UNDERSTANDING HOW HEREDITY "WORKS" ...THE BUILDING BLOCKS

Plants and animals are constructed from cells. These small fundamental units arise from other units by the process of cell division. The cells grow then split to give two roughly equal daughter cells. Cells usually contain an inner structure called the nucleus which divides at the same time so that each daughter cell receives its own copy of the nucleus. Every nucleus encloses a fixed number of linear structures called chromosomes, which are actually strings of genes. Each chromosome is made up of a very long chain of the chemical DNA (deoxyribonucleic acid). DNA has special properties, which make it ideal as the medium of the genetic record. It has a composition which allows it to he replicated exactly and it has four types of chemical budding blocks which can occur in different groups of three to make a code. § TOP §

WHAT ARE GENES?

Genes, the determinants of heredity, control the production of proteins. Proteins are made up by connecting amino acids in a specific sequence. The code of DNA specifies the sequence of the amino acids in a protein. Proteins are able to dictate the functions of an organism, including its appearance, because they act as enzymes, or catalysts, which control the chemical reactions in every cell. The actions of many enzymes determine such things as hair colour, bone and muscle growth patterns and lung capacity. The presence or absence of an enzyme is heritable and is governed by the expression of a single gene. Research has also shown that some genes code for proteins that are not enzymes (e.g. hormones and structural proteins such as collagen) and some control the production of special nucleic acid molecules needed to assemble the amino acid components into protein chains. § TOP §

CELL DIVISION

In higher plants and animals, each specific type of chromosome is normally present as a matching pair of two similar chromosomes. The two members of a chromosome pair have a very important functional relationship they contain duplicate sets of genetic information. Before a cell divides, each chromosome of a pair duplicates to form two identical chromosomes, and the pairs become fours. During division of the nucleus, one of each pair of the duplicated chromosomes moves into each daughter nucleus. As a result of these events, collectively called mitosis, the number of chromosomes in the two daughter cells is usually to that of the parental cell. There is one important exception to e process of mitosis. Two specialised cell divisions are needed to form the sex cells, or gametes. In gamete formation, the resulting sperm cell or egg usually contains only one of each pair of each chromosome type. This set of events is called meiosis and takes place in specially adapted areas of the sex organs, or gonads. Union of sperm and egg during fertilisation creates a zygote, which again contains the right number of chromosomes, one of each pair from the male parent and the other from the female parent. Complex organisms like man and the horse are made up of a large number of cells, up to a million times five million, but A these cells are descended from a single cell. This zygote contains A the information necessary for the growth and development of an adult animal. This living cell is able to transmit hereditary properties from one generation to another. § TOP §

DISCOVERING THE PHYSICAL BASIS OF HEREDITY

For thousands of years humans have witnessed the passing of characteristics from parents to offspring, from one generation to another. Theophrastus, a student of Aristotle, first recognized a similarity between animal and plant reproduction and coined the words male and female to describe the participants in sexual reproduction. Other Greek philosophers believed that, because offspring resemble both parents, both sexes must contribute to the formation of a new individual. These contributions were thought to be information of some kind, collected into the male or female semen from parts of the mature parents. Democritus, ahead of his time, suggested that the information was carried in particles whose shape, size and arrangement influenced the make up of the offspring. The physical basis of heredity was not understood until the first years of the twentieth century, although the rules of heredity were proposed first, in 1865, by Gregor Mendel, a German monk and plant breeder. His ideas did not gain general acceptance for another 35 years. Mendel conducted breeding experiments using strains of peas with well-defined differences like seed shape (round or wrinkled) or seed colour (yellow or green). He made a number of genetic crosses between true-breeding parental strains differing in a single characteristic such as seed colour. All the progeny in the first generation (or FI) had the appearance of only one of the parents, For example, when peas with yellow seeds were crossed with peas having green seeds, all Their progeny had yellow seeds. The characteristic which appeared in the Fl progeny is called dominant, while the trait that does not appear in the Fl is called recessive. In the peas, the yellow, trait was dominant while the green trait was recessive. When Mendel made genetic crosses (I between the yellow-seeded Fl offspring, the results proved to be very important. About one quarter of the offspring had w green seeds. The recessive trait had reappeared in the next generation, the F2 progeny. The dominant trait, yellow seeds, appeared in the remaining three-quarters of the second generation. When these experiments were carried to a third (F3) progeny generation, all the F2 peas with the recessive trait bred true and produced progeny with green seeds. Those with the dominant trait fell into two groups. One third bred true and produced only progeny, with the dominant trait, yellow seeds. The remaining two-thirds again produced mixed progeny in a ratio of 75% to 25 % of dominant to recessive.
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MENDEL'S DISCOVERIES

