What would cause a change in the phenotype of the offspring that was NOT present in the parent?

Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism. Often, it is impossible to determine which two alleles of a gene are present within an organism's chromosomes based solely on the outward appearance of that organism. However, an allele that is hidden, or not expressed by an organism, can still be passed on to that organism's offspring and expressed in a later generation.

littering streams           is the answer

i think that d. wind is the answer.   i missed the question when i answered a. currents.