Crosses involving sex-linkage, autosomal linkage, multiple alleles and epistasis
Some genes break the simple rules of inheritance. They can sit on sex chromosomes, travel together on the same chromosome, exist in more than two versions, or have one gene switch another gene off entirely.
Real World
Haemophilia A is an X-linked recessive disorder that affected several descendants of Queen Victoria, passing through carrier females to males across European royal families including those of Spain and Russia.
Exam Focus
Always show full genotypes with sex chromosomes (e.g. X^H X^h) in genetic diagrams — examiners penalise bare letters without the X.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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