Terminology of genetics
When breeding bettas, it can be very helpful to know
some basics about genetics. One thing that is really important with
genetics is to understand the terminology used. In this article, I will
try to clarify and cover some of the basics, which are indispensable to
understand betta genetics in a simplified manner.DNA: Deoxyribonucleic acid, the heritable
material of an organism.
Gene: The units of
inheritance that transmit information from parents to offspring.
Chromosome: A long threadlike association of genes in the nucleus of all eukaryotic cells
which are visible during meiosis and mitosis. A chromosome consists out of
DNA and proteins. An organism always has 2n chromosomes, which means that
all chromosomes are paired.
Genotype: This is the genetic makeup of an
organism: the genes.
Phenotype: The physical
and physiological traits of an organism. These are influenced by genetic
makeup (genes) and surrounding.
Allele: Another word
for gene. Each chromosome has a copy of this allel, thus a gene-pair.
Homozygous: This term
indicates that an organism has two identical alleles at a single place on
a chromosome. This results in an organism that breeds true for only one
Heterozygous: This term
indicates that an organism has two different copies of a gene on each
Dominant gene: In a
heterozygote, this allele (gene) is fully expressed in the phenotype. In
genetic schemes, these genes are always depicted with a capital letter.
Recessive gene: In a
heterozygote, this allele (gene) is completely masked in the phenotype. In
genetic schemes, these genes are always depicted with a lower case letter.
Intermediair gene: This is when in a
heterozygote, an allele (gene) is not fully masked in the phenotype.
You can already see some of the characteristics of the gene. Good examples
of this are the genes for crown- and doubletail.
How to indicate the different generations?
-Fish with only one copy of the crowntail (ct) gene (will most of the time) already show some ray
-Fish with only one copy of the doubletail (dt) gene (will most of the time) already show a broader
dorsal fin and fuller finnage.
When two unrelated parents (P) are crossed their hybrid offspring
is called the F1 generation (for the first filial
When the F1 generation is
interbred their offspring is called the F2 generation (for the
second filial generation).
When the F2 generation is
interbred their offspring is called the F3 generation (for the
third filial generation).
And so on........
to visualize this using for example the allele for hair color in humans:
Brown hair is a dominant trait. How is it possible that two parents with
brown hair get a blond daughter of son?
The allel for “brown hair” is dominant and depicted with “B”.
The allel for “blond hair” is recessive and depicted with “b”.
The answer lies here: Remember that all alleles come in pairs and that the
parents have to be heterozygous for the allel for haircolor. This means
that both parents have to posses the recessive trait for blond hair (“b”)
besides the dominant trait for brown hair (“B”), thus “Bb”. The best thing
to visualize this is by the use of a Punnet-square:
Inbreeding, linebreeding and outcrossing.
The offspring of two
parents carrying the heterozygous “Bb” genotype can result in the
following offspring: 25% homozygous for brown hair (“BB”), 50%
heterozygous for brown hair (“Bb”) and 25% homozygous for blond hair
In order to breed good quality Betta
splendens, different breeding methods are used. Inbreeding,
linebreeding and outcrossing play an important role in setting up a
quality line of Betta splendens.
Inbreeding A systematic program of breeding
closely-related animals. This generally refers to father x daughter,
mother x son, and brother x sister parings.
Linebreeding: This term is used to describe a less
intense program of inbreeding. This generally refers to closely related
pairings like uncle x niece and halfbrother x halfsister.
Outcrossing: Outcrossing refers to the breeding of
two unrelated (inbred) strains.
What does inbreeding do in the genetic sense?
Inbreeding will increase the probability that any given gene has two
identical copies derived from the same ancestor. It tends to make all
genes more homozygous. Remember each animal has 2 two copies of a given
gene (technically speaking, two alleles at each locus on the chromosome)
one from each parent. Unfortunately we are not able to only select the
desired genes we want because genes come as a package…….
One has to keep in mind that in the quest for fixating
the desired traits by inbreeding, there is always the chance that we
unintentionally loose some of the desired (“good”) genes and fixate some
undesired (“bad”) genes which surface throughout the process.
Good examples of this are for instance the inbred strains of laboratory
rodents. The process of inbreeding used to create this type of strains
most of the time kills the majority of the strains between the 8th
and 12th generations due to a loss of fertility (reduction in
litter size) and viability. The strains, which survive these critical 8th-12th
generations, form the inbred laboratory strains. These animals are homozygous for a
more or less random selection of genes derived form the initial pair.
Why outcrossing is needed ?
As described in the example of the laboratory rodents above, in general
inbreeding can be done up to F8 (8th generation). Most times
the rate of breeding success is really low at this stage.
When we extrapolate this
example to Betta splendens, extensive inbreeding can result in fish
which show a number of undesired characteristics like: smaller bodies,
decrease viability, decrease of aggressiveness, decrease of fertility, not
building bubble nests, fish which don’t know how to wrap themselves around
the female, etc. This is why it is advisable to use an out-cross
(unrelated partner, fresh blood) once in a while in order to keep the
lines healthy and viable.
When choosing the outcross candidate, the breeder always needs
to decide which outcross candidate possesses the desired traits that can improve the
established inbred line. Off course there is also a risk in outcrossing
because a breeder can loose the type of betta he has been worked on for a
long time. Breeders often decide to cross the hybrid offspring of an
outcross back to their original inbred line. This in order to add the new
or improven traits that were brought in by using an outcross, but also in
order to eliminate possible bad traits brought by the outcross.