Genetics: Information, Inheritance and Variation
What you must understand
Genetics runs from molecules to populations. At the molecular level: DNA is a double helix with complementary base pairing (A–T, G–C), replication is semi-conservative, and the genetic code is triplet-based and degenerate. Transcription makes mRNA; translation at the ribosome builds protein. Mutations (substitution, insertion, deletion) create new alleles; frameshifts from indels are usually more damaging than substitutions.
Inheritance: know genotype vs phenotype, dominant vs recessive, homozygous vs heterozygous, and the laws of segregation and independent assortment. Be fluent with monohybrid crosses (3:1 phenotypic, 1:2:1 genotypic from heterozygote × heterozygote) and with test crosses. Sex-linked traits behave differently in males and females because males are hemizygous for X-linked genes. Beyond Mendel: incomplete dominance, codominance, multiple alleles (ABO blood groups), polygenic traits.
Traps
The 3:1 ratio assumes independent assortment, complete dominance and no lethality - do not apply it blindly. A recessive phenotype is not "rare"; it is simply masked in heterozygotes. In pedigrees, an unaffected couple with an affected child indicates a recessive trait, but does not by itself distinguish autosomal from X-linked - check affected daughters and the sex ratio. Punnett squares give probabilities per offspring, not guarantees.
Worked example
Two heterozygous tall pea plants (Tt × Tt). Offspring genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short. Probability that a tall offspring is heterozygous = 2/3, because among the three tall outcomes, two are Tt. This conditional-probability twist is a classic trap.