GCSE Biology · Edexcel 1BI0 · Topic 3D
Inheritance
Key points
- Mendel bred pea plants and showed that characteristics are passed on in units (now called genes), even though DNA was unknown at the time.
- Alleles are different versions of a gene. A dominant allele shows when one copy is present; a recessive allele only shows with two copies.
- Homozygous means two identical alleles; heterozygous means two different alleles. Genotype is the alleles; phenotype is the characteristic shown.
- Use Punnett squares to predict ratios, e.g. Aa × Aa gives 3 : 1 dominant to recessive phenotype, and probabilities such as 25%.
- Sex: females are XX, males XY, so there is a 50% chance of each sex.
- Higher tier: sex-linked disorders (e.g. colour blindness) are on the X chromosome, so they are more common in males. Blood groups show codominance and multiple alleles (, , ).
- Family pedigree diagrams track a disorder through generations.
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Inheritance is about how alleles pass from parents to offspring and how to predict the results. This sub-topic covers Mendel's work, the key vocabulary, genetic diagrams, sex determination and family pedigrees, with sex-linked inheritance…
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