As you age, your DNA progressively accumulates mutations. Everything from the sun’s rays to the food you eat can slowly modify your DNA. As we’ve discussed, most of these mutations don’t affect protein production enough to alter phenotypes; in fact, most mutations are silent mutations. But if you live long enough, one of these random mutations will affect an important protein. If the wrong protein is altered, your heart could fail or you could develop cancer. This is why the older you get, the more likely you are to get cancer.
Sex offers a way around this problematic genetic decay. Only mutations that occur in the germ cells (the cells that undergo meiosis to create gametes) can be passed on to future generations. The mixing of parental DNA allows most recessive mutations to get masked by the other copy of DNA. Meiosis acts to create diversity and fertilization helps mask many potentially harmful side effects of that mixing.
This is where Dolly the cloned sheep is at a disadvantage. She was a new sheep created with a copy of DNA from a cell from the mammary gland of an adult sheep. That cloning process is dangerous for two reasons. The first problem is her age. The cell from the original adult sheep was the product of many mitotic divisions and a (sheep’s) lifetime worth of accumulated mutations and DNA damage. When Dolly was born, she was born with the accumulated DNA damage of a fully-grown adult. Dolly’s cells showed signs of premature aging, which would not have been the case if she had been a product of sex; rather, she would have begun life with “fresh” DNA.
The second problem with skipping sex is what this does to the population. If all the sheep in a continent were clones of each other, they would all have the same DNA. This sameness would impart the population with identical genetic strengths and weaknesses. A single disease could wipe out the entire population. This disease susceptibility is one of the reasons farmers and horticulturalists are concerned about growing monocultures—single crops that are virtually genetically identical, even if they aren’t necessarily clones. Genetic diversity helps a population be resilient in the face of change, and meiosis and outcrossing are the creators of this diversity.