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Related Concept Videos

Heritability01:06

Heritability

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Heritability is a statistical concept that measures the degree to which genetic differences among individuals contribute to trait variations within a population. It is a fundamental idea in genetics, often prone to misinterpretation. Heritability is expressed as a percentage, reflecting the proportion of variation in a specific trait across a population that can be linked to genetic differences. However, it's important to understand that heritability does not determine how "genetic"...
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Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural selection can be directional, stabilizing, or disruptive. Directional selection favors one extreme trait and shifts the population towards that phenotype while selecting against individuals displaying alternate traits. Stabilizing selection favors an intermediate trait with a narrow range of variation. Deviation from the optimal phenotype towards an...
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Organisms that are well-adapted to their environment are more likely to survive and reproduce. However, natural selection does not lead to perfectly adapted organisms. Several factors constrain natural selection.
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Genetic variation is the diversity in DNA sequences found among individuals of the same species. This diversity is crucial for a species' survival because it helps organisms adapt to environmental changes. Genetic variation begins with fertilization, where an egg and sperm cell merge. Each of these cells carries 23 chromosomes, up to 46 in the fertilized egg. Chromosomes are long DNA strands that contain genes, the basic units of heredity.
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Frequency-dependent Selection01:21

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When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to as frequency-dependent selection. Frequency-dependent selection may occur between species or within a single species. This type of selection can either be positive—with more common phenotypes having higher fitness—or negative, with rarer phenotypes conferring increased fitness.
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Natural selection—probably the most well-known evolutionary mechanism—increases the prevalence of traits that enhance survival and reproduction. However, evolution does not merely propagate favorable traits, nor does it always benefit populations.
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Realized genetic selection differentials in Canadian Holstein dairy herds.

B A Hagan1, J Moro-Mendez2, R I Cue3

  • 1Animal Science Department, McGill University, Ste-Anne-de-Bellevue, Montreal, QC, Canada H9X 3V9; Council for Scientific and Industrial Research, Animal Research Institute, Accra, Ghana.

Journal of Dairy Science
|November 25, 2019
PubMed
Summary

Canadian Holstein genetic selection differentials (GSD) decreased generation intervals significantly from 1980 to 2016. While production traits showed positive GSD, fertility and health traits had mixed results, highlighting herd variability and the need for monitoring selection progress.

Keywords:
Holsteingeneration intervalherdrealized genetic selection differential

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Area of Science:

  • Animal Breeding and Genetics
  • Quantitative Genetics
  • Dairy Cattle Improvement

Background:

  • Genetic improvement programs aim to enhance livestock populations through strategic selection.
  • Understanding realized genetic selection differentials (GSD) and generation intervals is crucial for evaluating program effectiveness.
  • Canadian Holstein cattle have undergone significant genetic selection over several decades.

Purpose of the Study:

  • To define and determine realized genetic selection differentials (GSD) and generation intervals in Canadian Holstein cattle using a 4-path model.
  • To analyze trends in GSD for production, durability, health, and fertility traits from 1950/1960 to 2016.
  • To investigate herd variability in GSD and the impact of non-genetic factors.

Main Methods:

  • Utilized estimated breeding values for a selection index and various traits in Canadian Holstein bulls and cows.
  • Calculated GSD and generation intervals across sire-to-bull (SB), dam-to-bull (DB), sire-to-cow (SC), and dam-to-cow (DC) pathways.
  • Analyzed trends over time and assessed the effects of non-genetic factors (year of conception, housing, region) and herd variations.

Main Results:

  • Mean generation intervals significantly reduced across all pathways between 1980 and 2016.
  • SB and DB paths showed higher GSD for production traits and lifetime index compared to SC and DC paths.
  • Positive GSD trends were observed for production traits (post-2009) and somatic cell score, while daughter fertility showed mixed results, with positive trends emerging in SB and DB paths after the early 2000s.

Conclusions:

  • Genetic selection in Canadian Holsteins has become more efficient, evidenced by reduced generation intervals.
  • Selection intensity varied across pathways and traits, with production and somatic cell score showing favorable trends.
  • Herd-level GSD analysis and benchmarking are essential for monitoring and optimizing genetic progress in dairy populations.