The correct answer is C. The plants had to be heterozygous, which means they expressed the dominant trait but also had an allele for the recessive trait. This is why it was expressed in the F2 generation. The homozygous dominant plants would not have contributed any recessive alleles to future generations. Segregation refers to the separation of alleles during gamete formation and does not refer to the type of allele. Autosomal dominant refers to a genetic trait or disorder that is passed on via non-sex chromosomes, so it does not account for the occurrence of recessive traits in Mendel's F2 generation.

C is the correct. answer. This law shows that parts of genes at different locations on chromosomes are distributed independently of each other during the formation of gametes. All traits arise from the pair of alleles inherited (one from each parent), but this is not relevant to Mendel's second law. Genes do separate into single alleles during gamete formation, but this states Mendel's principle of segregation, not his second law. Some traits are controlled by a single gene (consisting of two alleles), but this is not relevant to Mendel's second law.

B is the correct answer. The phenotype consists of the visible traits of an individual, which gain expression from dominant genes that may mask the presence in the genotype of recessive genes. Since the recessive genes are present but unexpressed, only part of the genotype is expressed in the phenotype. The phenotype does express the genotype, just not all of it. An individual homozygous for a recessive trait will express that recessive trait. Traits arise from pairs of alleles in gametes.

D is the correct answer. The second parent would have two dominant genes (SS) that mask the recessive gene of the first parent in order for their offspring not to have the disease. A heterozygous parent would have one recessive gene (Ss), so there would be a chance that some offspring would have the disease. If the second parent was homozygous recessive (ss), it is highly probable that some of their offspring would be affected. An individual cannot be heterozygous dominant or recessive because heterozygous means having two different forms of the allele.