Question

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.

Answer 1

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

Answer 2

Let's represent the locus representing the presence of a tail as T (tail) and t (no tail) while the texture of the tail as C (curly) and S (straight). We also know that T is dominant to t.

Three-fourths of the progeny yielded a tailed horse, presenting at least one dominant T allele for the unicorns, being either homozygous or heterozygous (TT or Tt). The remaining one-fourth unicorn (tailess) is homozygous recessive (tt). This phenotypic ratio of 3:1 is consistent with the heterozygous cross of Tt x Tt.

The tail texture is slightly more complicated with not knowing which allele is dominant. We were given a ratio of 2:1 of curly-tail to straight-tail: a slight variation of the typical genotypic ratio of 1:2:1 in a normal progeny between two heterozygous parents. The variation suggests that one genotype is missing within the progeny. Given that there more curly-tail than straight tail progeny, this suggests that C is dominant over S. This also suggests that homozygous dominant allele for curly tail (CC) is lethal.

The genotypes of the parents then can be concluded with both being heterzygous to both locus (TtCS x TtCS)

The progeny ratio would then be

6/16 : T- ,CS

3/16 : T- ,SS

3/16 : tt ,S-

4/16 : Lethal combination (CC included in allele)