Genetic Basis of Inheritance and Molecular Basis of Inheritance
What the exam tests
This domain combines Mendelian inheritance with molecular genetics: solving crosses, interpreting pedigrees, and explaining how DNA stores and expresses information. Questions often link a genetic ratio to a molecular mechanism rather than asking recall alone.
Core rules
- Mendel's law of segregation: alleles separate during gamete formation; law of independent assortment applies only to genes on different chromosomes or far apart on the same chromosome.
- Deviations: incomplete dominance gives an intermediate phenotype; codominance expresses both alleles; multiple alleles (ABO) produce more than two phenotypes; pleiotropy means one gene affects several traits; polygenic inheritance gives continuous variation.
- Chromosomal theory: genes are on chromosomes; linked genes do not assort independently; crossing over produces recombinants.
- DNA is antiparallel and complementary (A-T, G-C); replication is semiconservative; transcription uses the template strand; translation follows the genetic code, which is degenerate and non-overlapping.
Worked example
Cross AaBb × AaBb for two unlinked genes. Each parent makes four gametes (AB, Ab, aB, ab) in equal numbers. The Punnett square has 16 boxes: 9 A_B_, 3 A_bb, 3 aaB_, 1 aabb. Probability of A_B_ = 9/16. If genes were linked, the 9:3:3:1 ratio would shift toward parental combinations. Reasoning: independent assortment multiplies the single-gene probabilities (3/4 × 3/4 = 9/16).
Traps
Do not use 9:3:3:1 for linked genes or for epistasis. In transcription, do not copy the coding strand. In replication, remember Okazaki fragments are on the lagging strand. A test cross uses a homozygous recessive parent, not an F1 self-cross.