Final answer:
Concordance is used instead of heritability when dealing with small sample sizes or single-gene disorders to assess the genetic basis of a trait, as heritability requires larger population sizes to measure variance due to genetic differences.
Step-by-step explanation:
Concordance and heritability are both terms used in genetics to describe different aspects of genetic influence. Concordance relates to the presence of the same trait in both members of a pair of twins (identical or fraternal), which is particularly useful in twin studies when assessing the genetic contribution to a trait or condition. In instances like human genetic disorders, using concordance rates can be more informative than heritability, especially when large sample sizes, like those in Mendel's experiments, are not feasible. For example, if a small number of children are analyzed, the variability due to chance can significantly affect the observed ratios of a trait. Thus, concordance rates are preferred in such cases because they can provide evidence of a genetic influence even in small samples.
On the other hand, heritability is a measure that estimates the proportion of variance in a population that is due to genetic differences. It is more suited for traits influenced by a large number of genes and is typically used in studies where large populations can be examined to overcome the random effect of allele segregation, not for single-gene disorders like cystic fibrosis. So, concordance would be used instead of heritability when dealing with small sample sizes and single-gene disorders, where the focus is on the genetic basis of inheritance rather than population variance.