Trade Offs

Cancerous alleles are still very common within the human population today. An individual that has these cancerous alleles has a higher likelihood of eventually developing cancer. This increased likelihood of developing cancer, in turn, increases the likelihood of the disease becoming detrimental and fatal. In cases where individuals are affected this way by a cancerous illness, it would probably impact their reproductive output. Even if the disease begins to characterize itself after an individual is past their reproductive time, it would still result in a lower reproductive fitness.

So that raises the question; if individuals and families that carry these variant alleles, that lower one’s average reproductive fitness, then why is cancer still so prominent? It’s reasonable to wonder if those alleles lower an individual’s ability to reproduce, why those certain variant alleles aren’t selected against and out of the population. One explanation for this is that there is an antagonistic pleiotropy occurring. An antagonistic pleiotropy is a trade off, or to put it simply where a positive thing is offset by a negative thing. Antagonistic Pleiotropy is seen most commonly when there is a variant form of an allele that increases an individual’s fitness in one stage in life, while reducing fitness in a later stage in life. This is a possibility for cancer alleles, but only if having a variant allele that leads to cancer also leads to having a reproductive advantage at an earlier stage in life. 

Some research studies have shown that there is a possibility of this. In one study, researchers found that cisgender women in families with a BRCA1 or BRCA2 variant allele tended to have more children than women in families without a BRCA1 or BRCA2 variant allele. There have been some criticisms of this study, in that all the women in a given family were considered in the same category, instead of separating those that had a BRCA1 or BRCA2 variant alleles on an individual level. It’s likely that there were some women in the family who did not have a variant allele, in the event that one of the parents were heterozygous for BRCA1 or BRCA2 genes. 

Another study researched this same idea, instead they looked within families with variant BRCA alleles. By studying both women with a BRCA variant allele and women without a BRCA variant allele from the same family, these researchers concluded that the women with the variant alleles had 1.9 children on average, whereas women without the variant alleles had 1.5 children on average. So, this data sure seems to indicate the presence of antagonistic pleiotropy.

However, results from other studies show no form of correlation between variant BRCA alleles and fertility rates. So at this point in time, the answer has yet to be uncovered.