Average Genetic Correlations Among Offspring From Open-Pollinated Forest Trees

In estimating additive genetic variance and heritability, most forest tree geneticists assume that the genetic correlation among offspring (coefficient of relationship) of an open-pollinated tree is .25. Such offspring (often called a half-sib family) are all related as half-sibs only when none of them resulted from self-pollination and when all of them were sired by different males. When they are truly half-sibs and when the parents are not correlated or inbred, the correlation between all pairs of offspring is .25. Since these conditions are usually not fulfilled, geneticists are aware that the average correlation is likely to be greater than .25. Assumption that the correlation among offpring is .25 usually results in an overestimate of additive genetic variance (NAMKOONG, 1966). Assumption that the parents are unrelated may lead to either overor underestimates 01 additive genetic variance (FUJISHIMA and FREDEEN, 1972). Biases in estimates of additive genetic variance in turn cause biases in estimates of heritability and of genetic gains. Average genetic correlations among offspring from openpollination are computed herein for a range of hypothetical conditions. An attempt is also made to estimate the biases resulting from assumptions noted above, and recommendations are given for alleviating them.