Genetik und Gentechnik / Genetics and genetic engineering
Genetics items move from cross and pedigree analysis to molecular mechanisms and to the design of a gene-technological experiment. Marks are lost mainly through imprecise notation: gametes, base polarity, and the difference between template and coding strand must be written correctly.
Mendelian inheritance
- Law of segregation and law of independent assortment; a test cross (crossing with the homozygous recessive) reveals whether an individual showing the dominant phenotype is homozygous or heterozygous.
- A 9:3:3:1 dihybrid ratio requires independent assortment on different chromosomes. Deviations indicate linkage: recombination frequency = recombinants/total × 100%, and 1% corresponds to 1 centimorgan (map unit).
- Pedigree logic: affected children of unaffected parents indicate recessive inheritance; every affected child having an affected parent indicates dominance; X-linked recessive shows mainly affected males and no father-to-son transmission.
Molecular genetics
- Semiconservative replication: helicase unwinds, primase lays an RNA primer, DNA polymerase extends 5'→3'. The leading strand is continuous; the lagging strand is made as Okazaki fragments joined by ligase.
- Transcription: RNA polymerase reads the template strand 3'→5' and builds mRNA 5'→3'. Eukaryotic processing adds a cap and poly-A tail and splices out introns.
- Translation: start codon AUG, tRNA anticodons pair with codons, stop codons UAA, UAG or UGA terminate. The code is degenerate and non-overlapping, so a third-base substitution is often silent.
- Mutations: substitution → silent, missense or nonsense; insertion or deletion of a number of bases not divisible by three → frameshift.
Gene technology
- Restriction enzymes cut palindromic recognition sites and produce sticky ends; ligase joins fragments; plasmids supply origin of replication, selection marker and promoter.
- PCR: denaturation near 95 °C, primer annealing around 55–65 °C, extension near 72 °C with a thermostable polymerase; product doubles each cycle (2^n).
- Gel electrophoresis separates fragments by size, smaller fragments migrating further. CRISPR-Cas9 uses a guide RNA to direct a Cas9 double-strand cut followed by repair.
Worked example
A test cross of a doubly heterozygous individual gives 1000 offspring: 410 AB, 390 ab, 105 Ab, 95 aB. Reasoning: the parental classes dominate, so the genes are linked; recombinants = 105 + 95 = 200; recombination frequency = 200/1000 = 20%, that is 20 cM apart. Independent assortment would have produced roughly equal four classes.
Traps that cost marks
Mixing up template and coding strand; equating 'dominant' with 'more common'; claiming a 3-base insertion causes a frameshift; forgetting that PCR needs two primers and a thermostable polymerase; treating a silent mutation as having no possible effect on splicing or regulation.