(A Level) Inheritance

Learning objectives

Explain the terms haploid, diploid and homologous pair, why gamete production needs a reduction division, and how crossing over, independent assortment and random fertilisation make offspring genetically different.

Describe what chromosomes, the spindle, the nuclear envelope and the cell surface membrane do from prophase I to telophase II of meiosis, and identify the stages in photomicrographs and diagrams.

Construct and interpret genetic diagrams and Punnett squares for monohybrid and dihybrid crosses involving dominance, codominance, multiple alleles, sex linkage, autosomal linkage and epistasis, and for test crosses, using the terms locus, allele, genotype, phenotype, homozygous, heterozygous, F1 and F2.

Use the chi-squared test, χ² = Σ(O − E)²/E, with degrees of freedom and the critical value at p = 0.05 to decide whether observed and expected results differ significantly.

Explain how genes determine phenotype through the proteins they code for, using TYR and albinism, HBB and sickle cell anaemia, F8 and haemophilia, HTT and Huntington's disease, and the Le/le alleles that control gibberellin synthesis and stem length.

Explain how gene expression is controlled: structural and regulatory genes, repressible and inducible enzymes, the lac operon in E. coli, transcription factors in eukaryotes, and gibberellin triggering the breakdown of DELLA repressor proteins.