Genetics: Mendelian Inheritance, Linkage and Gene Expression
The genetics domain of the Basque PAU Biology exam tests inheritance ratios, linkage and mapping, the molecular basis of gene expression and the consequences of mutation. You need to move confidently between Punnett squares, recombination frequencies and the central dogma.
Mendelian inheritance
- The law of segregation describes allele separation in meiosis; independent assortment applies to genes on different chromosomes.
- A test cross uses a homozygous recessive individual to reveal an unknown genotype.
- Dominance may be complete, incomplete or codominant; multiple alleles and epistasis modify simple ratios.
Linkage and mapping
- Genes on the same chromosome tend to be inherited together.
- Crossing over produces recombinant gametes; recombination frequency (RF) = recombinants ÷ total offspring × 100.
- 1% RF equals 1 centimorgan (cM); RF cannot exceed 50%.
Molecular genetics
- DNA replication is semiconservative; transcription makes RNA; translation makes protein.
- The genetic code is triplet, degenerate, non-overlapping and read in frame.
- Substitutions may be silent, missense or nonsense; insertions or deletions can cause frameshifts; chromosomal mutations alter larger segments.
Worked example
A test cross AaBb × aabb gives 1000 offspring: AB 400, ab 400, Ab 100 and aB 100. The most frequent classes, AB and ab, are parental, so the alleles A–B and a–b are linked in coupling. Recombinants are Ab + aB = 200. RF = 200 ÷ 1000 × 100 = 20%, so the map distance is 20 cM. If the genes were independent, the expected number in each class would be 250.
Traps
- A 9:3:3:1 ratio requires independent assortment, not linkage.
- RF above 50% is not used; 50% means effectively unlinked.
- Dominant does not mean more common in the population.
- Somatic mutations are not transmitted to offspring.
- Degeneracy of the code means several codons can specify one amino acid, not that one codon specifies several amino acids.