Cell Biology and Genetics: From Organelles to Inheritance
Cell structure and division
Know the plant cell wall (middle lamella, primary and secondary walls, plasmodesmata), plastids (chloroplast, chromoplast, leucoplast), vacuole, Golgi, ER and cytoskeleton. Understand the cell cycle, mitosis and meiosis, and the differences between plant and animal cytokinesis (cell plate versus cleavage furrow). Learn the endosymbiotic origin of mitochondria and plastids.
Mendelian and non-Mendelian genetics
Master monohybrid and dihybrid ratios, test crosses, backcrosses and probability rules. Understand incomplete dominance, codominance, multiple alleles, lethal genes and epistasis. Know linkage, crossing over, recombination frequency and chromosome mapping. Learn sex determination in plants, cytoplasmic inheritance, and the chromosomal basis of inheritance.
Mutation and variation
Distinguish point mutations, frameshifts, chromosomal aberrations (deletion, duplication, inversion, translocation) and polyploidy. Understand autopolyploidy versus allopolyploidy and their role in crop evolution.
Traps
Examiners test whether a ratio is phenotypic or genotypic, and whether epistasis changes the expected 9:3:3:1. Another trap is confusing linkage with pleiotropy, or assuming cytoplasmic inheritance follows Mendelian ratios. Watch for questions on meiosis where crossing over is said to occur in mitosis.
Rule of thumb
Write the parental genotypes, draw the gamete array, then combine. For any ratio question, state whether you are counting phenotypes or genotypes before calculating.
Worked example
A dihybrid cross with independent assortment gives a 9:3:3:1 phenotypic ratio. If a recessive epistatic gene masks one locus, the ratio shifts to 9:3:4 - a common exam trap.