INBREEDING --- HORRORS!?
By W. F. Hollander
August 2000 

Self-fertilization is the “closest” inbreeding. Mendel’s peas normally do it.  Without crossing, selfing for about 7 generation produces pure lines completely homozygous and free of lethal and seriously detrimental genes.  Like clones they are reliable. Generally we like reliability, despite monotony. 

But most of our crop and domestic animal species are less inclined to pure-breed. Maize is normally wind-pollinated, with potential pan mixia.  Enforced self-fertilization of such plants produces varied progeny because of segregation and recombination from heterozygous loci.  Statistically this is termed inbreeding depression, but pure lines can be produced---reliable but commonly “inferior”. Crossing different pure lines produces uniform F₁, generally “superior” to the parents.  The program of crossing may be extended—3 way, 4-way, etc., and also rotational. 

Brother-sister mating is the closest inbreeding for animals.  Long-continued sibling mating---again reliable, but expensive and often “inferior” in some respect may produce pure lines. 

Cousin matings and parent-offspring matings may be fairly “close” inbreeding, and with selection may produce fairly pure lines—again reliable, and possibly not “inferior” 

So why is inbreeding usually considered “bad”?  Production of “defective”, lethal, or otherwise undesirable progeny obviously seems best avoided.  But how else can recessive lethals, and detrimental genes be exposed for elimination in an otherwise desirable stock? 

Close inbreeding when the parents are mutually heterozygous for many (say 10) deleterious genes may result in practically unavoidable homozygosity for one or more in the progeny.  That is scary.  With “mild” inbreeding the parents are unlikely to have many heterozygous deleterious gene in common, so the results are less drastic. 

So with domestic animals a happy medium seems the solution.  How about pretzels breeding—in, out, around, and back.  The recipe is flexible.