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|FGCA > About genetic diversity > What is genetic diversity?|
Genetic Diversity: A Primer
Excerpt From Genetic Diversity - Invisible but vital to Woodland Conservation and Restoration, Nielsen, Cathy, OMNR, Kemptville, June 2000.
One gene is inherited from the mother and one from the father. Heterozygosity is when the two alleles at a gene location are different and homozygosity means that the two genes are of the same allelic form (figure below). Genes that are recessive are only exposed when present in the homozygous state e.g. blue eyes. The relevance of this arrangement becomes apparent when an organism has a form of a gene that is detrimental, for example, an allele for no chlorophyll. If the plant is homozygous for this allele it will not survive. The number of gene locations that are homozygous (two genes the same) increase when related trees mate and if an allele is detrimental, it manifests itself as inbreeding depression.
Most tree species are outcrossing with high rates of pollen exchange, so they have high levels of heterozygosity. However this also means that trees have been able to carry a "high genetic load" of detrimental genes. They don't often occur in the homozygous state and therefore are not exposed to selection pressure, and hence purged from the population. A tree species that occurs naturally as scattered individuals or small groups may be adapted to inbreeding because a majority of detrimental recessive genes having been purged; there is no evidence of inbreeding depression when related trees mate.
In a species with a high genetic load, when two trees that are related mate, the resulting seedling can show signs of inbreeding depression. They exhibit less vigour or may not be able to survive at all. In the most extreme situation when self- pollination occurs (such as an isolated tree) little viable seed is produced. Under natural conditions, in most species of trees, female flowers are pollinated by many unrelated trees and relatively less related trees. Millions of seedlings are produced and seedlings of low vigour (in-bred seedlings) are eliminated over time. When we have only small isolated woodlots the chance that related trees will mate is greater and increases over each generation. This is another reason it is important to have larger woodlots and a less fragmented forest landscape. In addition, when we do manage isolated woodlots we must take care to maintain large numbers (at least 100 if possible) of mature individuals of one species to provide regeneration if we want to ensure a genetically healthy stand for the next generation.