Question

Imagine a pedigree that traces an inherited disorder found in individuals with two recessive alleles for gene D. The pedigree shows three siblings with the genotypes DD, Dd, and dd. Did the parents of these three children have the disorder? Explain.

Answer 1

No, the parents don't have a disorder, but they carry recessive allele.

Individuals with the disorder are recessive homozygous (dd). It is known that recessive allele produces its phenotype only when it is paired with another recessive allele. Thus, to have offspring with disorder (with genotype dd), parents must be heterozygous, or one parent is recessive homozygous, and the other one is heterozygous. 

Now, let's imagine both situations:

Parents:            Dd           x              dd
Offspring:       Dd       Dd         dd           dd

In this cross, there is no possibility to have children with genotype DD.

Parents:            Dd           x              Dd
Offspring:       DD       Dd         Dd           dd

In this cross all three genotypes DD, Dd, or dd could be inherited.

It can be concluded that three children with the genotypes DD, Dd, and dd, the parents must be heterozygous which do not have the disorder.