Name: Sex Determination (Drosophila melanogaster)
Uploaded: 2019-07-19
Duration: 7 min 20 s
Description: Drosophila melanogaster or the common fruit fly has contributed a lot to genetics. One of them is its sex determination system with more than five types of s...

Question

In Dalmatian dogs, the allele for black spots is dominant to the allele for liver spots. If a breeder has a black-spotted dog, how can he or she find out whether it is homozygous or heterozygous for this characteristic? Use genetic diagrams to explain the answer.

Accepted Answer

An organism that shows a dominant characteristic could have either of two possibilities. It could be homozygous for the dominant allele or heterozygous. Example: In Dalmatian dogs, the allele for black spots (B) is dominant to the allele for liver spots (b). The black-spotted dog has the genotype BB or Bb. It can find out the genotype of an individual with the dominant phenotype for a particular gene by crossing it with one known to have the homozygous recessive genotype for the same gene. This is called test cross. Hence, it can be possible to find out the genotype of the black-spotted dog by crossing it with a liver-spotted one.

The test cross is shown below:

If the black-spotted dog is heterozygous:

If the black-spotted dog is homozygous:

Based on the results, if any of the offspring show recessive phenotypic character (liver-spotted), then it must mean that the black-spotted dog has an allele for liver-spot. Hence, it should be heterozygous. If none of the dogs is liver-spotted, it means that the black-spotted parent dog is homozygous for the black-spot allele.

In Dalmatian dogs, the allele for black spots is dominant to the allele for liver spots. If a breeder has a black-spotted dog, how can he or she find out whether it is homozygous or heterozygous for this characteristic? Use genetic diagrams to explain the answer.

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