Final answer:
When both parents have a recessive phenotype, the genotype for their offspring can only be recessive. This is because only homozygous recessive individuals express recessive traits phenotypically. Therefore, the offspring will inherit two recessive alleles, one from each parent.
Step-by-step explanation:
If both parents have the recessive phenotype, the only possible genotype for their offspring would be recessive as well. This is because recessive traits are expressed phenotypically only when an individual is homozygous for that trait, meaning they carry two copies of the recessive allele. For example, if we consider albinism, where the allele for albinism is recessive, two parents with this trait would pass on the recessive allele to their offspring, resulting in children who are also albino.
Given this concept, the correct answer to the question, 'If both parents have the recessive phenotype, which genotypes are possible for their offspring?' is a. Only recessive.
Additionally, the recessive trait will only be expressed by offspring that have two copies of this allele. If two parents are known to have the recessive phenotype, it's certain that they are homozygous for the recessive allele.