The interpretation of Mendel's results is as follows: the various inheritable traits are controlled by pairs of factors which we now call genes. One factor is derived from the male parent and the other factor of the pair is derived from the female. For example, the pure-breeding strain of yellow peas contains a pair of genes for yellowness (YY) whereas the pure-breeding green strain has two genes for greenness (yy). The gametes of the yellow strain all have the gene for yellowness (Y). The green strain gametes all have the gene for greenness (y). In a cross between YY and yy strains with Y gametes and y gametes, fusion of the gametes at fertilization produces an Fl with both genes (Yy). The seeds are all yellow because Y is dominant over y. The appearance or physical structure of an individual is referred to as its phenotype. Its genetic composition is called its genotype. Individuals with identical phenotypes, like yellow seeds, may possess different genotypes, either YY or Yy. To discover the exact genotype of an individual, it may be necessary to perform several generations of genetic crosses. A gene pair in which both the paternal and maternal genes are the same is referred to by the term homozygous. Gene pairs with different paternal and maternal genes are called heterozygous. Alternate forms of the same gene, like the ones for the different colours of pea seeds, are called alleles. A particular gene may have only one, or two, three or more alleles. Alleles of a gene appear twice in the genotype of an animal, one of the alleles on each chromosome in a pair. Each chromosome is a string of different genes. The particular position of a gene in a chromosome is called the locus (plural loci). The word 'gene' is commonly used in the sense of either allele or locus. An allele is recessive for any characteristic if its effect with respect to that characteristic is not evident in the heterozygous pair. An allele is dominant with respect to a particular characteristic if its effect is the same in the heterozygous pair as in the homozygous. in the pea plant crosses reported by Mendel, one allele of each pair was clearly dominant and the other recessive. Such behaviour is not universal. Sometimes the heterozygous, or hybrid, phenotype is intermediate between the two homozygous phenotypes. It is possible, in this case, to distinguish beterozygotes from homozygotes by their appearance. The two alleles involved are said to be codominant, or incompletely dominant. The reappearance of the recessive green characteristic in the F2 pea generation indicates that recessive genes are neither modified nor lost in the hybrid (Yy) generation. The first law of heredity states that the dominant and recessive genes are transmitted independently to the next generation. The extension of Mendel's pea breeding experiments to strains differing by more than one characteristic led to the formulation of the second law of heredity. This is that there is no tendency for genes arising from one parent to stay together. (See Fig. 3.) Mendel started with two pure-breeding parental strains, one with round yellow seeds (RRYY) and the other with wrinkled green seeds (rryy). Since round and yellow are dominant over wrinkled and green, the entire Fl generation produced round yellow seeds (RrYy). When the F I generation was crossed within itself, two new phenotypes emerged in the F2 progeny together with the two original phenotypes. In addition to round yellow and wrinkled green seeds, wrinkled yellow (rryy or rrYY) and round green (Rryy or RRyy) seeds appeared. This independent assortment can be explained if the different allelic pairs for seed colour and seed shape are located on different chromosomes. The types of chromosomes normally occur in pairs and the number of pairs of chromosomes can differ among animal and plant species. The domestic cat has 19 pairs of chromosomes, the horse 32 pairs, the donkey 31 pairs, the vinegar fly 4 pairs, humans and one species of bat have 23 pairs, maize 10 pairs and the rhinoceros 42 pairs. All the chromosomes appear in matching pairs except in the male of mammals and many other organisms. The chromosomes that form the unequal pair are called the X and Y chromosomes, or the sex chromosomes. The X chromosome is much larger than the Y. Females have a matching pair of X-chromosomes. All male mammals are XY and all females are XX. All the chromosomes in a cell's nucleus apart from these two are called autosomes. An animal's sex is inherited in a simple Mendelian manner. Half the gametes from the male parent (XY) have an X chromosome and half have a Y chromosome. All the gametes (eggs) from the female parent (XX) carry an X chromosome. After union of the two parental gametes, the resulting offspring is male if a Y chromosome from the father is paired with one of the mother's X-chromosomes. A female results from the union of the egg (X) with an X sperm cell. § TOP §

