Selective Breeding in Animals: Genetic Improvement Examples

Selective breeding, also referred to as artificial selection, has revolutionized animal agriculture and biomedical research by enhancing specific traits through controlled mating practices. By choosing animals with superior genetic potential, breeders have increased productivity, improved animal health, and developed precise models for scientific study. BioVenic, a leader in animal research and breeding innovation, leverages these techniques to support both agricultural advancement and genetic discovery. This article explores the foundations of selective breeding, major case studies in domestic and laboratory animals, and the balance between efficiency and genetic diversity.

Fig.1 Schematic Diagram of Marker-assisted Selection in Animal Breeding.^{1, 2}

Principles of Artificial Breeding in Animals

Artificial breeding operates on a foundational principle: selecting parent animals with desirable traits to produce improved offspring. Unlike natural selection, where environmental pressures determine survival and reproduction, artificial selection is directed by human choice based on genetic and phenotypic evaluations. The process typically involves:

For traits influenced by a few genes—like coat color or presence of horns—improvement occurs quickly. However, for polygenic traits such as growth rate or fertility, selective breeding requires meticulous multi-generational planning and statistical analysis. The use of AI and embryo transfer not only amplifies desired traits but also accelerates genetic gain by enabling broader dissemination of elite genes.

Selective Breeding Strategies and Genetic Foundations

Modern animal breeding combines classical selection with advanced genetic principles. The two primary strategies include:

• Inbreeding and Linebreeding: Mating between relatives to consolidate traits, managed carefully to prevent inbreeding depression.
• Outbreeding and Crossbreeding: Introducing genetic material from unrelated or different breeds to increase heterozygosity and improve fertility and survival rates.

Breeders utilize tools from quantitative genetics to guide selection. Traits with high heritability respond more readily to selection pressure. Genetic evaluations incorporate data from relatives and offspring to increase accuracy. The development of best linear unbiased prediction (BLUP) and progeny testing has enhanced breeders' ability to predict genetic potential. These methods allow for the assessment of multiple traits, leading to the use of selection indexes that combine economically important traits into a single score.

In the last two decades, animal genomic selection has revolutionized breeding. By genotyping animals using thousands of genetic markers, breeders can estimate genomic estimated breeding value (GEBV). This allows for earlier and more accurate selection decisions, even before animals reach reproductive age. Genomic tools also aid in identifying markers for disease resistance, meat quality, and heat tolerance—traits that are difficult or costly to measure directly.

Marker-Assisted Selection (MAS) further enhances breeding precision by utilizing DNA markers linked to specific traits. MAS is particularly effective for traits that are challenging to measure directly, have low heritability, or manifest late in life. By identifying and selecting for these markers, breeders can enhance the precision and speed of genetic improvement.

Table 1. Common Breeding Strategies and Their Effects on Genetic Diversity

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Selective Breeding Animals Examples

Dairy Cattle: Unprecedented Milk Production Gains

Few industries demonstrate the power of selective breeding more clearly than dairy farming. In particular, Holstein-Friesian cows have undergone remarkable transformation:

• Milk Production: The average U.S. dairy cow now produces over 10,000 liters of milk per year, a fourfold increase since the 1940s. This has been driven by continuous selection of sires and dams based on production records and EBVs.
• Artificial Insemination: The use of AI has allowed a single elite bull to sire tens of thousands of daughters globally, rapidly disseminating superior genes.
• Genomic Selection: Since its introduction in 2009, genomic selection has further accelerated progress, particularly in traits like milk fat and protein percentage, udder health, and longevity.

Additionally, newer selection indexes now incorporate fertility, disease resistance, and calving ease to ensure long-term sustainability. These improvements have led to fewer cows producing more milk, reducing the environmental impact per unit of production.

Poultry: Specialized Lines for Meat and Eggs

Selective breeding in chickens has diverged into two specialized branches: broilers for meat and layers for egg production.

• Broilers: Modern broiler chickens reach market weight in just six to seven weeks, compared to over twelve weeks in the 1950s. Selection for growth rate, feed conversion efficiency, and muscle mass—particularly breast meat—has drastically improved performance. The feed conversion ratio in broilers has dropped from 3:1 to nearly 1.6:1 in recent decades.
• Layers: Commercial laying hens now produce 250–300 eggs annually, a sharp contrast to the 100 eggs per year of their ancestors. Breeding programs emphasize clutch size, shell strength, and disease resistance.

These improvements are achieved using multi-line breeding systems, where distinct pure lines are maintained and crossbred to produce commercial hybrids. This strategy combines additive genetic gain with hybrid vigor, maximizing both performance and resilience.

