Why is inbreeding so bad?

Answer 1

OK. I'm going to hope that you are not talking about people. But for animals it is not necessarily always a bad thing. The problem occurs when too much inbreeding or line-breeding has occurred, and bad genes are present. For example, animal breeders will often resort to inbreeding or line-breeding to enhance a certain trait. This is how come we have so many different breeds of dogs. If you breed a dog with a certain trait to a dog that it's related to, you increase the chances of the offspring having that same trait. Many of the problems that purebred dogs often have is also attributed to the inbreeding that occurred to make them "pure". Think about it, if you breed a dog that has inheritable Heart disease with an unrelated dog, the chances of the offspring having heart disease is reduced. But if you breed that same dog with it's parent, sibling, or offspring, then the chances of the resulting offspring having heart disease is increased exponentially. I hope that my answer helps you understand how inbreeding works.

Answer 2

Could be nothing, could be deformity, could be still born. Inbreeding reinforces good traits and bad alike. By going outside of a gene line, you increase the chances of not reinforcing a bad trait. Photocopy something and use the copy to make another one and keep doing that. After a while, you end up with a fuzzy barely recognizable copy. Same thing happens with inbreeding. Anyone's DNA needs to be mixed with others to supply the parts we lack or have damaged. Doesn't always work, but the chances are better than with offspring from the same gene line. Cow, hamsters or people.

Answer 3

Inbreeding actually has numerous benefits, despite all of its well known negative consequences. But this is actually a very complex question, which requires an elaborate answer.

First, inbreeding should not be confused with incest. Incest is the mating of individuals related in a direct line, and thus sharing half of their genomes ( brother-sister, parent-child ), or a quarter (child with grand-parent, or with uncle or aunt). Inbreeding is simply the mating of individuals that are more closely related than the average overall population. Hence, even in a theoretical completely panmictic population (i.e. mating completely at random ), a large proportion of matings will still be inbred.

The first advantage of inbreeding is that it reduces the cost of sex: when you breed with someone who is completely unrelated to you, each of your offspring only carries half of your genes. But if you share say 1/10 of your partners’ genes because you have common ancestors, your offspring will carry 55% of your genes.

The second advantage of inbreeding is that it cleanses the genome of recessive deleterious mutations: because DNA can mutate, and particularly because DNA replication is not a completely faithful process, new mutations will occur in the germ line every generation. In humans, recent estimates obtained through whole genome sequencing are of about 100 new mutations per generation, occuring more frequently in sperm, and increasing with the age of the father. The vast majority of those mutations will occur in silent DNA, however, but one or two may affect a gene’s functions. And of those, most will inactivate the gene and thus be deleterious rather than result in an improvement, but they will mostly be recessive because having just one functional copy of a gene suffices for most of them. Inbreeding will indeed bring those recessive mutations to light, and result in inbreeding depression, in other words in unfit offspring or even in lethal phenotypes. Somewhat counter intuitively, however, the overall result is that the mutation load ( i.e. the average number of mutations per genome ) will be reduced in the surviving offspring after inbreeding. This is why inbreeding depression is reduced in subsequent generations of inbred stocks, and how purebred stocks and varieties can actually be obtained and maintained by plant and animal breeders.

The third advantage is directly related to the second in that inbreeding allows the expression of advantageous but recessive phenotypes. This is particularly relevant for mutations that allow to become insensitive to pathogens such as viruses, or to evade predators because those usually correspond to the loss of a function or character, and those are usually recessive. In an essay published last year, I have actually advocated that this type of mutations was likely acting as a main driver of speciation, i.e. the preferential breeding

of individuals within small groups rather than with the ancestral stock.

The fourth advantage of inbreeding is that it allows to keep useful combinations of genes together rather than separating them every generation, and thus reduces what is called the recombination load

A fifth advantage of inbreeding is that it promotes cooperative behaviours and altruism. Indeed, in populations that are structured in small groups, the fitness of individuals will also depend on the fitness of the groups to which these individuals belong. This is called group-level selection, and is a more contentious concept which is not accepted by all population geneticists. What is clear is that, in panmictic populations, most models predict that selfishness will be promoted.