Basic Mendelian Inheritance

There are two basic types of genes when talking about basic Mendelian inheritance - dominant genes, and recessive genes.

Traits that follow these rules are made by the combination of two genes. One comes from the father, one from the mother. Each parent has two copies of the gene, but can only donate ONE of those, which is where the issue of chance comes into play.

There is a 50% chance that either of the genes will be "chosen" to give to the offspring. In the case of mice, there are multiple offspring and each one receives their genes independently. For example, if one pup gets gene A, another may get gene a, or another copy of gene A. It is entirely random - which is why even the best of predictions can never be 100% certain, and can be entirely wrong just because nature LOVES to play tricks on breeders!

Dominant genes are generally referred to by capital letters. Dominant genes need only ONE copy to be expressed. In general, they dominate other genes - hence the name. So if either parent donates a Dominant gene to the offspring, it will be expressed.

The genes that they dominate are called recessive genes. Recessive genes are represented by lowercase letters. Recessive genes need two copies to be expressed. Both parents must donate a recessive gene to the offspring in order for this to happen - which means both parents must have a copy of the recessive gene to begin with. The problem with recessive genes is that they are not always expressed. An example of this is when a parent has only ONE copy of a recessive gene. The recessive gene does not show in the parent - thus unless you know the genetic background of the parent, you never know it's there. Mice that have only one copy of a recessive gene are called carriers of that gene.

A simple way to predict the ratio of offspring that exhibit a specific trait is by the use of something called a Punnett Square.

The mouse in this picture is phenotypically agouti and genotypically Aa.

Let's use an example. Agouti, the "wild-type" color in mice, is a dominant gene. It is represented by the letter A (capitalization matters). The letter a (lowercase means recessive, remember?) represents anything except Agouti.

In a Punnett Square, one side represents the mother's genes, and one represents the father's genes.

In this example, both parents are not agouti. This is what the Punnett Square will look like before it is filled out:

The purpose of the Punnett Square is to show us the possibilities of how the genes can combine in the offspring resulting from this pairing. By matching up each column with the letters on each row, we get:

In this square we can see that 100% of the babies will have the genotype (genetic makeup) of "aa" When an offspring has a makeup of double any gene (ie. AA, aa, bb, BB, DD, dd, WW, ww) they are called homozygous for this gene.

In this case, where all of the offspring will be homozygous for the recessive non-agouti (remember, it takes two to be expressed), all of them will have the expected phenotype of non-agouti. Phenotype is the fancy word for how the genotype (genetic makeup) is expressed. A mouse with the genotype of Aa has the phenotype Agouti. A mouse with the genotype of aa has the phenotype of non-agouti.

Okay, that one was simple. Another?

In this case, one parent is a wild mouse or any other mouse that is homozygous for the dominant Agouti gene (their genotype is AA) and another is a non-agouti mouse (genotypically aa). This is what happened with my own wild hybrids like BB and Cici. Wild mice almost always have the genotype of AA because being brown is a genetic advantage for wild mice; it helps them hide from predators.

As you can see, all of the boxes contain a copy of the A gene. Because agouti is a dominant gene, the phenotypic expression of ALL of these babies will be Agouti - as all of the babies in my hybrid litter were. When two wild mice breed, both are homozygous AA, which results in all of the babies being homozygous AA (just like the non-agouti, only all the a's are now A's). This is why the wild mouse population is brown.

So far these have been simple. 100% of the offspring have been either non-agouti - like the first Punnett Square, or Agouti - like the second Punnett Square.

This one is just a bit more advanced:

In this case, one parent is Agouti, and one is non-agouti.

The agouti parent is NOT homozygous AA in this case, but heterozygous Aa. Where homozygous means the mouse carries two copies of the same gene, heterozygous means the mouse carries two different copies (like Aa instead of AA).

By looking at the Punnett Square, we can predict the ratio of Agouti to non-agouti babies. Any square that contains an A predicts one Agouti baby for every four born. Any square that does not contain an A (the aa's) represents one non-agouti baby for every four born.

As you can see, 2 of the 4 squares contain an A. This means that, theoretically, 2 out of every 4 babies born will be Agouti - or 50% of the offspring will be Agouti.

Just one last example. In this case, both parents are Agouti, but neither is homozygous. They are both heterozygous Aa. How do we know when a mouse is heterozygous? You must have at least some pedigree history to do this. If one of the parents of the mouse does not display the dominant gene (they are genetically aa), then they necessarily have at least one copy of the recessive a.

Using this punnett square can you predict the ratio of Agouti to non-agouti?

What percentage of pups should be (theoretically) Agouti?

We can see that 3 of the 4 squares contain at least one A - thus 3/4 or 75% of the babies will be Agouti, and 1/4 or 25% will be non-agouti. In this case, 1 out of the 4 squares has TWO copies of the A gene, so 25% will be AA - homozygous for Agouti. Any offspring from an AA parent will necessarily be Agouti - remember the wild mouse?

So this is how basic Mendelian inheritance works - for the most part.

With agouti, which resides on the A-locus, there are other genes that can mask agouti. Avy, for example, is viable yellow, aka American brindle. Anytime an Avy gene appears, the baby will be Brindle - whether it shows its stripes or not (unmarked brindles are quite common). Avy is epistatic to (in other words, it suppresses the actions of) even A, so an Avy brindle can be a hidden agouti. This is why some brindle litters have surprise Agoutis. Because A is dominated or supressed by Avy, A is considered hypostatic to Avy.

 
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