HOW A KNOWLEDGE OF HEREDITY CAN WORK FOR YOU DESIGNING A HORSEBREEDING PROGRAM

The aim of the horse breeder is not really to increase the number of horses in ie world but also to produce horses which have worthwhile characteristics of Information and ability. This goal is achieved not by chance but by design. Lost responsible breeders have some concept of the likely working careers of ie horses they produce. Future performance horses will have at least one parent dented in that direction; hopeful halter champions will have forebears with a good show ring record. While much of the process of breeding outstanding horses is as predictable as Saturday's Lotto numbers, it is not all luck, fashion and crossed fingers. There re some breeders who consistently present horses with the winning touch. There re sires and darns able to produce good progeny even when the other parent has few saving graces at all. Particular crosses ad combinations are known to produce good saddle or halter horses. What is good breeding' and how important is it? : is both something that a particular horse might have, as well as something that studs and horse breeders have to do. The breeding of a horse involves taking one half of the genetic material from one parent and combining it with half the genetic material from the other parent. When a horse is homozygous for a ?ne, the same allele of that gene occurs in both members of a chromosome pair. here is only one version of the gene in ic horse's genetic information but it is resent in a double dose. When the chromosome pairs are split to form the metes, both half-sets of genes will carry ie same allele of the gene and it will be found in all the gametes. This means that I the offspring of the homozygous animal ill inherit that allele. When an animal is heterozygous for a gene, the two alleles re different and its offspring will inherit either one allele or the other. It is not possible to predict which allele of the two will be passed on. It depends entirely on chance. § TOP §

