Genetics: Inheritance, DNA, Genetic Engineering and Biotechnology
What this domain tests
Genetics questions run from DNA structure through protein synthesis to crosses, linkage and genetic engineering. You must be able to set out a cross, interpret ratios and describe each step of a genetic engineering procedure.
DNA and gene expression
- A DNA nucleotide is deoxyribose, phosphate and a base; bases pair A–T and G–C by hydrogen bonds in a double helix.
- Replication is semi-conservative. Transcription makes mRNA in the nucleus; translation at the ribosome uses tRNA and the triplet code to assemble amino acids.
- A mutation is a change in base sequence. A substitution may alter one amino acid; an insertion or deletion causes a frameshift.
Inheritance
- Alleles are alternative forms of a gene; genotype is the alleles present, phenotype the observable trait.
- A monohybrid cross of two heterozygotes gives a 3:1 phenotype ratio; a test cross uses a homozygous recessive to reveal an unknown genotype.
- Sex-linked genes sit on the X chromosome, so males show recessive X-linked conditions more often.
- Linked genes lie on the same chromosome and do not assort independently, so ratios depart from Mendelian expectations.
Genetic engineering
Isolate the gene using a restriction enzyme, insert it into a vector such as a plasmid, transform a host cell, then select the successful cells. Human insulin is produced this way.
Worked example
A carrier mother (X^C X^c, normal vision) and a normal father (X^C Y). Sons receive Y from the father, so half inherit X^c and are colour-blind. Daughters receive X^C from the father and are all unaffected, but half are carriers. Reasoning: the father's X is normal, so no daughter can be affected; all risk lies in the mother's X^c.
Traps
Writing 3:1 as a genotype ratio; expecting exact ratios from small numbers; confusing gene and allele; forgetting a test cross needs a homozygous recessive partner; describing the vector step as 'mixing' genes.