attractive. A classic example of a mutation is the original Pigeon Blood Discus.

Recessive - When different gene variations (alleles) occur at the same location (locus) on the chromosome and one of the variations (alleles) is not expressed when the other is present, the trait which is not expressed is considered recessive.

Strain - A group of organisms of the same species, having distinctive characteristics, but not usually considered a separate breed or variety.

    Genetics means the study of inherited variation also known as polymorphism. There has been limited study of Discus genetics, however, Angelfish genetics have been widely studied and some of that body of research is analogous to Discus. It is important to remember that a Discus’ color, growth rate and adult size are a product of not only its genetic material but also environmental factors. These factors include tank size, temperature, frequency of feeding, nutrition, and water quality. Substrate color can effect the color of some fish as can the length of day called the "photo period". Substrate color and photo period have not been scientifically shown to influence Discus color patterns, however there is anecdotal evidence in the aquarium literature which should be further explored in a controlled environment, particularly the photo period’s influence on the vertical bars exhibited by many Discus.


    Discus, the descendants of which in many successive generations are exactly like themselves, form a pure strain. A pure strain continues to breed true and is described as genetically stable or "homozygous". Conversely, where breeding does not result in successively pure descendants, but in offspring with a number of differing characteristics the fish concerned are genetically mixed or "heterozygous". Almost all cultivated Discus strains today are genetically mixed or heterozygous. The only documented exception to this may be the original Wattley Turquoise Discus, developed by Jack Wattley in the United States, through strict selective breeding, many years ago. Wattley Turquoise Discus are virtually duplicates of each other having a very distinctive shape, size and color. Almost all other cultivated Discus "strains" are genetically mixed or " heterozygous".


    Genetic variation generally results when there is a mutation or permanent change in a gene. " Genotype" refers to the genetic material of an individual while "phenotype" refers to the observable traits. The "genotype" of a Heckel Discus in which the letter H is used as the symbol for the dominant Heckel bar gene is "H H". The "phenotype" or appearance of this fish is a Discus with a prominent middle vertical bar.

    Mendelian principles state that certain factors retain their individuality from generation to generation. Mendel’s first principle states that when individuals which are homozygous for a particular characteristic (such as a red eye) are crossed, all F1 descendants are identical with regard to the characteristics examined. That is, if a Discus is homozygous (a genetically pure) line, selected only for red eyes, all F1 descendants will have red eyes.

    Mendel’s second principle (the law of segregation) holds that the F2 individuals are not identical among each other and characteristics of the parent generation reappear. An example of this would be mating two phenotypical identical red eyed F1 Discus with each other. The grandparents are a genetically true breeding red eyed Discus and a genetically true breeding amber eyed Discus. The subsequent generation (F2) consists of not only red eyed fish but, once more there will be amber eyed fish which look like their grandparent. Nonetheless, in the F2 generation the red eyed fish are in the majority by roughly 75% or 3:1.

    A recessive gene is expressed only when present in a double dose, i.e. from both parents. A very rare exception occurs when a chromosome carries a recessive gene and the other chromosome of the pair breaks and uses some genes. An individual that is homozygous for a recessive factor exhibits that trait and breeds true for that trait when mated to another individual that is homozygous for the same gene.

    The third Mendelian principle, (the law of independent assortment) or the rearrangement of hereditary factors. In this case all pairs of factors by which the original parents differed from one another can, in the F2 generation, appear in any combination and cause the development of a completely new phenotype or appearance. One trait does not influence others which are transmitted, i.e., inheritance of a red eye has nothing to do with inheritance of short gill plates.

    An incomplete dominate gene is expressed differently when present in either a single or a double dose. In this case, each gene is expressed equally and appears to blend, although in actuality they do not. This is known as intermediate inheritance.

    There has been little scientific research done to genetically "map" Discus and their colors to determine which traits are dominate or recessive. The genetic characteristics which are either dominate or recessive must be studied more intensely and meticulous records kept to allow a breeder to predict percentages and characteristics of a spawn between proposed parent fish. Many characteristics are affected by a number of pairs of genes and this type of inheritance is termed polygenic or polyfactorial. This is generally the case in Discus. Research is being done at the National University of Singapore (NUS) in an attempt to identify genetic makeup of Discus through DNA "fingerprinting". Further work in this area has the potential to greatly broaden the possibilities of Discus breeders, particularly in commercial ventures.

