Genetics and Molecular Biology
Domain 3 - Genetics and Molecular Biology
This domain covers inheritance patterns, the chromosome basis of inheritance, gene expression from DNA to protein, mutation, and core molecular techniques.
Transmission genetics
- Mendel's law of segregation: allele pairs separate in anaphase I of meiosis. Law of independent assortment: genes on different chromosomes assort independently because of random chromosome orientation in metaphase I.
- Monohybrid Aa × Aa → 1 AA : 2 Aa : 1 aa, phenotypes 3:1. Dihybrid AaBb × AaBb (unlinked) → 9:3:3:1. A test cross (× aabb) reveals an unknown genotype.
- Incomplete dominance gives blended heterozygotes; codominance expresses both alleles (AB blood type); multiple alleles and polygenic traits complicate ratios.
- X-linked traits appear at different frequencies in males, who are hemizygous.
- Meiosis produces four haploid cells with crossing over in prophase I; nondisjunction yields aneuploid gametes (trisomy 21).
- Recombination frequency between linked genes estimates map distance (1% recombination = 1 map unit).
Molecular biology
DNA is an antiparallel double helix with A–T and G–C pairing. DNA polymerase synthesizes 5′→3′, continuously on the leading strand and in Okazaki fragments on the lagging strand; replication is semiconservative. Transcription by RNA polymerase makes mRNA, and eukaryotic introns are spliced out. Translation at the ribosome reads codons 5′→3′ against tRNA anticodons, beginning at AUG. Mutations: silent, missense, nonsense, frameshift (insertion or deletion not in multiples of three). Prokaryotic regulation is illustrated by the lac operon, where lactose inactivates the repressor.
Worked example
Cross AaBb × AaBb with the genes unlinked. Treat each gene separately: P(aa) = ¼ and P(bb) = ¼. Multiply because the loci assort independently: ¼ × ¼ = 1/16, so 6.25% of offspring are aabb. The probability of at least one dominant allele at both loci is ¾ × ¾ = 9/16.
Traps
Treating linked genes as independently assorting; mixing genotype with phenotype ratios; forgetting that males express X-linked recessive traits from a single copy; reading the template strand in the wrong direction; labeling a frameshift as a point mutation; assuming a dominant allele must be the more common one in a population.