INBREEDING

It follows then that increasing homozygosity in breeding stock will bring some level of predicability to the breeding program and strengthen the influence of desirable genes by increasing their frequency. A stallion or mare which is homozygous for a particular allele of a gene passes it on to the offspring. If both parents are homozygous for the same Allele the outcome is highly predictable. The best way to establish homozygosity in breeding stock is by inbreeding. The definition of inbreeding is the mating of animals which are more closely related than the average in the population. Three factors can determine the success or failure of an inbreeding program: the first factor is the quality of the foundation material, that is the performance of the stallion and mares, their fertility, and their freedom from genetic defects. The second factor is the efficiency of the breeder's selection procedure. Few breeders are as free from sentiment as they should be and many will keep inferior stock 'because she owes us nothing' or 'she's easy to get in foal' or 'he was our first stallion and is a sweet old thing'. This situation may be allowed to exist in spite of the fact that the, mare or stallion in question no longer conforms to a progressive standard of excellence. The third factor is the rate of inbreeding. Rapid inbreeding involves breeding sire to daughter, son to dam or brother to sister. Seven generations of brother-sister matings are required before homozygosity rises above 90 percent. Constant breeding of first cousins for about 15 generations increases the percent of homozygosity to about 65 at which value the percentage tends to level off to stay at less than 20 percent higher than the average population. The average population is generally assumed to be about 50 percent homozygous, especially in purebred horses. Studies with most animals have shown that mild inbreeding with rigid selection results in improvement, but by 62.5 to 70 percent homozygosity there is some loss of vigour. It is possible to calculate the amount of inbreeding in a particular animal especially if the pedigree to five generations is known. A high percentage of inbreeding indicates a greater level of homozygosity in the individual. Nevertheless, as homozygosity increases, some genes will cause a decrease in vigour, which may be so slight that the breeder cannot select against them. Stock seems to become more fragile, size decreases, soundness and fitness for work declines. At this stage, homozygosity has started to work counter to the aims of the breeder. Outcrossing to a new bloodline can bring about a dramatic increase in strength and energy. This is known as hybrid vigour, or here. A high percentage of genes in the offspring of such crosses have become heterozygous. Since most deleterious genes are recessive and many of these are being expressed in highly homozygous stock, the switch to heterozygosity results in loss of expression of many of these 'harmful' recessive genes. A diligent breeder hoping for long-term success must be able to keep the balance between the opposing principles of homozosity and heterosis. MANY BREEDERS ARE RELUCTANT TO PRACTICE THE 'INCEST' FORM OF INBREEDING TO INCREASE HOMOZYGOSITY IN THEIR HERDS. LINEBREEDING OFFERS AN EFFECTIVE ALTERNATIVE TO THIS SYSTEM. § TOP §

LINEBREEDING

Many breeders are reluctant to practice the 'incest" form of inbreeding to increase homozygosity in their herds. Linebreeding offers an effective alternative to this system. The process of linebreeding allows preservation of certain desirable characteristics of a particularly outstanding animal, increasing the homozygosity of the genes conferring these traits, while at the same time avoiding the pitfalls of very close breeding. Linebreeding attempts to get as many genes of a particular stallion or mare into the herd as possible while keeping inbreeding to a minimum. The theory is excellent. But if a breeder decides that a particular horse is outstanding and seeks to concentrate the blood of that horse in the breeding stock, the value of the program is no better than the breeder's judgement of an outstanding horse. In some instances, fashion dictates the line of choice but a truly great breeder can lead the way and make the fashion by choosing a good breeding base and sticking to it. It is possible to linebreed to members of a particular family without restricting bloodlines to one individual. In this way, several sources of the desired genetic material can be used in different combinations. Offspring may have more variety at first but careful selection will result in strong linebred stock. § TOP §

HOMOZYGOSITY BY SELECTION

The term prepotency means the strong ability of a particular breeding animal to stamp its likeness on its offspring. The prepotency conferred by a high degree of inbred or linebred homozygosity can also be achieved more slowly by careful selection of breeding stock for several generations. Although the horses in the breeding program may not be closely related, homozygosity for a certain desirable set of characteristics can be achieved by elimination of the undesirable alleles of the set as they occur. Many desirable characteristics are the result of a number of genes acting together. There is only one way to assure some degree of homozygosity among the several genes coding for a particular trait, such as stamina. For example, potential endurance horses should be bred from proven parents. The second generation of horses, if they are strong, can be bred to others that can take the pace. If lines of successful endurance horses are bred together for several generations, producing a reasonable proportion of offspring with the required soundness and stamina, then more and more of the genes that work together to confer these traits on a horse will be pairing in each new chapter of the breeding book. § TOP §

