outline the following examples of selective breeding: the introduction of disease resistance to varieties of wheat and rice, inbreeding and hybridisation to produce vigorous, uniform varieties of maize, improving the milk yield of dairy cattle

Natural and Artificial Selection

Artificial selection is the intentional breeding of organisms by humans to produce desired traits. It is widely used in agriculture to improve crop yields, disease resistance, and animal productivity.

Selective Breeding Examples

1. Introduction of Disease Resistance in Wheat and Rice

Breeders identify genes that confer resistance to specific pathogens and incorporate them into commercial varieties through cross‑breeding or marker‑assisted selection.

• Wheat – Stem rust resistance (Sr genes)

  • Source: Wild relatives such as Aegilops tauschii.
  • Method: Introgression of Sr genes via back‑crossing and selection for resistance under controlled inoculation.
  • Outcome: Varieties such as ‘Kavkaz’ and ‘Mace’ show durable resistance to Ug99.

• Rice – Blast disease resistance (Pi genes)

  • Source: Landraces and wild species Oryza rufipogon.
  • Method: Pyramiding multiple Pi genes using molecular markers.
  • Outcome: Cultivars like ‘IR64‑Pi9’ exhibit broad‑spectrum resistance.

2. Inbreeding and Hybridisation in Maize (Zea mays)

Maize breeding combines inbreeding to create homozygous lines and hybridisation to exploit heterosis (hybrid vigor).

1. Inbreeding (selfing or sib‑mating)

   • Produces uniform, genetically stable lines (e.g., S1, S2, … generations).
   • Allows precise evaluation of traits such as ear size, kernel colour, and disease resistance.

2. Hybridisation (crossing two inbred lines)

   • Combines complementary alleles, resulting in increased vigor, yield, and uniformity.
   • Hybrid maize accounts for >90 % of commercial maize production in many countries.

3. Improving Milk Yield in Dairy Cattle

Selective breeding in cattle focuses on traits that enhance milk production, composition, and animal health.

• Trait measurement: Daily milk yield, fat and protein percentages, lactation length.
• Selection methods

  • Phenotypic selection – choosing bulls and cows with highest recorded yields.
  • Estimated Breeding \cdot alues (EB \cdot s) – statistical prediction of an animal’s genetic merit.
  • Genomic selection – using DNA markers across the genome to predict performance.

• Result: Modern Holstein‑Friesian cows can produce >10 000 L of milk per lactation, a three‑fold increase over early 20th‑century breeds.

Comparison of Breeding Strategies