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?

750 offspring

Explanation:

The correct answer would be 750 offspring.

The D allele has been labeled with radioactive substance.

If you cross Dd and Dd:

         Dd    x    Dd

     DD   Dd   Dd   dd

3/4 of the resulting offspring would have, at least, the D allele.

Hence, out of 1000 offspring,

     3/4 x 1000 = 750

750 would possess the radioactive label.

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