PUNNETT SQUARE PROBLEMS

Incomplete Dominance

When examining the rules of simple dominace, black and white mice, for example, always produce offspring that are either black or white. Another type of dominance, called incomplete dominance, governs some traits and creates heterozygote offspring that have a third phenotype! In incomplete dominance, the heterozygote offspring of black and white mice are actually grey! In another example of incomplete dominance, a homozygous dominant red flower crossed with a homozygous recessive white flower may produce heterozygote flowers that are pink! Red doesn't totally block (dominate) the white color, instead there is incomplete dominance, and we end up with something in-between.

We can still use the Punnett Square to solve problems involving incomplete dominance.  The only difference is that instead of using a capital letter for the dominant trait & a lowercase letter for the recessive trait, the letters we use are both going to be capital (because neither trait dominates the other).  So the cross I used up above would look like this:  

R = allele for red flowers W = allele for white flowers RR= red flower WW= white flower RW= pink flower red x white ---> pink RR x WW ---> 100% RW

Try to look for clues and recognize when you are dealing with a question involving incomplete dominance. Seeing two types of capital letters (R and W) and/or three phenotypes (red, white, and pink) are instant clues that you are not dealing with simple dominance.

1. A cross between a blue blahblah bird & a white blahblah bird produces offspring that are silver.  The color of blahblah birds is determined by just two alleles.
a) What are the genotypes of the parent blahblah birds in the original cross?
b) Describe the genotype(s) of the silver offspring?
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?

2.  The color of fruit for plant "X" is determined by two alleles.  When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit.  What are the genotypes of the parent orange-fruited plants?