A male heterozygous for the cystic fibrosis (CF) mutation 'G542X' marries a female heterozygous for the CF mutation 'G551D'. The G542X mutation causes defective CFTR protein (an ATP- and phosphorylation-dependent chloride channel) production, while the G551D mutation causes defective regulation of CFTR protein. Determine the probability of the couple having an affected child.