Step Seven: Answer the question you've been asked.

The mating scheme we've just worked through is called a monohybrid cross. This means that we were paying attention to only one gene (mono=1), and both of our parents were heterozygous for that gene (hybrid=heterozygous).

Here again are the steps we followed:

Step 1:	Figure out the genotypes of the parents.
Step 2:	Figure out what kinds of gametes the parents can produce.
Step 3:	Set up a Punnett Square for your mating.
Step 4:	Fill in the babies inside the table by matching the egg allele at the top of the column with the sperm allele at the head of the row.
Step 5:	Figure out the genotypic ratio for your predicted babies.
Step 6:	Figure out the phenotypic ratio for your predicted babies.
Step 7:	Answer the question you've been asked.

Of course, the alleles you are using don't always have a complete dominance relationship. Other dominance relationships don't affect any of the early parts of the problem — they only enter the calculations when it comes time to look for the phenotypic ratio. Here's how you handle the most common situations:

Incomplete Dominance: If two alleles have incomplete dominance, the heterozygote has a phenotype which is intermediate between the phenotypes of the two parents. For example, in many flowering plants, alleles for red and white flowers have an incomplete dominance relationship. If C^R is the allele for red flowers and C^W is the allele for white flowers, a C^RC^R plant will have red flowers, a C^WC^W plant will have white flowers, and a C^RC^W plant will have pink flowers.

If our gerbil problem involved two pink flowering snapdragons, instead of two brown gerbils, we would have exactly the same kind of mating as we've just been through. You'd use different symbols for the alleles — capital and lower case letters usually predispose you to think in terms of complete dominance — but the procedure for working the problem would be precisely the same as the gerbil problem, right up to that moment when you ask yourself that phenotype question, "Do any of these genotypes produce the same phenotypes?" In this case, your answer would be, "No." This heterozygote has its own phenotype. So your phenotypic ratio would be 1 Red:2 Pink:1 White.

There is another dominance relationship called co-dominance which works pretty much the same as incomplete dominance does. In co-dominance, the heterozygote expresses both of the alleles, essentially independently of each other. That's where blood type AB comes from — a heterozygote between the A-type allele and the B-type allele, which are co-dominant to each other. Again, these problems are solved exactly like our gerbils, except when it comes to figuring out the phenotypic ratio.

Lethal Genotypes (Pseudodominance): In some cases, one of the three possible genotypes creates a condition which is lethal. In other words, the embryo which is conceived with that genotype doesn't survive to birth. Once again, problems involving lethal genotypes are worked exactly like our brown and black gerbils above until we get to the phenotypes. In fact, we can illustrate this situation with a different gerbil gene.

Besides the fur color gene we dealt with earlier, gerbils also have a gene which can cause the gerbil to have white markings on his or her fur. Gerbils who have no white markings are homozygous for the wild type solid color allele; gerbils which are marked with white patterns are heterozygous. The homozygous white-marked genotype is lethal; these gerbils don't survive the development period, and are not among the babies delivered when a litter is born.

If two white spotted gerbils mate, this is obviously a monohybrid cross, since both are automatically heterozygous. To figure out the prospects for their offspring, you solve the problem precisely the same way you solved the brown/black problem for Honey and Ritz. Again, the difference comes when you figure out the phenotypic ratio.

Since homozygous white-marked gerbils are simply missing from the babies who are actually born, you have to subtract them from the final phenotypic ratio. Thus, that ratio becomes: 2 white-marked gerbils:1 solid gerbil. Or 2/3 white marked, 1/3 solid.

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