Genetics For the Discus Breeder
By: Julia A. Mann, Esq.
BLUE MOON DISCUS
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
GLOSSARY OF BASIC GENETIC TERMS
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
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
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
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.
"PURE" DISCUS STRAINS
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 "
BASIC INHERITANCE AND MENDELIAN PRINCIPLES
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
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
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.
BRIEF HISTORY OF THE PRINCIPLES
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
The Punnett square below represents
the basic crosses.
T = dominant tall gene
S = short plant (tall gene is recessive)
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
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.
When we cross the F1s together, the resulting F2
generation looks like this:
In the F2 generation, we have 25%
phenotype, 50% pink and 25%
PUTTING THE PRINCIPLES TO WORK
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".
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
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.
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
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
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.
FUTURE OF DISCUS GENETICS
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
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