A level Biology B · Edexcel 9BI0 · Topic 8A
Inheritance and genetic crosses
Key points
- Genotype = alleles present; phenotype = observable characteristic. Homozygous means two identical alleles; heterozygous, two different.
- Monohybrid cross of two heterozygotes gives a 3:1 phenotype ratio; a dihybrid cross of two double heterozygotes gives 9:3:3:1.
- Codominance: both alleles are expressed (e.g. AB blood group). Multiple alleles: more than two alleles exist (I, I, I).
- Sex linkage: genes on the X chromosome; males have one X, so they show recessive X-linked conditions (e.g. haemophilia) more often.
- Autosomal linkage: genes close together on the same chromosome tend to be inherited together, unless separated by crossing over.
- Chi-squared: ; compare with the critical value at and (classes − 1) degrees of freedom.
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Genetic crosses let you predict the offspring of two parents and explain the ratios seen in real families and experiments. You need to handle one gene, two genes, sex linkage and linkage, and test results with chi-squared.
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