Genetics and Heredity
What you must understand
Master Mendel's laws: uniformity of the F1 generation, segregation of alleles, and independent assortment of genes on different chromosomes. Know the vocabulary: gene, allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive, codominance, incomplete dominance. Be able to use Punnett squares for monohybrid and dihybrid crosses and predict phenotype ratios (3:1 monohybrid, 9:3:3:1 dihybrid). Understand sex-linked inheritance (X-linked recessive traits affect males more) and the difference between autosomes and sex chromosomes. Know mutations (gene, chromosomal, genomic), karyotype, and the basics of human genetic disorders (e.g. Down syndrome = trisomy 21). Understand that meiosis halves chromosome number and creates genetic variation through crossing-over and independent assortment.
Traps candidates fall into
- Confusing genotype (alleles present) with phenotype (observable trait).
- Forgetting that a recessive trait appears only when homozygous recessive.
- Mixing up mitosis (identical daughter cells, 2n → 2n) with meiosis (four haploid gametes, 2n → n).
- Assuming a dominant allele is "more common" - dominance is about expression, not frequency.
- Missing that X-linked recessive disorders rarely affect daughters unless the father is also affected.
Worked example / rule of thumb
Cross two heterozygotes Aa × Aa: offspring genotypes are 1 AA : 2 Aa : 1 aa, so phenotype ratio is 3 dominant : 1 recessive. Rule of thumb: count the capital letters - at least one dominant allele gives the dominant phenotype for a simple dominant/recessive trait. For a dihybrid AaBb × AaBb, multiply the independent 3:1 ratios to get 9:3:3:1.