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.

Explanation:

As you can see in the picture below, this is a dihybrid cross with a twist.

We can accomplish that mutually parentages are heterozygotes for both characters 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.

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   curly tails, straight tales, tailless

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