In simple genetic crosses, as described by Gregor Mendel (the Father of Genetics), each characteristic (trait) is controlled by a pair of genes. An individual inherits one of each gene from each parent.
Genes can be either dominant or recessive. A dominant gene will always mask (suppress) a recessive gene.
If both of the genes for a trait are the same, the individual is said to be homozygous for that trait. If the pair of genes for a trait are different, the individual is said to be heterozygous for that trait.
In our example of albinism in bichirs, let's suppose that the dominant gene is the one for 'normal' color. We'll let the letter A stand for this gene. The recessive gene for albinism will be represented by the letter a.
The following combinations are possible:
A A : an individual with this combination is said to be homozygous,
dominant. This will be a normal-colored fish.
A a : an individual with this combination is said to be heterozygous
because the two genes are different. This will also be a normal-colored
fish because the dominant 'normal' gene masks the recessive 'albino'
gene.
a a : an individual with this combination is said to be homozygous,
recessive. This is the only combination that can give an albino fish.
Now....let's look at some possible crosses:
In our first test cross, let's cross a normal-colored, homozygous dominant (A A) fish with an albino, homozygous recessive (a a) fish.
A A X a a
(remember that each parent only contributes ONE of each gene pair to the offspring)
The box below (called a Punnett Square) shows the possible gene combinations for the babies from this cross. The two A at the top represent the genes from the first parent. The two a on the left side of the square represent the genes from the second parent. The four combinations inside the square represent the possible combinations that the babies COULD get.
You can see that ALL of the babies from this cross are A a, or heterozygous. They will ALL be normally colored.
Now....say we raised the babies to adulthood and then mated two of them together. Since both are heterozygous (A a) the cross would be:
A a X A a
The Punnett Square below shows the possible genetic combinations for the babies from this cross.
You can see that there is a 1-in-4 chance that a baby will be homozygous, dominant (A A) which will be a normally-colored fish.
There is a 2-in-4 chance that a baby will be heterozygous (A a) which will also produce a normally-colored fish.
There is a 1-in-4 chance that a baby will be homozygous, recessive (a a) and THIS baby will be an albino, even though neither of the parents were!
You should now be able to figure out that if you cross two homozygous dominant fish (both A A) then ALL of the babies will be just like the parents for this trait. Also, if you cross two homozygous, recessive albino fish (both a a) then ALL of the babies will be albinos.
Now that you are an expert at simple genetic crosses, I need to remind you that not all characteristics are controlled in such a simple manner. Some traits are controlled by more than one gene pair. Some genes exhibit incomplete dominance, where the dominant gene does not COMPLETELY mask the recessive one. There are many other things to take into consideration when looking at how genes work to control traits.
However, most cases of albinism SEEM to be controlled by simple crosses like the ones in this example. If albinism in bichirs follows the usual then this information will hold true for those of you who would like to breed albino bichirs of your very own.