Genetics and Molecular Basis of Inheritance
Genetics and Molecular Basis of Inheritance covers Mendelian inheritance and its extensions, linkage and sex determination, and the molecular flow of information from DNA to protein, including mutation and gene regulation.
Mendelian and chromosomal rules
- Law of dominance and segregation: a monohybrid cross of heterozygotes gives 3:1 phenotype and 1:2:1 genotype; a test cross gives 1:1.
- Independent assortment: a dihybrid cross of RrYy × RrYy gives 9:3:3:1.
- Incomplete dominance gives an intermediate phenotype; codominance is shown by ABO alleles IA and IB, with multiple alleles (IA, IB, i).
- Sutton and Boveri linked genes to chromosomes; Morgan showed linkage, with recombination frequency reflecting distance. Sex determination is XX–XY in humans; X-linked recessive traits (haemophilia, colour blindness) affect males more.
Molecular rules
- DNA is an antiparallel double helix (Watson and Crick, 1953) with A=T (two hydrogen bonds) and G≡C (three); one turn is 3.4 nm with 10 base pairs.
- Replication is semi-conservative; DNA polymerase adds nucleotides 5′→3′, producing leading and lagging strands with Okazaki fragments.
- Transcription makes RNA; translation uses a degenerate, non-overlapping triplet code, with AUG as start and UAA, UAG, UGA as stops.
- The lac operon is repressed without lactose; allolactose inactivates the repressor and induces genes z, y and a.
Worked example
Cross IAi × IBi. Possible gametes IA, i and IB, i. Offspring: IAIB (AB), IAi (A), IBi (B), ii (O), each 1/4. The reasoning is that IA and IB are codominant, while i is recessive to both, so four phenotypes appear in equal ratio.
Traps
A 3:1 ratio is not the same as 1:2:1; linked genes do not assort independently; an X-linked recessive allele cannot pass from father to son; Chargaff's rule applies to double-stranded DNA; UGA is a stop codon.