Answer:
an autosomal recessive disorder
Step-by-step explanation:
Autosomal recessive disorders are inherited diseases caused by the presence of two copies (alleles) of an abnormal gene, thereby only homo-zygous recessive individuals are affected by this type of disease. In consequence, although both parents don't express the disease, they can be carriers and pass this recessive disorder to their offspring who express the disease. In this case, the pedigree shows that heterozygous parents are not affected but they are carriers of the recessive allele for this disease, thereby some offspring express the trait. The black circle (female) and the black square (male) indicate affected individuals in the pedigree chart. Since both parents are heterozygous, 1/4 (25%) of all offspring would be expected to have the autosomal recessive disorder.