Genetic history, pedigree construction and inheritance patterns — formative SAQs

SAQ 1 (10)

A three-year-old boy is referred for global developmental delay. On taking a family history you learn that he has two unaffected older sisters, his parents are first cousins, and his mother had two first-trimester miscarriages. A maternal uncle died in infancy of an undiagnosed illness. You decide to construct a pedigree. ^{[1] [2]}

a) Outline the essential elements of a standardised three-generation pedigree, naming the minimum generations and the specific items you would document on each parental line. (3 marks) ^{[1] [2]}

b) Describe the standardised pedigree symbols for: a male, a female, an affected individual, a carrier, a deceased individual, the proband, and consanguinity. (3 marks) ^[1]

c) Given that the parents are first cousins, state the approximate additional risk of a serious genetic or congenital disorder in their offspring over the population baseline, and explain the genetic reason. (2 marks) ^[3]

d) Explain why an isolated affected child does not exclude autosomal recessive inheritance, naming two reasons. (2 marks) ^[4]