    The NUS study done at the National University of Singapore suggested that the genetic diversity on the wild forms of Discus is not being fully utilized in the present development of cultivated Discus varieties.

    The study further suggests that frequent out crossing of cultivated Discus with wild Discus yields greater probability of new fin and color morphs than the line breeding and inbreeding presently utilized by most breeders.


    Gregor Mendel was an Austrian monk who is considered the father of genetics. He studied how genetic characteristics were passed down to pea plants. Mendel developed two pure strains of pea plant - short and tall. He then cross pollinated a tall pea plant with a short plant. All of the F1 offspring of the plants were tall, despite the fact that one of the parents was a genetically pure short plant. Mendel theorized the tall plant genes overpowered the short plant gene, and thus were dominant. The result of this was that all of the offspring were tall.

    To further his theory Mendel interbred the F1 offspring whose phenotype (appearance) was tall. This produced an F2 generation which was a mix of tall and short in a ratio of approximately 3:1.

    The Punnett square below represents the basic crosses.

T = dominant tall gene

S = short plant (tall gene is recessive)

      First Cross or F1












All F1 offspring have the same genotype (one gene for tall which is dominant, and one gene for short which is recessive) and phenotype (appearance).

Second Cross or F2s

             Ts X Ts












Thus, in the second cross of F2s the recessive gene for short appears with approximately 25% of the offspring’s phenotype (appearance) being short. These 25% would be a true breeding strain of tall plants and their phenotype is tall. Approximately 50% will express the dominant tall gene and carry the recessive short gene.

    These first two Mendelian principles, or laws, did not explain why when pea plants with red and white flowers were crossed pink flowers resulted. This is an example of what is termed "intermediate inheritance". The mix of red and white creates a cross of genes which are neither dominant or recessive but in essence blend. Each gene affects the flower color resulting in pink flowers.

R = Red

W = White










F1 offspring is a blend of incomplete

dominant genes. All offspring is pink.

RW = pink

  When we cross the F1s together, the resulting F2 generation looks like this:









In the F2 generation, we have 25% red

phenotype, 50% pink and 25% white, or

1:2:1 ratio.


    The study of genetics is best commenced by looking at a few specific traits one wishes to enhance. Producing high quality Discus is rarely the result of only good fortune or luck, although one needs a certain amount of both in addition to skill. In order to produce consistently high quality fish with desirable characteristics the successful Discus breeder uses certain genetic strategies, whether consciously or not. In order to strengthen a certain trait, such as red eyes or body shape, the following basic genetic principles are utilized.


    Mating of closely related Discus such as siblings, mother to son or father to daughter. A technique used to intensify traits in a particular Discus line. It often has the result of intensifying negative characteristics as well. A strain can deteriorate quickly when this is the exclusive method of development. This technique is only recommended to those breeders who understand and are knowledgeable of the genetic makeup of their Discus. In such a person’s hands inbreeding can be used to develop some attractive strains such as the "Blue Diamond", "Snakeskin" or "Leopard".

Line Breeding

    This involves the mating of related fish such as sister to grandfather, cousins, etc. This technique is used in much the same way as inbreeding, but is more forgiving if regularly used. As with inbreeding negative as well as the desired traits are enhanced.

    With both inbreeding and line breeding genetic problems are inevitable. This is the result of the small gene pool and cannot be avoided without infusing new genetic material into the strain or out crossing.

Out Crossing

    Used to strengthen a strain as an antidote for undesirable traits which may have been strengthened by inbreeding or line breeding. An unrelated fish is introduced into the strain every few generations to improve the overall vigor of the strain. This is sometimes referred to as "hybrid-vigor". If out crossing is used, it is more difficult to "fix" a characteristic which is being developed through line breeding. When attempting to "fix" a certain characteristic through line breeding some Discus breeders keep two or more separate but related lines of the same strain. By crossing these lines every third to sixth generation, the need to out cross can be delayed.

