Genetics and Molecular Biology
This domain tests inheritance patterns and the molecular basis of gene expression. Expect monohybrid and dihybrid crosses, test crosses, blood groups, sex linkage, DNA replication, transcription, translation and mutation.
Mendelian inheritance
Alleles are alternative forms of a gene; genotype is the allele combination and phenotype is the observable trait. A monohybrid cross of two heterozygotes gives 3:1; a test cross with a heterozygote gives 1:1. A dihybrid cross of unlinked heterozygotes gives 9:3:3:1. Codominance and incomplete dominance alter phenotypic ratios. ABO blood groups involve multiple alleles I^A, I^B and i, with I^A and I^B codominant. X-linked recessive traits appear more often in males because they have one X chromosome.
Molecular biology
DNA is a double helix with antiparallel strands; A pairs with T and G with C by hydrogen bonds. Replication is semi-conservative and uses DNA polymerase. Transcription occurs in the nucleus, producing mRNA; translation occurs at ribosomes, where codons pair with tRNA anticodons. The genetic code is degenerate and non-overlapping. Substitutions may alter one amino acid; insertions or deletions can cause frameshifts.
Worked example
Two parents of blood group A have genotype I^A i. Each can pass i with probability 1/2, so a child of genotype ii has probability 1/2 × 1/2 = 1/4, or 25%, and is group O. A group AB child is impossible because neither parent carries I^B.
Traps
Keep genotype and phenotype ratios separate. Remember that sons inherit their X from the mother. Do not apply 9:3:3:1 to linked genes, and do not confuse a codon with an anticodon.