Heritability can be defined as the proportion of the phenotypic variation seen in a population that is due to differences in breeding values. In this case that means that only about 10% of the variation seen in litter size is due to (additive) genetics. In turn that means that to select for a trait like litter size lots of data is needed to make an accurate estimate. Heritabilities for different traits can range from approximately 5% to 50%. Typically heritability for traits involving reproduction have lower heritabilities than traits involved with growth or carcass quality.