Outcrossing

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This article is about the technique used in animal and plant breeding. For naturally occurring outcrossing, see Allogamy.
Labradoodles, a crossbreed between a Poodle and a Labrador Retriever

Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity.

Outcrossing in animals

Outcrossing can be a useful technique in

deleterious genes that may be expressed by subsequent inbreeding. There is now a gamut of deleterious genes within each individual in many dog breeds.[1]

Increasing the variation of genes or alleles within the gene pool may protect against extinction by stressors from the environment among inbred animal populations. For example, in this context, a recent veterinary medicine study tried to determine the genetic diversity within cat breeds.[2]

A degree of outcrossing to avoid mating between very close relatives is believed to happen in the wild.[1]

Outcrossing in plants and fungi

Outcrossing in plants is usually enforced by

hybrid vigor or heterosis. Once outcrossing is established in a lineage of flowering plants due to the benefit of genetic complementation, subsequent switching to inbreeding becomes disadvantageous because it allows expression of the previously masked deleterious recessive mutations, i.e. inbreeding depression
.

Outcrossing in fungi involves

syngamy between haploid cells produced by separate diploid individuals.[4]

Life-history traits are said to increase the probability of outcrossing in fungi such as long-distance dispersal and persistence of the haploid stage. Some studies even show that fungi favor outcrossing in comparison to other mating types. In a study performed with the commercial button mushroom, Agaricus bisporus, outcrossed populations of the fungi showed higher fitness than inbred ones in several fitness components.[5]

General practice

Breeders

cull those traits that are undesirable. When undesirable traits begin to appear, mates are selected to determine if a trait is recessive or dominant. Removal of the trait is accomplished by breeding two individuals known not to carry it.[6]

Gregor Mendel used outcrossing in his experiments with flowers. He then used the resulting offspring to chart inheritance patterns, using the crossing of siblings, and backcrossing to parents to determine how inheritance functioned.[7]

Darwin's perspective

Charles Darwin, in his book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom,.[8]: 462  stated regarding outcrossing that "the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigor and fertility over the self-fertilizing offspring from either one of the same parents". He thought that this observation was amply sufficient to account for outcrossing sexual reproduction. The disadvantages of self-fertilized offspring (inbreeding depression) are now thought to be largely due to the homozygous expression of deleterious recessive mutations;[9] and the fitness advantages of some outcrossed offspring are thought to be largely due to the heterozygous masking of such deleterious mutations except when such mutations lead to outbreeding depression.

See also

References

  1. ^ a b Sharp, C.A. (26 February 1999). "The Downside of Inbreeding: It's Time For a New Approach". Canine-Genetics.com. Double Helix Network News Vol. VII, No. 1 (Winter 1999). Archived from the original on 26 January 2012.
  2. ^ "Feline Genetics". UC Davis Veterinary Genetics Laboratory. Archived from the original on 2009-03-12.
  3. ^ Bernstein H, Byerly HC, Hopf FA, Michod RE. Genetic damage, mutation, and the evolution of sex. Science. 1985 Sep 20;229(4719):1277-81. doi: 10.1126/science.3898363. PMID: 3898363
  4. S2CID 25007801
    .
  5. PMID 8536962
    .
  6. ^ David M. Hillis. "Inbreeding, Linebreeding, and Outcrossing in Texas Longhorns". University of Texas at Austin.
  7. ^ "Mendel's Paper (English - Annotated)". www.mendelweb.org.
  8. ^ "Darwin, C. R. 1876. The effects of cross and self fertilisation in the vegetable kingdom. London: John Murray". darwin-online.org.uk.
  9. PMID 3324702
    .
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