INBREEDING, LINE BREEDING and MINIATURE HEREFORDS
An issue for ‘closed herds’, like the majority of miniature Herefords in Australia, is the prevalence of inbreeding. High inbreeding coefficients can reduce fertility (for example) and increase the incidence of birth defects.
Inbreeding has always been of lively interest to livestock breeders. Widely differing opinions exist between breeders and even scientists on this subject. It is therefore not uncommon to find many diverse breeding practices concerning inbreeding in the livestock industry. Although some breeders claim success with the use of inbreeding, it is generally accepted that practicing inbreeding holds very few advantages and should at best be considered risky if the large number of disadvantages are taken into account.
Inbreeding occurs when animals related to each other are mated. An animal is inbred when both its parents have a certain proportion of identical genes. The inbreeding coefficient is defined as the probability that two identical genes (one from each of the parents) is found at the same locus. The inbreeding coefficient varies between 0 (0%) and 1 (100%) – a higher value indicating that the parents of the animal concerned were more closely related. This common heritage is expressed by a parameter called the inbreeding coefficient, first proposed by Sewell Wright in 1922.
The inbreeding coefficient is a function of the number and location of the common ancestors in a pedigree. It is not a function, except indirectly, of the inbreeding of the parents. Thus, one can mate two highly inbred individuals who share little common ancestry and produce a calf with a very low IC. (Because the potential number of ancestors doubles every generation, eventually you reach a point where the number of ancestors exceeds the number of individuals alive at that time. Therefore, you are bound to find some common ancestors if you go back far enough.) Conversely, it is possible to mate two closely related individuals, both of which have low ICs, and boost the IC substantially.
Unfortunately, in the average pedigree, there are a large number of shared ancestors.
Therefore, the total inbreeding for a calf cannot generally be calculated manually and appropriate software must be used. Calculating inbreeding for only the first few generations is not particularly useful. If there are more than one or two common ancestors in four or five generation pedigree, the inbreeding is probably already higher than desirable. Unfortunately, having none is no guarantee that common ancestors will not occur in abundance further back, and some pedigrees of this type still achieve moderately high inbreeding coefficients. Neither can be number of shared ancestors be used as a reliable guide, as the inbreeding coefficient is very sensitive to when and where they occur in a pedigree.
A low inbreeding coefficient indicates that a calf has few common ancestors, thus minimising the chance of genetic defects. Inbreeding and line-breeding really differ only in degree. Line-breeding is less likely to cause harm than inbreeding. Inbreeding is not for novices. Knowledge of genetics and the breed is required for success. For good results, it must be well-planned and breeders must be ready for whatever problems it presents.
An excellent discussion paper can be found at http://www.CompuPed.com/bell.htm , although this talks about dog pedigrees, the ideas can be applied to miniature Herefords.
Another site is http://www.inform.umd.edu/EdRes/Topic/AgrEnv/ndd/genetics/INBREEDING.html where they talk of inbreeding in relation to dairy cattle.
Inbreeding: Its Meaning, Uses and Effects on Farm Animals http://muextension.missouri.edu/xplor/agguides/ansci/g02911.htm
Inbreeding in Swine http://persephone.agcom.purdue.edu/~agcom/Pubs/NSIF/NSIF-FS4.html
Genetics of Reproduction: Considerations for Sire Selection http://muextension.missouri.edu/xplor/agguides/ansci/g02039.htm
Effects of Inbreeding and Heterozygosity on Preweaning Traits in a Closed Population of Herefords Under Selection http://jas.fass.org/cgi/content/abstract/76/5/1303