This is late, but it may be useful in the future if not now.

Some freely downloadable sources for genetics information:
No-frills genetics guide. This is a sticky in the Boa Genetics forum at http://http://www.redtailboas.com/f1...s-guide-53782/. The definitions use a minimum of jargon. They are mostly paraphrases of the definitions in King and Stansfield's A Dictionary of Genetics.

Project on Genetics. See the Contents page at http://www.ringneckdove.com

A Survey of Genetics (4 parts), by Wilmer Miller. See the Contents page at http://www.ringneckdove.com (This book has a page on appropriate gene symbols.)

Mendelism, by R. C. Punnett. http://www.gutenberg.org/ebooks/28775. Printed in 1911 so much of the text is badly out of date. But the pictures are still useful, and they are out of copyright, with no restrictions on their use.

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Before studying genetics, a student should have a grounding in cell division, both mitosis and meiosis. Meiosis explains how a baby gets one member of each gene pair from each parent.

A smart student may ask why humans, mice, and other organisms have thousands of genes, but there is only one gene pair in that Tt example. The scientific method advises using one variable in an experiment and holding everything else constant. But there is no way to breed an organism without having all those other genes present. Geneticists can ignore all but one pair of genes if they make two assumptions:
1. Each gene pair in the parental male and female is homozygous.
2. The parental male and female are genetically identical except in one gene pair.

These two assumptions can trip us up if there are two pairs of mutant genes in a mating when we think there is only one. This is especially true when the effect of one pair of genes masks the effect of the genes in the other pair. And the results of two different genetics studies are difficult to combine unless a stock in one study is the same as a stock in the second study.

To avoid these problems, mouse and fruit fly geneticists use wild type (AKA normal) as the standard. The wild type phenotype is the most common phenotype in the wild population. (In the wild house mouse, the wild type coloration is a brownish color called agouti.) A wild type gene is the most common gene at a given location in the chromosomes in the wild population.

Using wild type simplifies the above assumptions to the following:
1. Each unspecified gene pair in the parental male and female is homozygous wild type.
2. One gene pair in one parent is specified as homozygous mutant.

Those assumptions are technically incorrect in the real world. But they work out in practice because phenotypes act like assemblages of independent characters. The gene that causes albino fur in mice does not affect blood type, and the blood type genes do not affect fur color (or lack of color).

The biochemists say that one gene does not produce what we see as coloration. One mutant gene can change the color or even prevent its formation. But that does not mean that the corresponding normal gene produces the normal coloration by itself. Color production is more like an assembly line with many stations. Many genes provide directions to the assembly line, with each gene telling one or a few stations what to do. Each gene corresponds to one line in the operating procedure book. We look at what rolls off the end of the line. If what comes off is what was expected (normal), then all the machines are getting the standard line from the OP. If something unexpected comes off, there is a typo in least one line. That typo is making the machinery malfunction -- either to do nothing or to do something unexpected.

As there are pairs of genes, there are two copies for each line in the operating procedure. Those may be two copies of the standard line, one copy of the standard line and one with a typo, two copies with the same typo, or two with different typos. Using breeding tests, we can figure out a number of things. Does the malfunction come from a typo in one line in the OP book or in more than one? Do two malfunctions come from different typos in the same line or from typos in two different lines? Are two lines close together in the book? Etc.