Level 5: A rancher buys a brown bull with curved horns, but needs to know the bull's genotype for breeding purposes. He knows that brown is dominant to black in these cows. 1: How can the rancher determine (find) the genotype of this bull? 2: What other information does the rancher need, if any, to determine the bull's genotype?

Explanation:

1. Since brown is dominant to black, in order for the farmer to determine the genotype of the brown bull, he will need to test-cross the bull with a black bull. The phenotypes of the offspring from the testcross will determine whether the brown bull is a true-breeding one or a heterozygous one.

If all the offspring from the testcross are brown, it means the bull is a true-breeding brown. If the offspring is a mix of brown and black, it means the bull is heterozygous for the brown genotype.

To further illustrate the explanation, let us assume that the brown allele is B and the black allele is b. B is dominant over b, hence, a brown bull can BB or Bb while a black bull can only be bb.

The rancher is not sure if the bull he bought is BB or Bb, hence, he needs to testcross with another bull that is bb. Assuming that the rancher's bull is BB

           BB     x     bb

       Bb    Bb    Bb    Bb (all brown)

Assuming the rancher's bull is Bb

            Bb     x     bb

        Bb    Bb    bb    bb (50% brown, 50% black)

2. All the information that the rancher needs have been explained above. No other information is needed

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