Answer:
D. If all of the offspring are short-tailed, the unknown individual is the homozygous dominant genotype.
Explanation:
The short tail phenotype is completely dominant over the long tail phenotype. When a short tailed salamander with unknown genotype is crossed with a homozygous recessive long tailed salamander, the cross is called testcross. Lets assume that the allele "S" gives "short tail" phenotype while the allele "s" gives "long tail" phenotype.
If the "short tailed salamander" with unknown genotype is homozygous dominant (SS), the process of the testcross will express only dominant trait (cross 1). On the other hand, if the "short tailed salamander" with unknown genotype is heterozygous dominant (Ss), segregation of alleles during gamete formation would form two types of gametes (cross 2). The 50% progeny would express the "long tail" phenotype while rest 50% will express the "short tail" phenotype.
