Exam Tips: Sex Determination

Father determines sex — state this explicitly: Examiners want to see "the father's sperm determines the sex of the child because the father produces X-bearing and Y-bearing sperm in equal numbers." Do not say "the mother determines sex" even if the question tries to mislead you.

Genetic diagram method marks: Always write parent genotypes, then gametes, then the Punnett square, then offspring genotypes, then offspring phenotypes. Each step can carry individual marks.

X-linked disorders — why males are affected more: Males are hemizygous for X-linked genes (only one copy). A single recessive allele on the X chromosome causes the condition in males. Females need two copies. Always state this when explaining X-linked inheritance.

Higher tier notation: Write X-linked alleles as superscripts on X: X^H (normal allele) and X^h (haemophilia allele). Do not use plain letters for X-linked questions — examiners need to see the allele is on the X chromosome.

Each pregnancy is independent: Previous children's sexes do not change the probability for the next child. Probability is always 50:50 per pregnancy regardless of family history.

Edexcel pedigree charts for X-linked conditions: Edexcel 1BI0/1 frequently uses three-generation pedigree charts as stimulus material for X-linked questions. Practice reading these: circles = females, squares = males, shaded = affected, half-shaded = carrier female. Work from the pedigree data to deduce genotypes before writing your Punnett square — Edexcel mark schemes reward logical deduction steps.