Suppose that in unicorns, two autosomal loci interact to determine the type of tail. one locus controls whether a tail is present at all. the allele for a tail, t, is dominant over the allele for no tail, t. if a unicorn has a tail, then alleles with an unknown dominance relationship at a second locus determine whether the tail is curly or straight. define the alleles for tail texture as us for a straight tail and uc for a curly tail. farmer baldridge has two unicorns with curly tails. when he crosses them, one-half of the progeny have curly tails, one-quarter have straight tails, and one-quarter do not have a tail.

As you can see in the picture below, this is a dihybrid cross with a twist.
We can conclude that both parents are heterozygotes for both traits because the genotypes are 
9 unicorns with curly tails (shown in green)
3 with straight tails (shown in red)
3  curly tales trait carriers with no tail (shown in dark blue)
1  straight tail trait carriers with no tail ( shown in light blue)
If this was an ordinary case of the dihybrid cross, we would get a 9:3:3:1 phenotype ratio.
However, since the first trait, the presence of the tail, is affecting the other trait, the trait texture, the ratio is altered.
1/4 of the offspring (genotypes tt C_   and tt SS) has no tail, therefore we cannot observe the texture of the tail (some of the tailless individuals have Curly alleles -blue squares marked with green, some of them have Straight alleles -blue square marked with red ).

Therefore, the altered ratio is  1/2 curly tails, 1/4 straight tales, 1/4 tailless