Overview
This lecture covers key concepts in inheritance for A-level Biology, including genotypes, phenotypes, genetic crosses, linkage, epistasis, and the chi-squared test, with worked exam-style questions and mark scheme explanations.
Genotype, Phenotype & Alleles
- Genotype refers to the genetic constitution (combination of alleles) of an organism.
- Phenotype is the observable expression of the genotype, influenced by the environment.
- An allele is a variant form of a gene.
- Multiple alleles exist when a gene has more than two forms (e.g., eye color).
- Mutations at different loci cause multiple alleles for one gene.
Dominance, Codominance, Homozygotes & Heterozygotes
- Homozygous individuals have two identical alleles at a locus; heterozygotes have two different alleles.
- Dominant alleles are expressed if present; recessive alleles are only expressed when homozygous.
- Codominance occurs when both alleles are equally expressed (e.g., blood group AB).
Monohybrid & Dihybrid Crosses
- Monohybrid crosses involve one gene; use Punnett squares to predict genotype/phenotype ratios.
- In monohybrid crosses, crossing two heterozygotes gives a 3:1 ratio of dominant:recessive phenotypes.
- Dihybrid crosses involve two genes; crossing heterozygotes yields a 9:3:3:1 ratio of phenotypes.
Genetic Linkage
- Autosomal linkage: genes on the same non-sex chromosome are inherited together unless separated by crossing over.
- Sex linkage: genes on the X chromosome show different inheritance patterns in males (XY) and females (XX).
Epistasis
- Epistasis is when one gene masks or suppresses the expression of another gene.
- Epistatic gene: masks expression; hypostatic gene: is masked.
- Recessive epistasis alters expected ratios to 9:3:4 instead of 9:3:3:1.
Chi-Squared Test
- The chi-squared test checks if observed genetic ratios differ significantly from expected ratios.
- Null hypothesis states any difference is due to chance.
- Use formula: χ² = Σ[(O−E)²/E]; O = observed, E = expected.
- Compare χ² to critical value (usually at p=0.05) to determine significance.
Exam-Style Questions & Approaches
- For sex-linked traits, recessive alleles are more likely in the sex with one copy of the X chromosome.
- Pedigree diagrams are used to infer genotypes and dominance/recessiveness.
- Always refer to alleles, not genes or chromosomes, in exam answers when discussing inheritance.
- Mark schemes require specific reference to alleles and genotypes for full marks.
Key Terms & Definitions
- Genotype — The genetic makeup (alleles) of an organism.
- Phenotype — The observable traits resulting from the genotype and environment.
- Allele — A different form of a gene.
- Homozygous — Having two identical alleles at a locus.
- Heterozygous — Having two different alleles at a locus.
- Dominant allele — Expressed if present in the genotype.
- Recessive allele — Expressed only when homozygous.
- Codominance — Both alleles are equally expressed in the phenotype.
- Autosomal linkage — Genes located on the same non-sex chromosome.
- Sex linkage — Genes located on a sex chromosome (usually X).
- Epistasis — One gene affects the expression of another.
- Chi-squared test — Statistical method to compare observed and expected genetic ratios.
Action Items / Next Steps
- Review Punnett square practice for monohybrid and dihybrid crosses.
- Memorize key phenotype ratios (3:1, 9:3:3:1, 9:3:4).
- Practice applying the chi-squared test with sample data.
- Review past paper questions on sex linkage and pedigree analysis.