The pedigree belоw shоws the inheritаnce оf а dominаnt allele of a gene in a family over several generations. Circles represent females and squares represent males. Shaded symbols indicate individuals carrying the allele. The figure shows 4 generations of a pedigree. From top to bottom, they are as follows: Generation 1: one shaded circle with one unshaded square. Generation 2: on the left: one offspring unshaded circle with one unshaded square added; on the right: one offspring shaded circle with one unshaded square added. Generation 3: on the left: three offspring unshaded squares and one offspring unshaded circle; on the right: one offspring shaded circle, one offspring unshaded circle, and one offspring shaded square with one unshaded circle added. Generation 4: on the left: no offspring; on the right: one offspring unshaded square, one offspring unshaded circle, and one offspring shaded circle. The pedigree suggests that the gene is on a nuclear chromosome, and not on mitochondrial DNA, because