Genetics and Heredity
What you must understand
Genetics studies inheritance. Know the vocabulary: gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive, codominance, incomplete dominance. Understand Mendel's laws: segregation (alleles separate during meiosis) and independent assortment (genes on different chromosomes assort independently). Be able to complete monohybrid and dihybrid crosses using Punnett squares and predict phenotypic and genotypic ratios. Know sex-linked inheritance (e.g., colour blindness and haemophilia are X-linked recessive) and the difference between autosomes and sex chromosomes. Understand mutations as changes in DNA sequence and their possible effects. Know the difference between the terms phenotype ratio and genotype ratio.
Traps that appear in questions
- Forgetting that a recessive phenotype appears only when homozygous.
- Mixing up genotype (e.g., Aa) with phenotype (e.g., tall).
- Assuming a dominant allele is always more common in a population - dominance and frequency are independent.
- Treating X-linked recessive traits as if males can be carriers - they cannot (they have one X).
- Confusing codominance (both alleles expressed, e.g., AB blood) with incomplete dominance (blended phenotype).
Worked example
Cross two heterozygous tall plants (Tt × Tt). Punnett square gives TT, Tt, Tt, tt. Genotypic ratio 1:2:1; phenotypic ratio 3 tall : 1 short. Rule of thumb: for a monohybrid cross of two heterozygotes, expect 3:1 phenotype and 1:2:1 genotype.