Up tp now we have been dealing with situations where we assumed a gene
did something to produce a phenotype that we were able to observe.
This is usually accomplished by a gene being transcribed into an
enzyme that took some precursor chemical and converted it to a
product. In the example of the white vs red flower we have been using
with the R allele producing a red flower and the rr genotype producing
a white flower, we can say that the R allele takes some colorless
precursor and converts it into a red pigment.
We are now going to introduce inhibitor genes that block a process from occurring rather than mediate a process. Inhibitor genes are fairly common, especially in flower pigments, and they can greatly complicate the understanding of how certain traits are inherited. They can even make dominant traits appear as recessive traits.
As a starter, lets take the simple one gene red vs white flower example we have been working with. You, as a hybridizer, have taken a red flower and crossed it to a white flower, grew out the F1, F2 and backcross populations and have proved that this is a simple one gene system with red dominant over white: RR or Rr gives red flowers and rr gives white flowers. However, someone else repeats the red x white cross, but they use a different white flowered parent. Now, in their cross white is dominant in the F1 generation. We can explain this by assuming an inhibitor gene that prevents allele R from converting some colorless precursor to a red pigment. Lets say gene R is responsible for red/white flowers and gene I is an inhibitor gene that prevents the R allele from producing a red pigment. The parents in this cross are then RRii and rrII and the F1's are RrIi. Even though the R allele is present in the F1's, the dominant inhibitor allele I prevents the R allele from functioning, thus, the RrIi phenotype is white. In the original example where red was dominant we would assume the cross to be RRii x rrii where the inhibitor gene was homozygous recessive in both parents, and in this case the dominant I allele is not present to inhibit the R allele.
In the RRii x rrII cross with the RrIi F1's, if we self the F1's we get gametes RI, Ri, rI and ri. By now everyone should be able to figure out the Punnett Square for these gametes, so I will leave that to the reader. The only genotype that can produce a red pigment is R_ii, which is 3/4(R_) x 1/4(ii) = 3/16 R_ii. Thus, when we self these white "dominant" F1's we get a white:red ratio of 13:3. A 13:3 ratio would be very difficult to statistically separate from a 3:1 ratio, and if we didn't know better we would think we were dealing with a "dominant white" gene.
If we backcross the white F1's to the red parent we have the cross RRii x RrIi. In this case, all the gametes from the red parents are Ri, so we only need to look at the gametes from the F1 parents - RI, Ri, rI and ri. Of these 4 gametes the Ri and ri gametes can produce red flowers when combined with the gametes from the red parent. Thus, in this red x white backcross we get a 1:1 ratio for white:red. Mathematically, we can ignore the red gene in the F1's because all the resulting backcross progenies will have the R allele from the red parent. Thus, we only need to look at the segregation of the inhibitor gene in the F1's which is 1/2 I and 1/2 i. Notice that this gives us the same 1:1 ratio that we would expect from a single gene.
If we backcross the white F1's back to the white parent, rrII x RrIi, all the gametes from the white parent will carry the dominant I allele, so all the backcross progenies will be white.
lets assume that we know that red is dominant and that an inhibitor gene can prevent the R allele from producing the red pigment. Here are a few interesting situations that can arise. We can cross two true breeding white flowers and get white F1's. However, when we self or sib mate these F1's we get red flowers! Consider the cross RRII x rrii. In this case the pod parent is white even though the R allele is present because the I allele is also present. In the pollen parent the lack of the inhibitor allele doesn't matter because the R allele is also lacking. Now, the F1's are RrIi, which will be white. Selfing or sib mating these will give a 13:3 ratio for white:red.
Now, here is a case where we can get some real confusion as to which is dominant and which is recessive. Lets say we make the cross RRii x RRII. In this case the pod parent is red because it lacks the inhibitor gene. The pollen parent is white because the inhibitor gene is present. Notice that both parents are homozygous dominant for the red gene, so in effect the red gene is not segregating in this example. Now, when we make the cross we get RRIi F1's, which are white because of the presence of the inhibitor gene. When we self or sib mate these F1's we get a 3:1 ratio for white:red, 3/4(I_) and 1/4(ii), and in the backcross we get the expected 1:1 ratio. From this we would conclude that we have a single dominant gene with white being dominant. We would probably call this gene "W" with the W allele producing white flowers and the w allele producing red flowers. Just the opposite of our original example!
>From this we can make some generalities. If two people are working on the same trait it is easy for them to come to completely different conclusions as to which is dominant and which is recessive if inhibitor genes are present. Also, if a "new" phenotype suddenly appears when you don't expect that phenotype, and then it becomes relatively easy to stabilize that trait, consider the possibility of inhibitor genes being involved.
I will leave this lesson with one further complication. Inhibitor genes can also be temperature sensitive. At high temperatures the inhibitor gene may be active while at colder temperature the gene may be inactive, or vice versa. Temperature sensitive inhibitor genes can help explain why some flowers have a slightly different color in different areas. A flower that opens up in a 80 degree Florida night may well appear to have a slightly different color compared to when that same flower opens up in a 50 degree night in the Pacific Northwest. Many white daylilies have a definite yellow color when they open up under cool conditions. If there seems to be some difference in the expression of a trait in different environments, consider the possibility of temperature sensitive inhibitor genes.