INHERITED ABNORMALITIES, INCLUDING CID

Excess inbreeding can allow too many undesirable recessive genes to become prevalent and result in too great an occurrence of genetic abnormalities. Abnormalities may be classified as either lethal or non-lethal. A lethal factor leads to the death of the aff6eted animal while a non-lethal inherited weakness may only make the horse less fit for its purpose. Lethals may be true lethals, those like the LwLw 'white lethal' or the oeoe white foal syndrome, which are expressed prior to or shortly after birth. They may be delayed lethals such as heart defects may be partial lethals which only become lethal under certain circumstances. Some inherited weaknesses are partial lethals and they may include reproductive disorders, cataracts, and umbilical and scrotal hernias. Weaknesses which may be accounted for more by heredity than environment include contracted heels, weak or sway back (a recessive gene wb), parrot mouth and the bursting of nasal blood vessels. Inheritance is partly responsible for many causes of unsoundness, in particular ringbones, sidebones, curbs, spavins and weak flexor tendons. One of the most costly delayed lethal factors in horses causes the disorder combined immune deficiency (CID). A foal suffering from CID lacks both B-lymphocytes and T-lymphocytes. These are the cells responsible for the production of antibodies and other elements of the immune system's defence against infections by bacteria and viruses. The foal is protected by antibodies received from its mother during the first month or so of life. Once this period of insurance has expired, the foal will surely die. Sometime in the past a single gene mutation occurred in an Arabian. The mutant gene was recessive, so it was passed undetected through many generations until it ended up on one of the chromosomes of a very popular stallion. Many breeders probably used this horse for his superior qualities, thus rapidly increasing the incidence of the CID gene in the population. Linebreeding increased the chances of the gene occurring on both sides of a pedigree. When two of his descendants carrying the recessive gene were bred together, a CID foal was the result. A mating between two carriers will produce a CID-affected foal one out of four times, on average. Two out of four foals will be unaffected but will be carriers of the gene. Only one of the four will carry two non-defective alleles and will not pass the CID gene on to its progeny. Suspected carriers of sufficient value can be progeny tested by breeding them to known carriers. The appearance of a CID foal confirms a heterozygous genotype. Non-appearance of CID foals simply increases the probability that the horse is not a carrier. Even after eight normal foals are born from matings with known carriers, there is still about a 10% chance that the horse is heterozygous for the defect. (Test your understanding of all the above why would it be impossible to breed to a horse which is homozygous for CID?) In theory, it is a simple matter to cull all mares and stallions known to have produced a CID foal. In practice, culling is frequently a painful and expensive option as many of these horses carry other valuable genes for desirable characteristics. Breeders keep on with the carriers, perhaps hoping to produce the one-in-four that is genetically free of the defect but this practice only increases the number of carriers. Gelding of male potential carriers effectively limits the spread of the gene but the breeding future of female carriers is more difficult to control. The development of molecular biology has given some hope to breeders of Arabians. If the gene coding for the defect can be identified and its chromosome location determined then relatively simple testing will tell whether or not a horse is a carrier. But first the researchers have to catch their gene. They have a lot of DNA to cover in those 32 pairs of chromosomes. It will take time and money before the goal is reached and an Arabian colt or filly can be sold certified genetically free of the CID defect. § TOP §

THE IMPORTANCE OF THE MARE

Discoveries in cell biology and genetics have recently verified a principle long upheld by traditional breeders. The mare is more important in inheritance than the stallion. It is estimated that she contributes about 10 percent more to her foal than its sire does. This is due to a number of factors, in particular the differences between the male and female gametes and the sex chromosomes and the influence that the uterine environment has on the developing foetus. The ovum is about 40 times bigger than the sperm cell. The egg is a large cell which provides 0 the necessities for the establishment of a new life including the machinery to begin the process of cell growth and division. Messages from the maternal genes are set in place in the egg to give the new cell a good start. The 'power packs' of the cell, called mitochondria, have their own simple genetic material which is independent of the chromosomes. The genes in these little generators in every cell of the body are all inherited from the mother, because the originals were present in the egg before it was fertilized. Sperm cells have a long way to go, so they travel light. They bring only their half-sets of chromosomes with them when they fuse with the ovum. The sex of a horse, and a human, is determined by the X and Y-chromosomes. These chromosomes can make a pair in the male (XY) but they are very different in size. The X chromosome is much larger than the Y and accounts for about 5 percent of the total chromosomal material in the horse. All the gametes from a mare have an X chromosome, either one the same as the original from her sire or one as inherited from her dam. The X chromosome carries inheritance that comes directly from the female line. Although fillies receive an X chromosome from their sire, this has been passed intact from the sire's dam with no mixture of material from the sire's sire. The X chromosome, which carries genes other than those related to gender, is truly female in every respect. Unfortunately, there is no sure way yet to determine which family or foundation mare in a horse's pedigree was the source of their X-chromosomes. The importance of the female in determining the type of her offspring is well illustrated when there is outcrossing between contrasting equine types. In the most extreme case of interspecies mating, between horse and zebra or horse and donkey, the progeny resemble the dam much more than the sire. Apart from her genetic contribution, the mare influences the development of her foal through the quality of the uterine environment, the quantity and nutritional value of her milk, and her effectiveness as a 'mother'. So, significant genetic information is passed through the ovurn and is sustained by appropriate maternal qualities. These factors ensure that the tail female line in a horse's ancestry takes on a special significance. The tail female is the bottom mare at each generation of the pedigree. § TOP §

