Biology 30: Cells, Genetics, and Molecular Biology
Core concepts
Biology 30 tests how genetic information flows from DNA to trait. You must know the structure of DNA (sugar-phosphate backbone, complementary base pairing A–T and G–C), the stages of mitosis and meiosis, and how meiosis plus fertilization generates variation. Be able to distinguish genotype from phenotype, dominant from recessive, and to predict outcomes with Punnett squares and pedigree analysis. Molecular topics include transcription in the nucleus, translation at the ribosome using mRNA codons and tRNA anticodons, and mutation types (substitution, insertion, deletion) with their effects.
Traps to watch
- Mitosis vs. meiosis: meiosis has two divisions and halves chromosome number; mitosis preserves it.
- Crossing over occurs in prophase I and creates new allele combinations; independent assortment occurs in metaphase I.
- Dominance is not frequency: a dominant allele need not be common.
- Pedigree logic: unaffected parents with an affected child indicate a recessive trait; every affected child of an affected parent suggests dominance.
- Reading codons: always translate mRNA codons, not the DNA template strand, unless told otherwise.
Rule of thumb
Use a systematic checklist: identify the trait, assign alleles, determine parent genotypes, then predict ratios. For a monohybrid cross of two heterozygotes (Aa × Aa), the expected phenotype ratio is 3:1 and the genotype ratio is 1:2:1. For a testcross, cross an unknown dominant phenotype with a homozygous recessive to reveal its genotype.
Worked example
Two heterozygous parents (Aa × Aa) have a child. Probability of a recessive phenotype is the aa box: 1 of 4, or 25%. Probability of a dominant phenotype is 3 of 4, or 75%. If the trait is sex-linked recessive, remember males express it with one copy while females need two.
Molecular flow
DNA → mRNA → protein. Each step is a candidate for mutation questions; a frameshift from an insertion or deletion usually changes more amino acids than a substitution.