Why do individuals obtained by parthenogenesis turn out to be weaker and more painful than ordinary ones?

In fact, this is not entirely true. Parthenogenesis is a method of sexual reproduction. It is characteristic of some species of insects and lizards. When a female does not find a sexual partner, she can produce offspring without him. That is, the DNA of the mother and offspring are identical. There is a drawback here, since the mixing of genetic information does not occur and the pathologies that the parent organism has are transmitted to the daughter organisms. This doesn’t happen when genes are mixed, so they often get useful genes.