ARABIAN BREEDING

The secret of the Arabs' success at horse breeding may lie in their emphasis on a good mare. (One legend has it that the Arabian breed was descended from the Al Khamseh, five superb mares.) Only stallions from good mares were used for breeding and all records were kept with reference to mare lines, Following the lineage of the mare led to the establishment of 'strains' such as the Kuhaylan, the Ubayyan, the Saqlawi, the Hayndani, the Hadban, the Munique and the Jilfan. From the early nineteenth century, European travellers and empire-builders were curious about the many strains of the Asil, or pure, horses. Rosetti, a resident of Syria, Northern Arabia and Egypt for 40 years wrote: "The strains belong to one breed or genus (Arabian) but they differ in points of conformation and individual characteristics and peculiarities of the shape, which they transmit..." A few Arabian breeders have become interested in trying to recreate the strains, to regroup the genes as they were in the strain-bred desert horses. One way to do this is to breed only to certain lines of the pedigree, the lines, which carry the desired tail-female X chromosome. If each of the dams printed in bold type in fig.1 were the same mare, then the pedigree would show inbreeding for family. By the fourth generation, only these nine lines could have provided the tail-female X chromosome for the filly whose pedigree is represented. Nevertheless, because a female obtains an X chromosome from both her sire and darn, there is no way to trace a strain with total accuracy. For a mare or filly, these are the nine lines, which the breeder should aim to a have as the same strain. In this way, it I should be possible after about six generations to have Arabian horses bred relatively pure in the strain along the correct lines. § TOP §

