Genetics and Heredity
Genetics and Heredity examines how information passes from DNA to protein to offspring, and how inheritance ratios can be predicted and interpreted. The exam favours genetic diagrams, pedigree and family data, and short explanations of molecular events such as transcription and translation.
DNA, genes and the code
- A nucleotide is phosphate + deoxyribose + base; A pairs with T and G with C by hydrogen bonds in an antiparallel double helix.
- Semi-conservative replication: helicase unzips the helix, each strand acts as a template, DNA polymerase joins free nucleotides.
- A gene is a base sequence coding for a polypeptide; the code is a triplet, degenerate and non-overlapping.
- Transcription in the nucleus makes mRNA; translation at the ribosome uses tRNA and codons to assemble amino acids.
Inheritance patterns
- Alleles, genotype, phenotype, homozygous, heterozygous, dominant and recessive.
- Monohybrid heterozygote × heterozygote gives 3:1; a test cross of a heterozygote with a homozygous recessive gives 1:1.
- Dihybrid crosses give 9:3:3:1 only with independent assortment; linked genes distort the ratio.
- Co-dominance (AB blood group) and multiple alleles; incomplete dominance gives an intermediate phenotype.
- Sex linkage: genes on the X chromosome; males are hemizygous, so recessive alleles appear more often in males.
Meiosis and mutation
- Meiosis halves the chromosome number; crossing over in prophase I and independent assortment in metaphase I create variation; fertilisation restores diploidy.
- Substitutions may be silent because the code is degenerate; insertions or deletions cause frameshifts; chromosome mutations change whole segments.
Worked example
A carrier woman for red–green colour blindness (XᴮXᵇ) has children with a man of normal vision (XᴮY). Gametes: mother Xᴮ or Xᵇ, father Xᴮ or Y. Daughters are XᴮXᴮ or XᴮXᵇ - all normal vision, half carriers. Sons are XᴮY or XᵇY - 50% colour-blind. So '50% of sons' is correct, but '50% of children' is not.
Traps
- Writing gene when allele is meant, or failing to define symbols and parental phenotypes.
- Assuming the dominant phenotype is the commonest in a population.
- Confusing the four products of meiosis with the two of mitosis.
- Giving a ratio when a probability is asked for, or forgetting the sex split in sex-linked answers.