Question

how can a loss-of-function allele in a gene encoding the cohesin protein could be dominant to the wild type allele ?

Answer 1

Loss of function alleles are usually recessive as human are diploid and they have two copies of each gene. So if one gene have experienced mutation and the product synthesized have lost its function, the other gene can easily compensate for it.

But in some cases both the copies of genes are required for the proper expression of the protein product. Why?

With one gene mutated, there's only one active copy of the gene and this result in the knockdown of translation. Knockdown here means that the protein is being synthesized but at a lower rate.

When two copies are present, both genes can provide mRNA and this means that double the amount of protein is available. But in loss of function mutation, one protein is non functional and the product turnover isn't enough for the cell to meet its requirement.

As the genes which are mutated in Bushy syndrome are proteins subunit of Cohesin, this means that less Cohesin is synthesized than what is needed. This leads to the change in phenotypes and symptoms of disease.