Genetics and Molecular Genetics
Genetics and Molecular Genetics tests Mendelian inheritance, deviations from Mendelism, sex determination, mutation, DNA replication, transcription, translation and gene regulation.
Core rules
- Mendel's law of segregation explains a 3:1 monohybrid ratio; independent assortment explains a 9:3:3:1 dihybrid ratio only for unlinked genes.
- Incomplete dominance gives an intermediate phenotype; codominance expresses both alleles, as in ABO blood group alleles IA and IB.
- DNA replication is semiconservative; DNA polymerase adds nucleotides to a 3' end, so one strand is synthesised continuously and the other as Okazaki fragments.
- Transcription uses the template strand; translation reads mRNA codons 5' to 3' and uses tRNA anticodons.
Worked example
In a dihybrid cross between YyRr and YyRr, each parent makes four gamete types in equal frequency: YR, Yr, yR, yr. A 4 x 4 Punnett square gives 16 combinations, with phenotypes in the ratio 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled. The 9:3:3:1 result depends on independent assortment of the two gene pairs, not on dominance alone.
Traps
- Linked genes do not assort independently, so dihybrid ratios deviate.
- Codominance is not incomplete dominance: both phenotypes appear distinctly in codominance.
- Replication and transcription both use complementary base pairing but only replication is semiconservative.
- The codon is on mRNA; the anticodon is on tRNA.