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What traits do ASI select for?Since 2014, ASI has primarily focused on fast-tracking resistance to Pacific Oyster Mortality Syndrome (POMS) in our genetically improved oysters, while still maintaining the performance and survival gains that have been made. Currently EBVs are calculated for five performance traits (length index, width index, depth index, total weight, and condition) and two resilience traits (POMS survival, and South Australian survival). ASI chooses broodstock based on EBV rather than year class.
What is an EBV?EBV is short for “Estimated Breeding Value”, which is an indicator of genetic merit and is one of the primary metrics used in breeding programs. In a statistical context an EBV is an ‘effect’ of an individual relative to an overall mean value of the trait within a population. We can interpret this as a genetic effect because of the use of specialised statistical models that account for the influence of environmental factors on a trait. The units of an EBV can be reported in several ways depending on the target audience. For example, EBVs are commonly reported in the same units of a measured trait (such as grams) but can also be reported as the number of (genetic) standard deviations from the mean value of a population. The ASI program reports EBVs as percentage deviations from the unselected population mean for performance traits, and for survival traits EBVs are reported as percent survival. ASI typically presents EBVs at the family level, which is the mean EBV of all individuals within that family.
What is heritability?Heritability (denoted h2) is the proportion of a variation in a trait that can be attributed to genetics. Because it is a proportion, heritability is a value that falls between 0 and 1. If a trait has a heritability of 0, none of the variation that is observed in that trait is due to genetics, whereas a heritability of 1 means that all variation in that trait is caused by genetics. Variation in a trait that is caused by environmental factors and not genetics cannot be improved by breeding. Consequently, heritability is a central focus of selective breeding because it is the component of trait variation that can be inherited from one generation to the next. It is also important because the greater the heritability, the faster we will be able to achieve a genetic response to selection. In an applied breeding context, a trait having h2 < 0.1 is generally considered as being lowly heritable, h2 from 0.2 – 0.3 is a moderate heritability and a trait with h2 > 0.4 is considered to be highly heritable.
What is inbreeding?Inbreeding occurs if (and only if) an organism has a mother and father that have one or more ancestors in common. The level of inbreeding of an individual can be quantified when pedigree records are known and is denoted as F. It is the proportion of an individual’s genome that is identical-by-descent from a common ancestor in its lineage. You could think of this as genes from one or more individuals “descending” down a family tree and then meeting up again in an inbred individual. Inbreeding can also be interpreted as the likelihood that the two alleles (i.e. one from the dam and the other from the sire) of any given gene within a genome are identical-by-descent. Managing inbreeding is important in a breeding program because, if left unchecked, it can increase the likelihood of undesirable recessive genes ‘pairing up’ and affecting the overall fitness of an animal. When this occurs, it is known as “inbreeding depression”. In selective breeding programs, an average increase in inbreeding of 1% per generation is a generally accepted rule-of-thumb and is what is following by ASI.
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