Linkage, recombination and deviations from Mendelian ratios

Mutation

Original source of genetic diversity

Independent Assortment (Meiosis I)

• Homologous chromosomes align randomly on the metaphase plate.

• With 23 pairs:
  → 2²³ = 8,388,608 combinations per gamete

Crossing Over (Prophase I)

• Exchange of genetic material between homologous chromosomes.

• Occurs at pachytene phase via the synaptonemal complex.

• Results in recombinant chromosomes

Random fertilisation

• Any sperm can fertilize any egg.

• With independent assortment:
  → ~70 trillion (2⁴⁶) combinations possible in offspring

Crossing over

• Occurs during Prophase I of meiosis.

• Involves synapsis and formation of tetrads.

• Leads to new allele combinations (recombinants)

Recombination frequency

• % of offspring with a non-parental allele combination.

• Estimated via testcross (e.g., AaBb × aabb).

• 50% frequency → genes are unlinked (on different chromosomes)

T.H. Morgan’s Discovery

• Observed gene linkage in fruit flies.

• Showed that some genes do not assort independently

Sturtevant’s Linkage Maps

• Recombination frequency ∝ distance between genes.

• >50% recombination not observed (due to multiple crossovers)

Coupling heterozygote

Dominant alleles on the same chromosome

Repulsion heterozygote

Dominant allele of one gene with recessive allele of another

Three-point cross

• Largest classes = parental types.

• Smallest classes = double crossovers

Sex-linkage

• Genes on sex chromosomes

• e.g. white eyes in male fruit flies

Incomplete dominance

• Blended phenotype

• e.g. Red x white = pink flowers

• Human: Familial Hypercholesterolaemia

Co-dominance

• Both traits show distinctly

• e.g. AB blood group

Pleiotropy

• One gene gives rise to multiple traits

• e.g. sickle cells, cystic fibrosis

Polygenic inheritance

• Many genes gives rise to one trait

• e.g. height, skin colour

Epistasis

• One gene masks another

• e.g. Mouse coat colour (9:3:4) ratio from 9:3:3:1 dihybrid cross

Independent assortment

Recombinants from genes on different chromosomes

Crossing over

Recombinants from genes on the same chromosome

Multiple crossovers

Underestimate map distance