Question

drawing a pedigree

• this will be a recessive pedigree
• three generations
• parents of generation 1 have three kids in this order: boy, boy, girl
• boy 1 gets married and they have two daughters
• the two sisters have a recessive disease.
• no-one else has the disease.
• the grandmother got tested and was found to be homozygous dominant.
• write the genotype for everyone you can possible identify, using letter d,d

Explanation:

Step1: Define recessive disease rules

Recessive disease requires genotype $dd$ to express phenotype. Unaffected individuals have at least one $D$.

Step2: Set Gen 1 Grandmother genotype

Given grandmother is homozygous dominant: $DD$

Step3: Deduce Gen 1 Grandfather genotype

Since no other family member has disease, but his son (Boy 1) must pass $d$ to affected daughters. Grandfather must be $Dd$ (to pass $d$ to Boy 1, while being unaffected).

Step4: Deduce Gen 1 children genotypes

Parents: $DD$ (grandmother) × $Dd$ (grandfather). Offspring genotypes:

• Boy 1: Must be $Dd$ (he passes $d$ to affected daughters)
• Boy 2: $DD$ or $Dd$ (unaffected, no disease-linked offspring)
• Girl 1: $DD$ or $Dd$ (unaffected, no disease-linked offspring)

Step5: Deduce Boy 1's spouse genotype

Boy 1 ($Dd$) and spouse have two $dd$ daughters. Spouse must pass $d$ to each daughter, so spouse is $Dd$ (unaffected, so has one $D$).

Step6: Deduce affected daughters' genotypes

Affected recessive disease = $dd$

Answer:

• Generation 1:
• Grandmother: $DD$
• Grandfather: $Dd$
• Generation 2:
• Boy 1: $Dd$
• Boy 2: $DD$ or $Dd$
• Girl 1: $DD$ or $Dd$
• Boy 1's Spouse: $Dd$
• Generation 3:
• Boy 1's Daughters (both): $dd$