Mokoro
Digital Growth Engine

Biology · Topic

Genetics, inheritance and variation

Show the gametes and the cross; the ratio earns a mark only if the working is visible.

1 original question in the bank, worth 3 marks · still building (1 more to reach the floor)

Drill this topic All genetics, inheritance and variation questions

The questions, with the examiner reasoning

State3 marksBGCSE

Two parents are both carriers of a recessive condition (genotype Bb). State the expected ratio of unaffected to affected children, and explain your reasoning.

Show the mark scheme and the examiner note

Answer: 3 unaffected : 1 affected (genotypes 1 BB : 2 Bb : 1 bb)

  1. Gametes from each parent: B and b
  2. Cross: BB, Bb, Bb, bb
  3. Three have at least one B, so three are unaffected; one is bb and affected

Examiner technique: Draw the cross. A correct 3:1 with no visible gametes and no square is usually awarded fewer marks than the same answer with the working shown.