Betta breeders are most interested in understanding and manipulating genes for either color or tail type. Understanding the various genes and traits and how they are manipulated can be complicated. One of the first things you need to understand is the difference between dominant and recessive traits and how they affect what you will get when you breed two bettas together.
A betta gets one gene from his mother and one from his father for any given trait. Let’s say we want to breed a double-tail betta. Whether a betta fish has a single tail or a double-tail depends on the genes it receives from his parents.
Genes can be dominant or recessive. The single-tail gene is a dominant gene. The double-tail gene is a recessive gene. Genes are often given abbreviations. When the gene is dominant it is shown in capital letters. The single-tail gene is dominant and is abbreviated as (ST). The recessive double-tail gene is abbreviated with a lowercase (dt).
If a betta receives the dominant single-tail (ST) gene from either parent or both parents it will have a single-tail. It will only have a double-tail if it receives the recessive (dt) gene from both parents. Remember, each parent carries two genes but can only pass one down to its young. For a double-tail betta to be born, both parents must have and pass down a (dt) gene.
Let’s pretend that the mother carries (STdt) genes (one single-tail and one double-tail). The mother will have a single-tail but will be a carrier for the double-tail trait. If the father also has (STdt) genes the same thing is true. Using a simple table you can see that 75% of the brood will have single-tails and 25% will have double-tails.
If the mother carries (STdt) genes (she has a single-tail) and the father has two double-tail genes (dtdt) (he has a double-tail) then approximately 50% of the brood will have double-tails.
If either parent has two single-tail genes (STST) then it is not possible for the young to inherit two dt genes and no young will be produced with double-tails.
Breeding two double-tail (dtdt) to (dtdt) parents together will result in 100% double-tail bettas, however this comes at a high cost. Double-tail bettas bred from two double-tail parents are prone to bent spines and other deformities.
This is the basics of dominant and recessive genes. It gets a lot more complicated but already you have enough information to estimate about how many bettas with a given trait you’ll get from a brood if you know the traits the parents carry.