In the pedigree shown, we can see that 2 parents that are unshaded shapes have 2 children that are shaded shapes. This would be impossible if shaded shapes were dominant and unshaded shapes were recessive. This is because parents that have a recessive trait have 2 of the same alleles. So if 2 parents that both have a recessive trait have a child, the child will definitely have that trait. But we can see that this is not the case, the parents have 2 offspring that have different phenotype from the parents. This must mean the parents are heterozygous, meaning that they both have 2 alleles that are different. The dominant allele masks the recessive allele, so the dominant allele is expressed. In this case, the children inherited 2 recessive alleles, 1 from each parent, making them have a different phenotype from the parents.
