Simple Genetics

Although it can seem daunting at times, the basic genetic theories behind selective breeding are really very simple.

In its simplest form, every genetic trait is controlled by one or more pairs of alleles. An allele is one of two or more possible alternative forms of a given gene. One allele is provided by each parent during fertilization of the egg.

Visually speaking, the double helix, is a model of DNA structure proposed by molecular biologists James Watson and Francis Crick. The major features of this model are as follows:

There is one double helix molecule of DNA per chromosome. One member of each homologous pair of chromosomes comes from each parent. Which means that each parent contributes one double helix of DNA to make a pair of double helixes in a pair of homologous chromosomes. One allele is in one double helix, and the other allele is in the other double helix.

Each bar in the double helix is a base pair linked by hydrogen bonds. The standard model is one gene to one protein. A protein is made up of from 30 to several thousand amino acids. And it takes three base pairs to specify one amino acid. A change in one base pair can make a new mutation, but it takes a hundred to thousands of base pairs to be a gene that is able to specify the makeup of a single protein.

The Punnett Square

The Punnett Square (developed by English geneticist Reginald Punnett) is the basic tool used by breeders to determine the mathematical probability of a breeding producing offspring that will inherit a specific trait.

In this very simple example, the male {recessive trait - let's use Albino) is bred to a normal female {dominant trait}.

The letters used to represent each allele to be passed on by the parents are placed in the top row and left column. Capital letters are used to represent dominant traits and lower case letters are used to represent

recessive traits.

The remaining boxes of the Punnett Square are determined by taking one letter from each parent and combining them. The result of that combination is displayed in the intersecting box. As we get into more elaborate pairings, different letters will be used to represent different traits.

Since the offspring will receive one allele from each parent, the results of this punnett square reveal the following:

All of the resulting offspring will be Aa which in this case would be 100% het for Albino.
They will appear visually as normal because the dominant allele controls the appearance, but each baby is carrying the recessive gene for Albinism.

Heterozygous x Heterozygous

In the next example, we'll cross a heterozygous animal with another heterozygous animal.
Both parents appear as normal but carry the recessive trait for albinism.

The results are as follows:

25% are AA or normal
50% are Aa or het for Albino {heterozygous}
25% are aa or Albino {homozygous}

Because the appearance of the offspring is controlled by the dominant allele, both the het albino and normal babies will all appear as normal.
These offspring will be considered 66% possible het for Albino. They are 66% possible het because statically speaking, two of every three of the normal appearing babies are het Albino - 2/3 = 66%.

Heterozygous x Homozygous

Now let's look at a heterozygous X homozygous pairing.

In this case, the male is visually albino (homozygous} and the female is normal looking but het for albino {heterozygous}.

The results are as follows:

50% are Aa or het for Albino {heterozygous}
50% are aa or Albino {homozygous}

Homozygous X Homozygous

It should be pretty obvious by this point what the next pairing will accomplish
A Homozygous X Homozygous pairing.
In this case, both the male and the female are visually albino (homozygous}.

The results are as follows:

100% are aa or Albino {homozygous}