The woman have heterozygous (Dd) gene so she should have one dominant gene and one recessive gene. When her cells undergo meiosis, the gene would be split and the result of the child would be 50% cell with dominant gene and 50% cell with recessive gene.
Her child that have dominant gene will have 50% chance to inherit the dominant gene into the woman grandchildren. So, the total chance would be: 50% * 50%= 25%
