B. 1:2 – Explanation
Pay special attention to the final line of the question. In this question they arespecifically
asking for chances that this boy is a carrier (not an affected child with cysticfibrosis). In
other stems and questions, they may ask for the chance that this boy willhave the
disease.
Both parents here would have to be carriers in order to produce a child with
cysticfibrosis as it is an autosomal recessive disease.
Therefore, the chances of having:
– An unaffected child is 1:4
– An unaffected ‘carrier’ child is 1:2
– An affected child is 1:4
asking for chances that this boy is a carrier (not an affected child with cysticfibrosis). In
other stems and questions, they may ask for the chance that this boy willhave the
disease.
Both parents here would have to be carriers in order to produce a child with
cysticfibrosis as it is an autosomal recessive disease.
Therefore, the chances of having:
– An unaffected child is 1:4
– An unaffected ‘carrier’ child is 1:2
– An affected child is 1:4
