Joel is comparing two slides showing chromosomes in a sperm cell and a skin cell of a dog. he finds that the sperm cell has 39 chromosomes. how many chromosomes can he expect in the
skin cell?

78 chromosomes can be expected in the skin cell

Explanation:

The sex cells i.e sperm and ova cells are haploid in dogs. Being haploid means, the number of chromosomes is "n".

While a skin cell in dogs in diploid which means that the number of chromosomes is equal to twice the chromosomes in haploid cells i.e "2n"

Hence, if the total number of  chromosomes in haploid sex cell is "n" or "39"

Then the number of chromosomes in diploid skin cell is equal to "2n"

or

the answer to your question is b