Genetics and Molecular Biology
What the domain covers
Genetics and molecular biology tests Mendelian inheritance, gene and allele concepts, linkage, the central dogma, DNA replication, transcription, translation, the genetic code, mutation, and common genetic disorders. You should be able to follow a cross, read a pedigree, and explain information flow from DNA to phenotype.
Core rules and concepts
- Law of segregation: two alleles separate during gamete formation. Law of independent assortment applies to genes on different chromosomes or far apart on the same chromosome.
- A test cross uses a homozygous recessive parent to reveal the genotype of a dominant phenotype.
- Central dogma: DNA → RNA → protein, with replication as information copying. Transcription uses the template strand; translation uses mRNA codons read 5′→3′.
- The code is triplet, non-overlapping, degenerate, and nearly universal; AUG codes methionine and often acts as start.
- Mutations include substitutions, insertions, deletions, and frameshifts; chromosomal changes include deletion, duplication, inversion, and translocation.
- Pedigree clues: dominant traits often appear in every generation; X-linked recessive traits affect males more and are not transmitted father-to-son.
Worked example
Cross Tt × Tt for a dominant trait. Each parent gives T or t with equal probability, so offspring genotypes are TT:Tt:tt = 1:2:1 and phenotypes dominant:recessive = 3:1. The reasoning uses segregation and random fertilisation, not the mistaken idea that three offspring out of four must be dominant in a small family.
Traps
Do not write 3:1 as a genotype ratio. Do not confuse template and coding strands. Do not assume a dominant allele is always more common in the population. Do not treat a frameshift and an in-frame substitution as equivalent.