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Causes of Genetic Abnormalities in Cattle
DPI
Breeders may see various genetic abnormalities in cattle from time to time. Examples are albinism, hairlessness, mulefoot and dwarfism. Usually they are harmful and we may want to eliminate them, but occasionally there can be some advantages associated with even such harmful characteristics.


What types of genetic abnormalities are found?   Top
Any defect that is inherited is described as a genetic abnormality, and may take one of many forms - the extreme being where it is visible and lethal. Other effects can be less obvious and can vary from premature abortion and early embryonic death, to animals that are weak or poor doing, slow growing and inefficient, with lower vigor, fertility, and longevity.

What are the causes of genetic abnormalities?   Top
Genetic defects usually show what is called recessive inheritance. A parent can carry the defective gene and appear normal; the parent is then known as a carrier. If both parents pass the defective gene to the offspring, the genetic defect shows up and the genetic condition in the offspring is called homozygous recessive. The defective gene in the carrier animal is present along with the normal gene and this condition is termed heterozygous. The underlying problem of genetic defects is that parents that appear to be perfectly normal can be carriers and so can produce offspring that are defective. Parents that never produce defective progeny are in the majority and are called homozygous normal.

At first glance it seems that if we could identify carrier animals and eliminate them from the breeding population the problem of recessive genetic defects would be solved. However, it is necessary to explore the problem a little further before deciding that identifying carriers is necessarily worth the money and effort.

There are a few defects that exhibit dominant inheritance, which means that a carrier animal with one defective gene can be identified directly. Cattle breeders are unlikely to retain animals carrying such genes. Some embryos with severe genetic defects are never seen, since they are lost soon after fertilisation and we would normally assume that the cow had not conceived. So far we have discussed single gene defects. All genes, normal or unfavourable, are organised onto chromosomes and these are arranged in pairs. There are 30 pairs in cattle cells. Occasionally a major defect has been noted that is attributable to the mispairing of chromosomes, but there are few defects in cattle that can be explained in terms of such chromosomal abnormalities.

Although it is possible to see chromosomes under the microscope, the genes which they carry cannot be seen. Thus it is not possible at present to screen individual animals for all known genetic defects by chromosome studies (karyotyping).

The criteria for deciding what defects are best studied and perhaps controlled by karyotyping are as follows:

  • that there is evidence that a chromosomal abnormality causes a particular production defect, rather than just happening to be associated with it
  • that the abnormality actually occurs in the nation's cattle population
  • that there is no simpler means of studying or controlling the defect.
  • While karyotyping is unlikely to be of benefit in individual herds, there are circumstances where the procedure is of value. Many aspects of semen 'quality' of AI bulls are checked at the AI centre, including the karyotype, and this is sound practice because of the possible widespread use of semen from a particular bull.

    How widespread are defective genes?   Top
    Every animal species, breed and individual animal carries some genes that, when in the pure or homozygous condition, may prove to be harmful. Furthermore, no individual or breed is genetically pure or homozygous for all gene positions on the chromosomes. Inbreeding increases the amount of homozygosis and results in decreases in vigour and general fitness. Our livestock are not meant to be homozygous and cannot stand being homozygous.

    Two forms of recessive genes   Top
    Two kinds of genes that can lead to problems of recessive genetic abnormalities can be defined. One kind, which is not wanted, shows no effect in the heterozygous form, but when it occurs pure in the homozygous form, the defect shows up. The other kind of gene may have a beneficial effect when present in the carrier or heterozygous animal, but when it occurs as a homozygote a deleterious effect is visible.

    Unwanted genes are usually present in very small numbers and kept in a breeding population by chance, recurring mutations, and by migration of new animals into the population. Examples are those that cause mannosidosis, spastic paresis and albinism. Such unwanted genes can increase in frequency if a particular animal that happens to carry them is used too extensively, as may often occur when linebreeding is followed. The result is that some genetic defect will result further down the line, which may not have been encountered before in that strain.

    The best way of keeping the number of individuals affected by unwanted genes down is to prevent inbreeding, or conversely, to use outbreeding. In small herds of say less than 50 cows, this is best accomplished by examining the pedigrees and avoiding mating animals that have common ancestors in their pedigree at least in the two or three most recent generations of their pedigree.

    In the absence of a monitoring plan for defects and in the absence of outbreeding, it is necessary to identify the carrier animals if any control is desired. In larger herds, 'turning over' sires in the herd as quickly as possible and not taking too many replacement sons from any one sire will be important in minimising inbreeding. Using fewer cows per bull will also help, although there is an optimal level because some selection intensity would be forfeited at the same time.

    The other kind of gene, which shows some advantage in the heterozygous condition, creates a special problem. This type of gene complements its partner gene in the heterozygote, producing some superiority over the homozygous normal; but when it is paired with one of its own kind in the homozygous recessive, there is some loss of function and abnormality results.

    Such genes can increase in frequency because of the relative advantage they confer on the carrier animals. Among all known genetic abnormalities, however, these are rare. If they confer a very strong advantage on the farm or in the market place they may increase in frequency very rapidly.

    Such was the case with dwarfism in beef cattle a couple of decades ago in North America. The heterozygote had the type that was favoured by breeders and show judges at that time. When two carriers were mated the result could have been a dwarf calf. This was expected to occur in 25% of joinings. Eventually dwarfism was eliminated because the characteristics preferred in the carrier became unfashionable. Higher performing, faster growing animals were preferred to the dwarf carrier animal and as a result, the dwarf genes in the population were diluted by new bloodlines being introduced .

    Depending on the particular characteristic and the economics involved, it may be desirable to utilise genes that produce an advantage in heterozygotes. This can be done by consciously producing carriers. Special test matings with known carrier females are the most appropriate method for detecting carriers of such specific genes.

    Thus we see that there are two kinds of genes that might be considered defective. The first, arising by mutation or migration, has no observable effect in the heterozygote, but is deleterious as a homozygote in its pure form. It merely spreads by chance brought about by excessive use of particular animals. The second kind of gene shows an improvement in performance in the carrier condition, and its spread is brought about because of the advantage conveyed to any individual that is a carrier.
     

    Mike Harrison, Melbourne
    Adapted from an article by Professor R. T. Berg, Department of Animal Science, University of Alberta, Edmonton, Canada
    © The State of Victoria, Department of Primary Industries, 2002.
    This material may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or consequences which may arise from your relying on any information contained in this material.



     
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