Genetics and Molecular Biology
What this domain tests
Genetics and Molecular Biology tests inheritance patterns, nucleic acid structure and replication, gene expression, mutation, and core molecular techniques such as PCR and gel electrophoresis. Questions often combine probability with mechanism.
Core rules and concepts
- DNA replication is semiconservative: each new double helix has one parental and one new strand. DNA polymerase adds nucleotides to a 3′ end, so synthesis proceeds 5′→3′ on the new strand.
- Transcription makes RNA from a DNA template; translation uses mRNA codons and tRNA anticodons. Ribosomes read codons 5′→3′, and the genetic code is degenerate but not ambiguous.
- Mendel’s law of segregation applies to alleles at one locus; independent assortment applies to unlinked loci. Linked genes deviate from independent ratios and are more likely to be inherited together.
- Mutations are changes in DNA sequence. Point mutations may be silent, missense, or nonsense; frameshifts usually alter all downstream codons.
Worked example
Cross AaBb × AaBb with independent assortment. Find the probability of offspring showing dominant A and recessive b. For A: A_ occurs in 3/4 of offspring. For B: bb occurs in 1/4. Multiply independent probabilities: 3/4 × 1/4 = 3/16. In 320 offspring, expected number = 320 × 3/16 = 60. Reasoning: each locus is treated separately, then probabilities are multiplied because the loci assort independently.
Traps
Do not assume a dominant allele is more common in a population. Do not confuse a test cross with a self-cross. Check whether genes are linked before multiplying probabilities. Remember that PCR amplifies DNA exponentially, while gel electrophoresis separates fragments by size, not by sequence.