Genetics and Molecular Biology
Genetics and Molecular Biology
This subarea tests information flow from DNA to phenotype and the inheritance patterns that explain phenotypic ratios. Questions commonly combine replication, transcription/translation, mutation effects, meiosis, and Mendelian or non-Mendelian crosses, with some biotechnology lab reasoning.
Core concepts
- Information flow: DNA replication is semiconservative; helicase unwinds, DNA polymerase adds nucleotides 5′→3′, and the lagging strand is made in Okazaki fragments. Transcription produces mRNA in the nucleus; RNA processing adds a 5′ cap and poly-A tail and removes introns. Translation at the ribosome reads codons 5′→3′ and uses tRNA anticodons.
- Mutations: silent mutations do not change the amino acid; missense changes one amino acid; nonsense creates a stop codon; frameshift mutations alter the reading frame and usually have larger effects. Only germline mutations are heritable.
- Inheritance: Mendel's laws of segregation and independent assortment apply to unlinked genes. Use a testcross to reveal an unknown genotype. Non-Mendelian patterns include incomplete dominance, codominance, multiple alleles, sex linkage, and epistasis.
- Meiosis and chromosomes: crossing over in prophase I and independent assortment in metaphase I create variation; nondisjunction produces aneuploid gametes.
Worked example
In a cross between two individuals heterozygous for two independently assorting genes, AaBb × AaBb, what is the probability of offspring that are aabb? Treat each gene separately: Aa × Aa gives aa with probability 1/4; Bb × Bb gives bb with probability 1/4. Because the genes assort independently, multiply: 1/4 × 1/4 = 1/16. The common error is to double the heterozygote probability or to assume linkage without evidence. If the genes were linked, the 1/16 expectation would not hold.
Traps
Do not confuse transcription (DNA→RNA) with translation (RNA→protein); dominant does not mean more common; a Punnett square gives probabilities, not guaranteed ratios in small families; and a mutation in a somatic cell is not passed to offspring.