Basics of Genetics

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Sherolyn-Basement Bettas

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.

Basic terminology of genetics

    – 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 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 allele, thus a gene-pair.

    – Homozygous: This term indicates that an organism has two identical alleles at a single place on each chromosome. This results in an organism that breeds true for only one trait.

    – Heterozygous: This term indicates that an organism has two different copies of a gene on each chromosome.

    – 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. 
    – Fish with only one copy of the crowntail (ct) gene (will most of the time) already show some ray extensions.
    – Fish with only one copy of the doubletail (dt) gene (will most of the time) already show a broader dorsal fin and fuller finnage. 
    How to indicate the different generations?
    When two unrelated parents (P) are crossed their hybrid offspring is called the F1 generation (for the first filial generation). 
    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…….. 

    Now try 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 allele for brown hair is dominant and depicted with B.
    The allele 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 allele for haircolor. This means that both parents have to posses the recessive trait for blond hair (Bb) besides the dominant trait for brown hair (B), thus Bb. The best thing to visualize this, is by the use of a Punnet-square:


    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 (bb)

    Inbreeding, linebreeding and outcrossing
    In order to breed good quality Betta splendens, different breeding methods are used. For instance when working with halfmoons we try to fixate and improve the desired traits in our lines (like 180 degree spread of the caudal, straight rays and edges, broad and high dorsal, broad anal fin, etc.). Here inbreeding, line breeding and out crossing play an important role. 

    – Inbreeding: A systematic program of breeding closely related animals. This generally refers to father x daughter, mother x son, and brother x sister parings.

    – Line breeding: This term is used to describe a less intense method of inbreeding. This generally refers to closely related pairings like uncle x niece and half-brother x half-sister. 

    – Out crossing: Out crossing 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?
    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 not 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. 

    When using inbreeding, linebreeding and outcrossing, it’s important to realize that it all comes down to a matter of selection (by knowledge and experience), but the “luck”-factor also plays an important role here. 
    It is the responsibility of each breeder to closely observe the offspring of every generation very thoroughly for possible abnormalities (smaller bodies, less viability, less aggressive, less-fertile, etc.). This should not be underestimated our in goal to breed balanced fish. 

    I hope this article gave you a better insight and understanding about some aspects concerning genetics.


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