While Mendel discovered that a gene determines a physical feature, Dobzhansky realised the full picture – the intersection of genes, environment and chance/triggers determines the physical outcome.
An excerpt from The Gene:
In humans, a mutant BRCA1 gene increases the risk for breast cancer- but not all women carrying the BRCA1 mutation develop cancer. Such trigger-dependent or chance-dependent genes are described as having partial or incomplete “penetrance” – i.e., even if the gene is inherited, its capacity to penetrate into an actual attribute is not absolute. Or a gene may have variable “expressivity” – i.e., even if the gene is inherited, the extent of it becoming actualised into an attribute varies from one individual to another. One woman with the BRCA1 mutation might develop an aggressive, metastatic variant of breast cancer at age thirty. Another woman with the same mutation might develop an indolent variant; and yet another might not develop breast cancer at all.
We still do not know what causes the difference of the outcomes between these three women – but it is some combination of age, exposures, other genes and bad luck.
Our indeterminable fates must cause us to question our persistent clinging onto certainty in life, and our lack of examination of what it means to live.