well, the benefit of having two copies of the genome is that one gene is broken, you are not dead. the other one simply takes over. this mechanism provides resilience against genetic defects. inbreeding undermines this mechanism.


and the benefit of sexual reproduction is the recombination of genes. if one individual has one beneficial mutation, and a different individual has a different beneficial mutation, sexual reproduction makes it possible that in the end, all have both beneficial mutations. without sexual reproduction, one part of the species has one of the beneficial mutation, and the rest has the other one, and the only way to improve is to wait for another beneficial mutation to come along. so it greatly speeds up the process of evolution. again, inbreeding undermines the mechanism, one gene needs to be able to float around in the entire gene pool of the population. if you have a bunch of inbred families and not much cross-breeding going on, the process is slowed down and the benefit of sexual reproduction is diminished.

you never get a perfectly clean genome, defects get introduced by radiation, by carcinogens, and by retroviruses. the diploid set of chromosomes and sexual production both help adress this issue, inbreeding makes it worse.

but then, in captivity it has been done with lab rats, there are lines that have been inbred for hundreds of generations over many decades in the most extreme way: brother to sister in every single generation. it has no benefit for the rats, and early in the process there are a lot of deformities and health issues. the benefit is that after a few hundred generations, they are basically all genetically identical, which helps in scientific research. real drawbacks are that one infection can eliminate the whole colony, and they all have the same weaknesses. in one line, they all tend to be overweight, they all tend to get diabetes, they all drink alcohol the first time you offer it to them, and they all have the same behavioral quirks. personally i think thats quite messed up. in an evolutionary sense, they stopped evolving, they are like frozen in time. but hey, no need to sequence, you can just download their genome online.