Mental retardation or learning disability

Intelligence is a polygenic trait. Mild learning disability
(intelligence quotient 50–70) represents the lower end
of the normal distribution of intelligence and has a
prevalence of about 3%. The intelligence quotient of
offspring is likely to lie around the mid-parental mean.
One or both parents of a child with mild learning disability
often have similar disability themselves and may have other
learning-disabled children. Intelligent parents who have one
child with mild learning disability are less likely to have
another similarly affected child.