The doubletail gene is reccesive which means a dominant gene like Single tail. So this means that you need two genes of doubletail (dt/dt) inorder for it to show. A betta that has one doubletail gene and one other gene (let's say single tail) will show but single tail but will be called a doubletail carrier (ST/dt). It is called a carrier because though it does not show the gene it has the ability to pass it on to it's children. Also a dt/dt can only pas on the doubletial gene where as a ST/dt can pass on either gene.
Now let's look at some breeding combos and what is the chance that will occur from each one:
ST/ST x dt/dt
This is cross will acheive 100% ST/dt. This is usually how one woukd start out and then take an offspring and cross it back to the parent as seen below.
ST/dt x dt/dt
This cross is the best to use to breed Doubletails. It will yeild 50% ST/dt and 50% dt/dt. So all the offspring are either doubletail or doubletail carrier. Also the children will avoid deformities that sometimes occur from dt/dt x dt/dt.
ST/dt x ST/dt
This will yeild many types of bettas but mainly carriers. It willyeild about 50% ST/dt, 25% ST/ST, and about 25% dt/dt. So though you will get doubletails you will not be able to tell the carriers apart from the ST/STs. So this will work it is not the best option.
dt/dt x dt/dt
Though this may seem like the most practical, breeding a doubletail to a doubletail, I do not reccomend it. You willacheive 100% dt/dt but the offspring will have problems. Commonly swimbladder is a problem. Also the babies will be stunted and look like they have a compacted body. This is not reccomended.
So if you plan to breed doubletails I would surgest you use the ST/dt x dt/dt cross. Also it is said that the dt gene improves finnage.
