Mathematicl methods exist to correct this bias. Rate about 0.4 in all populations. Unfortunately it doesn't account for dominance or for co-variance. Measure of how 2 measures co-vary. Twin studies try to get at genetic and environmental variance by altering genetic variance in a controlled way.
Adoption studies try to randomize environmental variance and eliminate the covariance term.
Compare child with birth family and adopted family. Best is MZ twins adopted into separate families, but very rare. General linkage method. Look at pairs of markers in families. To compare families, first divide likelihood values by the likelihood for unlinked and then take logarithm.