Final answer:
When two heterozygous dogs with drooping ears are mated, 75% of the offspring are expected to have drooping ears because the trait is due to a dominant gene.
Step-by-step explanation:
If two heterozygous dogs with drooping ears are mated, the pups can inherit different combinations of alleles for the ear trait. Drooping ears are due to a dominant allele (let's call it 'D'), while erect ears are due to a recessive allele (let's call it 'd'). Since the dogs are heterozygous (Dd), each will pass either a 'D' or a 'd' allele to their offspring with equal probability.
Using a Punnett square, we can visualize the mating of two heterozygous dogs (Dd x Dd) and determine the expected genotypes and phenotypes of the offspring:
The genotypic ratio will be 1 DD : 2 Dd : 1 dd, which means 1 out of 4 will be homozygous dominant, 2 out of 4 will be heterozygous, and 1 out of 4 will be homozygous recessive. The phenotypic ratio for ear type will be 3 drooping : 1 erect. Therefore, we can expect 75% of the pups to have drooping ears and 25% to have erect ears.