To better discuss genetic principles, we need to introduce some new terms. Look up each of the following terms in the glossary:

• Phenotype
• Genotype
• Gene
• Allele
• Wild Type
• Homozygote
• Heterozygote
• Product Law

Another thing to know is the Punnett square. This tool is used to predict the probability of genotypes and phenotypes occurring in the offspring of a particular mating or cross. To use a Punnett square, Take the parental genotypes, and determine all of the possible combinations of alleles that could occur in gametes. List one parent's gametes across the top of the square, and the other parent's gametes down the side of the square. Then, within the square, combine all of the possible gametes from each parent to produce all of the possible combinations of alleles in the offspring. I'll illustrate this by showing a Punnett square of the tall/dwarf example we considered:

In this case, only one characteristic (plant height) is being considered. Both parents are heterozygous, and each gamete will receive one allele from the parent, D or d. This is true of both parents in this case. When gametes are combined to form new individuals (in the square), we see that 1/4 of the offspring should be DD, 2/4 (or 1/2) should be Dd, and 1/4 should be dd. These are the possible genotypes of the next generation, and the probabilities of each occurring. This illustrates the genotypic ratio of the monohybrid cross - 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. Because tall (D) is dominant to dwarf (d), 3/4 of the offspring should be tall , and 1/4 should be dwarf. This illustrates the phenotypic ratio of the monohybrid cross - 3 dominant : 1 recessive.

The Test Cross

One last tool to consider: If you perform the above monohybrid cross, and get the expected 3/4 tall plants and 1/4 dwarf plants, how can you tell which of the tall plants are homozygous and which are heterozygous? There's no way to tell just by looking at the plants. To tell for sure, Mendel devised a test cross. To do a test cross, you take the individual in question, and cross it to a homozygous recessive individual (test crosses are always done using homozygous recessives). To see how this works, lets consider our example again. Look at the possible results of testcrosses involving a homozygous dominant, and involving a heterozygote:

So if you take a tall plant, and cross it with a homozygous recessive, and all of the offspring are tall plants, then you know that the tall plant in question is homozygous. If, on the other hand half of the offspring are tall and half are dwarf (a 1 : 1 ratio), then the tall plant being tested is heterozygous.