Punnett Square

    The Punnett Square is a tool which is useful to help identify the probable outcome of proposed spawning. The Punnett Square is used by many students of genetics. Because the tank raised Discus of today has such a diverse genetic background, it is of limited use unless you have actual knowledge of the genetic background of the parent fish. Nevertheless, the Punnett Square is helpful in attempts to develop or fix a trait such as red eyes.

    To construct a Punnett Square place the alleles (genetic variations) of one proposed parent across the top and the alleles (gene variations) of the other proposed parent on the side. Then bring each allele (variation) down from the top row and place in the box directly below it. Working then from the side rows each allele is brought across into the box to the right allele.

    Each box shows the possible outcome of the cross. This is the genotype. The appearance of each fish or, its phenotype is determined by the dominant and recessive traits of each allele in its respective box.

Punnett Square for red eye:

R = dominant red

                                Y = recessive yellow

egg   y sperm
Y Ry yy

    In the example, each box is 25% of the total spawn for two-parent fish with the dominant allele for a red eye. Thus, 25% of the spawn will be true breeding red eyed

    Discus (RR). Another 25% will be true breeding yellow eyed Discus (yy). The largest portion of the spawn will be hybrid (Ry) and will not breed true. Among these "hybrids" will be Discus exhibiting "intermediate inheritance" and showing orange eyes, although the majority of the spawn should be red eyed Discus.

    The above example is a simplified model to assist the introductory process of using the Punnett Square. Two or more traits or characteristics can be graphed in the same way with a bit of practice.


    Presently there is limited knowledge of the genetic structure of the Discus species and its breeding stock and breeding stock management in terms of genetic identification.. Research has been conducted at the National University of Singapore, Department of Biological Sciences in using DNA "fingerprinting" to access genetic diversity among and between four wild forms of Discus (Heckel, Green, Brown and Blue) S. haraldi (Blue) and five cultivated forms (Turquoise, Pigeon, Ghost, Cobalt and Solid Red). Comparisons were made among the four wild forms and the five sample varieties of cultivated forms. The two groups – wild versus cultivated were then compared with each other. The test used in the comparisons showed the gene pool of the wild Discus forms to be broader than that of the cultivated varieties. The research suggests the Heckel Discus to be the most genetically divergent compared to the other three wild forms. The only two Discus forms to show statistically significant genetic clusters were the Heckel and the Wild Green. The research further suggested that the wild Green form (S. Aquafasciata) is a more likely genetic organ of the cultivated varieties. This research tends to support the widely held view of the importance of the wild form Green Discus in the development of many cultivated strains as reported in the aquarium literature.

    The most interesting finding in this research is no distinct genetic clustering of individuals from the same cultivated variety was observed. This would indicate there is no genetic basis for the present classification of Discus which is based upon their phenotype or appearance with regard to the cultivated varieties, i.e. solid, striated, spotted, Pigeon, etc.

    In regard to classifying cultivated forms, the only method continues to be the appearance or phenotype of the fish as opposed to its genetic background as suggested by some writers in the aquarium literature, as opposed to the scientific literature. Until more research is conducted there is no meaningful way to classify the different cultivated varieties beyond appearance because genetically, there is no scientific evidence of differences between the cultivated varieties studied. With more research the possibility exists that the cultivated varieties may some day be genetically classifiable.

   Should genetic identification of Discus advance, it is probable that a wider variety of colors can be produced more predictably. The Discus market commands premium prices for each new popular variety. The identification of Discus genetic traits will allow the market to continue to expand with a wider variety of colors and patterns than possible using the present "trial and error" selection of breeding stock.



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Langhammer, J. K. 1982. Albinism in Pelviachromis pulcher. Buntbarsche Bull. 93:8.

Schroder, Johannes Horst. 1976. Genetics for Aquarists.

Wohlfarth, G.W. 1983. Genetics of fish: applications to warm water fishes. Aquaculture 33:373-81.

Koh, T.L.; Khoo, G.; Fan, L.Q.; Phang, V.P.E. (1999). Genetic diversity among wild forms and cultivated varieties of Discus (Symphysodon spp.) as revealed by Random Amplified Polymorphic DNA (RAPD) fingerprinting. Aquaculture. 173:483-495.

Norton, J. 1992. Fish Genetics. Aquariology 6:95-124.

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