Mendelian Genetics

Mendel's first hypothesis was that "Alternative versions of genes account for variations in inherited characters." What he was talking about were alleles, or different versions of genes that affect the same characteristic. His second hypothesis was that, "For each character, an organism inherits two genes, one from each parent". This was refers to the fact that when somatic cells are produced from two gametes, one comes from each parent. The alleles are either the same, or they are different. His third hypothesis, stems directly from the second, "If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance." Finally his fourth hypothesis is that "The two genes for each character segregate during gamete production," which refers to the fact that each reproductive cell has only one of each pair of alleles, which will meet up with its pair when the reproductive opposites meet up and create a new organism.

Mendel's work was with peas, and most schoolchildren learn a little of Mendel's theories in school. However we will go into much greater depth, and apply this information specifically to mice.

Let's take a simple example. An albino mouse has two alleles for albinism labeled "c" and a certain non-albino mouse has two alleles for full color, labeled "C". The capital letters represent dominant alleles, and the lower case letters represent recessive alleles. When the genes are placed in a Punnett square we can see the possible offspring:

You can see from the table that all of the offspring have one full color allele, and one albino allele. We might say that these resulting offspring are all recessive for albino which means that when bred to another mouse with a recessive allele for albino, you may get albino offspring from the pairing. Let’s look at another Punnett square, where two mice that are recessive for albino are crossed:

According to this Punnett square one fourth of the offspring have no albino alleles, one half are recessive for albino, and one fourth are albino. Except for the albino offspring, the rest of the offspring will be identical to one another, due to the effect of the non-albino allele. Let’s look at one more combination, for when a mouse that is albino is crossed to a mouse that is recessive for albino:

In this situation, fifty percent of the offspring are albino, and fifty percent are recessive for albino.

Now lets take a more complicated situation, so as to understand the true complexity of Mendelian genetics. Let’s take an agouti mouse, and a yellow mouse, both recessive for non-agouti. The two loci, or locations of the alleles, that we will discuss will be the a-locus and the b-locus. The mice have the following alleles:

The Punnett squares for the two loci are:

In order to get a true sense of the possible outcomes, a larger Punnett square must be constructed:

As you can see the number of outcomes increases! There are sixteen possible outcomes, and for more complex combinations, this can result in sixteen different looking mice. In this particular case, the result is:

6 out of 16 will be yellow

2 out of 16 will be red

2 out of 16 will be agouti

2 out of 16 will be black

2 out of 16 will be cinnamon

2 out of 16 will be brown

This is an over-simplification of the facts, but makes it possible to see how Mendelian genetics, when applied to the vast number of alleles and loci out there, can result in so many different combinations.