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"The Founder Effect: Origins, Characteristics and Future of the New Zealand Dexter Herd - Part I"

by Tony Cutten and Alex Meades

Founder Effect

The founder effect occurs when a small group of individuals breaks away from an original population to found a new colony. By definition, the new colony is geographically and genetically isolated and does not return to its original territory. The particular finite pool of genes (alleles) represented by the founding cohort determines the genetic makeup of the pioneering population (herd). If these colonizing individuals are introduced into an existing population (base herd), the subsequent generation will have a new allele frequency somewhat intermediate between the original makeup of the recipient population and the donor group.

Genetic Drift

Genetic drift occurs when, by chance alone, generation by generation (in humans nominally each 25 years, in cattle around every 2-3 years), genes are lost. This is not a "good" or "bad" thing, it is evolution. After many hundreds of generations genetic drift may start to be seen in a visual sense by observable phenotypic traits, such as fair or red hair in Nordic people, and eye shape and dominance of straight black hair in Oriental people. Particular alleles will be lost by the process of genetic drift by chance alone. At each conception, the process of meiosis involves random breaks between the paired up chromosomes of sire and dam. The resultant daughter chromosome, which determines the offspring, results from the random ligations of the various segments of the DNA sequence. This process can be thought of as a bit from dad and a bit from mum. Depending on the breeding potential (how many offspring are produced by a particular animal) so will that animal influence to greater or lesser extent the gene pool. From a population perspective, particular alleles will be lost in each generation. In a small founding population, alleles, perhaps "desirable alleles", can disappear. The genotypic nature of the population drifts.

The world of controlled breeding such as is found in small Dexter herds is somewhat different. Matings are not necessarily random. New bulls are periodically introduced. There is the chance of reintroduction of sires possessing "lost genes" from "original" herds e.g. from the UK, USA, Canada, and Denmark.

In cattle solid colour has been "fixed" in certain breeds: black in Angus, white face in Herefords, red in Red Devon, black and white (mosaicism) in Holstein Friesian, and small stature, black, red or dun in Dexters. This has almost certainly occurred by the intervention of people in controlling cattle breeding.

The New Zealand Dexter Herd

The NZ Dexter herd may be considered as a modification of the founder effect. The embryonic herd remained isolated for a number of years by Department of Agriculture constraints against further introduction of Dexter genes because of the BSE mad cow disease outbreak which stopped importation for a period in the 1980s. A further period of constraint is in place during the Foot and Mouth outbreak in UK 1998 to 2002 (current).

The early selection of Dexter genes which established the NZ Dexter herd was dominated by the following sires: Bookhams Robert (1979, when 200 straws were imported), Shootlands Ben Macdhui (1984) and, after a six year interval, Saturn of Knotting (1990), then Bradner Hilltop Dunstan, Braco Shoeshine, and Cornahir Outlaw (see Appendix for genealogies).

Alex and Anne Meades graded up Dexters by AI (artificial insemination) into their Jersey herd to produce the first purebred Dexter bull born in New Zealand - Meadowpark Charles, born in 1988. The dam base which was used to establish the NZ Dexter herd was dominated by the Jersey breed and later by Angus foundation stock. As befits one of the major dairy product exporting nations in the world, it is not unexpected that dairy breeds have been chosen in the NZ breeding up program - the predominant breed being Jersey. Friesian and occasionally Shorthorn have also been used (see Table below). Preservation of milking quality is therefore a feature of the NZ Dexter herd. Of the beef breeds, Angus as a full colour black animal has predominated. To a lesser extent Red Devon, Red Poll or Murray Grey (an Angus variant) have been used.
 

Breeders who have subsequently come to the society in recent years are small block lifestylers (with a few notable exceptions, e.g. the Glenaan herd, described below), who started their breeding programmes with beef breed base cows and are producing larger beef style Dexter crosses. These animals show full meaty hindquarters and preserve the meat producing aspect of the Dexter dual-purpose animal.
 

On to Part 2
 

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