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Betta Genetics

To illustrate the basic rules of heredity, I use the double tail mutation of the Siamese fighting fish (Betta splendens).


Example of a double tail betta.
Betta with a wild-type tail. Pictures from Garage full of bettas (Butch Roe)
The double tail phenotype is controlled by one gene, when double recessive , (dtdt), produces bettas that have two tails and a longer dorsal fin.

There are most likely other genes affecting the appearance of the double tail since not all have the same length of the split in the caudal fin.



This table shows the relation between the alleles present in a given betta and the phenotype.
Alleles dt+dt+ dt+dt dt dt
Phenotype Normal caudal tail Normal caudal tail Split tail (double tail)
Genotype Homozygous Heterozygous Homozygous


dt dt
dt+          
dt+    
When we cross two bettas, the occurences of the double tail phenotype in the offspring depends on the alleles present in the parents. The best way to predict the offspring is by using a table where we put the two alleles in the upper row and the left column. In the right table we have one betta, let's say the mother, in the upper row. The mother has the double tail phenotype (dt dt). The father is represented by the two wild-type alleles (dt+dt+) in the left column.

dt dt
dt+ dt+ dt dt+ dt
dt+ dt+ dt dt+ dt
Next we put the alleles of the mother and father in the four squares. These four squares represent the possible offspring. In this case, the four squares contain the same result; all offspring will be of type dt+dt (heterozygous). The phenotype will be wild-type in all four cases.

A more variable offspring can be obtained if we cross a heterozygous female with a heterozygous male. In this cross we obtain the three possible genotypes, dt+dt+, dt+dt and dt dt.
dt+ dt
dt+ dt+ dt+ dt+ dt
dt dt+ dt dt dt
Each square represents 25% of the offspring. This means that among the total offspring, 25% will be of the genotype dt+dt+, 25% will be dt dt and 50% will be dt+dt. Because the phenotype of dt+dt+ and dt+dt are the same, 75% of the offspring will have the normal, wild-type tail and only 25% of the offspring will have the double tail phenotype.








Mutation

A mutation is a change in a gene and changes the nature of the information transmitted from parent to offspring. Mutations occur spontaneously - i.e without our doing anything deliberately to cause them - with low frequency.

The original gene is called the wild-type gene, meaning that most bettas found in the wild have a tail without a split, whereas a mutant form of this gene, which splits the tail in two halfs, is also known. The wild-type gene is often symbolised as + and the mutant gene by a italicised abbreviation, dt. Sometimes dt+ is also used to represent the wild-type gene.

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Locus

The physical location of a particular gene along a chromosome is called a locus.

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Allele

The dt+ and dt genes are said to be alleles to indicate that they represent two alternative genes that occupy the same chromosomal locus.

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Diploid

All fish, including bettas have two set of chromosomes, one set coming from the father, the other set coming from the mother. Cells (and organisms) that contain two sets of chromosomes are called diploid. Because a betta inherits two sets of chromosomes, it will also inherit two alleles of the double tail gene. Cells that contain only one set of chromosomes are called haploid.

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Phenotype

The phenotype indicates the particular expression of a gene, while the genetic makeup is called the genotype. A betta with genotype dt+dt+ will have a wild-type phenotype. An organism with two identical alleles at a given locus (for example dt+dt+), is said to be homozygous, while an organism such as dt+dt, is said to be heterozygous.

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Dominant and recessive

In a haploid cell, with only one copy of each gene, the presence of a given phenotype can be attributed to the presence of a particular gene. This conclusion cannot be drawn for diploid cells, because of the existence of dominant and recessive genes. A diploid cell possesses two alleles at every chromosomal locus. If one of these alleles turns out to specify a nonfunctional protein while the other specifies a normal protein, then enough normal protein may be produced to endow a cell with a normal phenotype, and the normal allele is said to be dominant to the abnormal allele. Or the abnormal allele is recessive (to the normal allele).

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