Pigs: Leaner, Faster-Growing, and Prolific

Pig breeding has focused on increasing growth rate, improving carcass quality, and enhancing reproductive traits:

• Growth and Feed Efficiency: The time required to reach market weight has decreased by over 30%, and pigs now convert feed to meat with much greater efficiency.
• Lean Meat and Carcass Yield: Selective breeding has reduced backfat thickness and increased lean muscle mass, meeting consumer demands for healthier pork products.
• Reproduction: Average litter sizes have grown from 9–10 to 13–14 piglets, and sow longevity has improved due to selection for maternal traits and structural soundness.

Selection indexes in swine now incorporate economic weights for traits like daily gain, backfat depth, and number of piglets weaned. Genetic companies also use genomic data to accelerate breeding and improve accuracy.

Performance Comparison: Pre- and Post-Breeding Improvement

Table 2. Performance Comparison: Pre- and Post-Breeding Improvement

Laboratory Mice: Breeding for Research and Trait Models

In biomedical research, mice have been selectively bred for over a century to provide genetically consistent models:

• Inbred Strains: C57BL/6 and BALB/c mice are among the most commonly used strains, bred through 20+ generations of sibling mating to achieve >99% genetic homogeneity.
• Behavioral and Physiological Models: Selective breeding has created mouse lines with exaggerated traits, such as the "High Runner" line bred for voluntary wheel running. These mice help study metabolism, exercise physiology, and neurological conditions.

Researchers also breed mice for susceptibility or resistance to specific diseases, enabling controlled studies of genetic mechanisms in cancer, immunology, and aging. Selective breeding ensures that phenotypic differences are due to experimental variables, not genetic background noise.

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Selective Breeding Enhances Animal Productivity and Genetic Research

Modern animal breeding combines classical selection with advanced genetic principles. The two primary strategies include:

Companies like BioVenic contribute by offering custom breeding strategies, genomic evaluations, and data analysis services. This integration of commercial breeding and scientific research ensures that animal populations continue to meet evolving industry and research needs.

Preserving Genetic Diversity in Modern Breeding Programs

While selective breeding has delivered significant gains, it can lead to reduced genetic variation if not managed properly. Risks include:

• Inbreeding Depression: Accumulation of harmful recessive alleles can reduce fertility, immunity, and overall performance.
• Loss of Adaptive Traits: Narrow selection goals may inadvertently remove genes valuable for stress resistance or climate adaptation.

To combat these risks, breeders adopt several safeguards:

• Genetic Monitoring: Breeding software tracks relationships among animals to avoid excessive inbreeding. Genomic data provides even greater accuracy.
• Cryopreservation: Gene banks store semen, embryos, and tissue samples from diverse and rare lines, ensuring future access to lost genetic material.
• Balanced Indexes: Including health, fertility, and welfare traits in selection goals helps maintain overall robustness and prevents overemphasis on single traits.

Crossbreeding and the conservation of heritage breeds also serve as genetic buffers. By maintaining multiple lines with distinct genetic profiles, breeders ensure a reservoir of diversity that can be tapped when needed.

Genomic technologies now make it possible to calculate inbreeding coefficients and diversity indexes at the DNA level, allowing breeders to optimize both short-term gain and long-term genetic health.

Optimizing Animal Breeding for Sustainable Genetic Improvement

Selective breeding has transformed animal production and research by enhancing desirable traits and providing insights into genetics. Advancements such as genomic selection and marker-assisted selection have increased the precision and efficiency of breeding programs. However, maintaining genetic diversity remains crucial to ensure long-term sustainability and adaptability.

BioVenic plays a vital role in supporting balanced breeding programs that maximize performance while safeguarding genetic diversity. By integrating traditional breeding methods with modern genomic tools, we help usher in a new era of data-driven breeding strategies. Find more BioVenic's animal breeding services:

• Genomic Selection in Animal Breeding
• Genomic Selection in Animal Breeding
• Multi-Omics Selection in Animal Breeding
• Microbiome Selection in Animal Breeding
• Genome-wide Association Studies (GWAS) in Animal Breeding
• Precision Animal Breeding

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References

1. Shriver, Adam. "Prioritizing the protection of welfare in gene-edited livestock." Animal Frontiers 10.1 (2020): 39-44. https://doi.org/10.1093/af/vfz053
2. under Open Access license CC BY 4.0, without modification.
3. Wiggans, George R., and José A. Carrillo. "Genomic selection in United States dairy cattle." Frontiers in Genetics 13 (2022): 994466. https://doi.org/10.3389/fgene.2022.994466
4. Paxton, Heather, et al. "The effects of selective breeding on the architectural properties of the pelvic limb in broiler chickens: a comparative study across modern and ancestral populations." Journal of Anatomy 217.2 (2010): 153-166. https://doi.org/10.1111/j.1469-7580.2010.01251.x