Suppose D were dominant and d were recessive, and you could label the D allele in the P generation with a radioactive substance that could be inherited over several generations. You create hybrids by mating your radioactive DD individuals to dd individuals. Finally, you cross two F1 individuals (with the Dd genotype). In 1,000 offspring, how many would you predict would be radioactive?

Question

Suppose D were dominant and d were recessive, and you could label the D allele in the P generation with a radioactive substance that could be inherited over several generations. You create hybrids by mating your radioactive DD individuals to dd individuals. Finally, you cross two F1 individuals (with the Dd genotype). In 1,000 offspring, how many would you predict would be radioactive?

a. 1,000
b. 500
c. 750
d. 250

Answer 1

b. 500

Since D is dominant over d, all offspring of the mating between DD and dd individuals would be Dd and thus express the dominant phenotype. Therefore, all 1,000 offspring from the initial mating would be radioactive. When crossing two F1 individuals (Dd genotype), 50% of the offspring would be Dd and express the dominant phenotype, and therefore be radioactive. So, out of 1,000 offspring, 500 would be radioactive.