Biology – Natural and artificial selection | e-Consult

Selective breeding is a process of intentionally breeding organisms with desirable traits to produce offspring with those traits. This alters the genetic makeup of a population over generations. Here's a breakdown of the examples:

Disease Resistance in Wheat and Rice

• Process: Farmers identify wheat or rice plants that show natural resistance to specific diseases (e.g., rust, blight). These plants are then selectively bred, meaning only their seeds are used to produce the next generation. Repeated selection over many generations increases the frequency of genes associated with resistance.
• Advantages: Reduces crop losses due to disease, leading to increased food production and reduced reliance on pesticides.
• Disadvantages: Disease pathogens can evolve to overcome resistance (e.g., a new strain of rust). Genetic diversity within the crop population can be reduced, making the crop vulnerable to other unforeseen threats. The resistance genes may be linked to undesirable traits.

Inbreeding and Hybridisation in Maize

• Inbreeding: Involves breeding closely related individuals. This increases homozygosity (the proportion of individuals with two identical alleles for a trait).

  • Advantages: Stabilizes desirable traits, producing more uniform offspring. Useful for fixing desirable recessive alleles.
  • Disadvantages: Increases the risk of expressing harmful recessive alleles, leading to reduced vigour and fertility (inbreeding depression).

• Hybridisation: Involves crossing two genetically different individuals (often from different varieties). This produces offspring (hybrids) with a combination of traits from both parents.

  • Advantages: Often results in offspring with increased vigour, larger size, and improved yield (hybrid vigour or heterosis). Can combine desirable traits from different parent lines.
  • Disadvantages: Hybrids are often sterile (e.g., many commercially grown maize varieties are sterile). The hybrid vigour may not be stable across generations.

Improving Milk Yield in Dairy Cattle

• Process: Farmers select cows that produce high volumes of milk and breed them together. This increases the frequency of genes associated with high milk production in the next generation. Genetic testing is increasingly used to identify cows with superior genetic potential.
• Advantages: Increases the efficiency of milk production, leading to higher profits for farmers and a greater supply of milk.
• Disadvantages: Can lead to health problems in cows due to the strain of producing large amounts of milk. Reduced genetic diversity can make the herd more susceptible to disease. Ethical concerns regarding animal welfare may arise.

In summary, selective breeding relies on understanding the heritability of traits and using controlled mating to increase the frequency of desirable alleles in a population. Each technique has its own advantages and disadvantages, and careful consideration must be given to potential unintended consequences.