SOME FUNDAMENTALS OF HEREDITY IN HORSES
Until very recent times , the general principle that “ like begets like ” was the only recognized concept of heredity . That the application of this principle over a long period of time has been effective in modifying animal types in the direction of selection is evident from a comparison of present-day types of animals within each class of livestock . Thus , the speed of representatives of the modern Thoroughbred — coupled with their general lithe , angular build and nervous temperament is in sharp contrast to the slow , easy gaits and docility of the Tennessee Walking Horse . Yet there is good and substantial evidence to indicate that both breeds descended from a common ancestry . Because of the diverstiy of genes carried by the original parent stock , it has been possible , through selection , to evolve with two distinct breeds — one highly adapted to fast running at extended distances and the other to a slow , ambling gait . Also , through selection accompanied by planned matings , this same parent stock has been altered into horses especially adept as hunters , jumpers , stock horses , polo mounts , three- and five-gaited park hacks , harness race horses , etc .
The following is a condensation of a few of the known pertinent facts relative to the genetics of horses :
The Gene is the unit of heredity . In the body cells of horses there are a number of pairs of bundles , called chromosomes . In turn , the chromosomes carry pairs of minute particles , called genes , which are the basic hereditary material . The nucleus of each body cell of horses contains 32 pairs of chromosomes , or a total of 64 ; whereas there are thousands of pairs of genes . When a sex cell ( a sperm or an egg ) is formed , only one chromosome and one gene of each pair goes into it . Then when mating and fertilization occur the single chromosomes from the germ cell of each parent unite to form new pairs , and the chromosomes and genes are again present in duplicate in the body cells of the embryo . Thus , with all possible combinations of 32 pairs of chromosomes ( the species number in horses ) and the genes that they bear , it is not strange that full sisters ( except identical twms from a single egg split after fertilization ) are so different . Actually , wc can marvel that they bear as much resemblance to each other as they do .
Because of this situation , the mating of i mare with a line track record to a stallion that on the average transmits relatively good performance will not always produce a foal of a merit equal to its parer ts . The foal could be markedly poorer than the parents or . happily ,
28 it could in some cases be better than either parent .
Simple and multiple gene inheritance occur in h as in all animals . In simple gene inheritance , only one n^ ’ of genes is involved ; thus , a pair of genes may be resno ible for color of hair in horses . However , most charact — such as speed — are due to many genes ; hence they a called multiple-gene characters . For most characters [ n
or more pairs of genes are involved . Growth rate in [ L \ for example , is affected by ( 1 ) appetite and feed con ’ sumption , ( 2 ) the proportion of the feed eaten that is ab ' sorbed , and ( 3 ) the use to which the nutrients are put-J whether they arc used for growth or fattening . Because multiple characters show all manner of graduation — from high to low performance , they are sometimes referred to as quantitive traits . Thus , quantitative inheritance refers to the degree to which a characteristic is inherited ; for example , all race horses can run and all inherit some ability to run , but it is the degree to which they inherit the ability that is important .
Dominant and recessive factors exist in horses . Some genes have the ability to prevent or mask the expression of others , with the result that the genetic makeup 0f such animals cannot be recognized with accuracy . This is called dominance . The gene that is masked is recessive . Thus , black is dominant to chestnut ; hence when a pure black stallion is crossed on a chestnut mare , all of the offspring will be black . The resulting black is not genotypically pure , however ; it is Bb , where B stands for the dominant black and b for the recessive chestnut . This black animal will produce germ cells carrying black and chestnut genes in equal proportion . Then if an F1 stallion is crossed on Fx mares , the F2 population will , on the average , consist of three blacks to one chestnut . The chestnut — being a recessive — will be pure for color ; that is , the mating of two chestnut horses will produce , according to the most authoritative work , chestnut offspring , which is the situaion in the Suffolk breed of draft horses where all animals of the breed are chestnuts . Of the three black , in the F2 , however , only one is pure for black ( with the genetic constitution BB ). The other two will be Bb in genetic constitution , and will produce germ cells carrying B and b in equal proportion .
As can be readily understood , dominance often makes it difficult to identify and discard all animals carrying an undesirable recessive factor . Also , in some cases dominance is neither complete nor absent , but incomplete or partial and expressed in a variety of ways . The best known case of this type in horses is the Palomino color .
Heredity and environment in quantitative traits function in horses , just as they do in all animals . Generally horse trainers believe that heredity is most impoiant , whereas horse owners believe that environment partic urly training , is most important — especially if they ose a race . Actually ; maximum development of characters^ economic importance — growth , body form , speed , etc . cannot be achieved unless there are optimum condition of nutrition and management .
The problem of the horse breeder is that of selecting
the very best animals available genetically — these 0 ^ parents of the next generation . The fact that on y 30 percent of the observed variation may be due to c * lty , and that environmental differences can produce leading variations , makes mistakes in the selectio breeding animals inevitable . a Sex is determined by chromosomes . The lTialC l ? cs ) pair of similar chromosomes ( called x chromoso
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VOICE of the Tennessee Walking Hors