THE NICK

All successful breeders are following a system of some kind. Some programs follow obvious strategies where bloodline preferences lead to family-based systems like inbreeding or linebreeding. Other methods appear to be more performance-oriented and may be based on judicious outcrossing. An intuitive feel for breeding (and a little bit of trial and error) leads some breeders to have faith in the power of the 'nick'. All of these systems depend on a thorough familiarity with the studbook and a good grasp of current performance statistics. The dictionary defines a nick as a favourable throw at dice. When the word is used about breeding, the sense is much the same. A nick is a mating where the strong points and the weak points of the sire and dam complement one another to produce offspring that are better than either parent. Many breeders have favourite nicks; outstanding horses can be produced when outcrossing is combined with a good nick. Occasionally the nick is so good that the combination goes beyond 'breeding for performance' and becomes 'good breeding'. This outcome is related to the previously discussed topic of breeding for homozygosity while avoiding inbreeding. The theory behind nicks can be explained genetically. There are many genes making up the message for 'winner' and the complexity of their interactions makes it impossible to list specific reasons for the success of a nick. In theory, and as an example, there might be five genes, or closely linked groups of genes, coding for lung capacity, temperament, size of heart, length of back, and straightness of legs. The st0ion involved in a good nick for endurance horses might be homozygous for three of the dominant genes necessary for outstanding endurance performance: large lung capacity, big heart and straight legs. But he might carry two undesirable recessive alleles for temperament and length of back. This would give him the necessary heart-lung stamina and leg soundness to perform well but leave him susceptible to back problems and disadvantaged by a nervous nature. The mare in the good nick might be homozygous for good genes for lung capacity, temperament and length of back, being a short-coupled filly and a steady stayer. However, she could carry recessive genes for inferior heart size and offset ?? stamina and lead to leg problems under stress. Part of the genotypes of stallion and mare would look like this (the alleles represented by upper-case letters are dominant). Sire: LL, tt, If H, bb, SS. Dam: LL, TF, hh, BB, ss. The alleles might be; L = large lung capacity, I = small lung capacity (which neither of them have); T = calm temperament, t = highly strung and excitable; H = big heart, h = small heart; B = short back, b = long back; S = straight legs, s = offset cannons. Progeny of these two will have the genotype: LL, Tt, Hb, Bb, Ss. Remember that the alleles represented by upper case letters are dominant and when an animal is heterozygous for a gene, with two different alleles, the dominant allele is the one that is expressed. So the progeny of the nick will inherit and express the genes for large lung capacity, calm temperament, big heart, short back and straight legs. They should be better endurance horses than either of their parents. The nick comes into its own when parents have good traits that cannot be fully utilised because a complementing trait, such as one for soundness, is lacking. § TOP §

SELECTION OF BREEDING HORSES

Most breeders, including the one-foal amateurs, have some goal for their program. It can range from hoping that a favourite mare will produce a replica of herself to the establishment of an outstanding dynasty. Successful breeders realise their ambitions by using a consistent selection system. Effort and funds spent on the careful arrangement of matings can lead to nothing if the breeder takes an uncritical view of the progeny. The selection process is especially important to the future of colts. A sire's reputation can he ruined if a breeder indiscriminately leaves all his colts entire at the time of sale. Good fillies can be kept for the next stage in the breeding plan but inferior stock must be identified and disposed of. The quality of progeny is a reflection of the genetic constitution of the sire and dam. Poor producers should be removed from the breeder's herd. Evaluation of each crop of foals in a systematic way serves two purposes, The foals themselves can be rated, and the value of their parents as breeding stock can he assessed. ntitonmf used or a fresh bloodline introduced can be evaluated. Several methods of systematic selection have been developed. An individual breeder may wish to institute his or her own processes. § TOP §

THE INDEX METHOD

The index Method is a good system for a breeder who has a long list of items that warrant selection. It involves the development of an index, or point system, for each trait considered important to the selection process. Traits with a high priority are assigned a higher number of points on the scale. Important traits must be carefully balanced against those deserving less consideration. This is the system that is used by most good breeders although they may never put it down on a page in black and white. Breeders using the Index Method construct a chart each year which lists the traits being considered and the total number of points assigned to that trait. The horses being examined are given a score for each trait, which is their fraction of the total possible score for that selectable attribute. (See fig. 2.) If the breeder of the four fillies listed in the grading chart intended to retain the two which scored the highest on the index then a decision would be needed which the system cannot bypass. The top-scoring filly, although perhaps a little small, has good conformation and valuable breeding and could be kept without question. The scores of the two runners-up need some scrutiny before the choice is made. Does the breeder opt for conformation or pedigree? According to the index, both fillies are equally suitable for the breeder's purposes. Maybe Filly 4 gets the nod, because of her good temperament and better movement. The more traits that are used, the less progress towards the breeder's goal can be made in any one-year. Selection for four traits is half as efficient as selection for one. Selection for as many as 16 traits will reduce the efficiency of selection by one-half again. The average serious breeder concerned with about a dozen defined traits will take four times longer to reach a goal than a breeder selecting for only one trait such as colour. Once an index system has been established, it may be appropriate to select breeding stock on the basis of their attaining a certain index score. In some years there may be none scored, as worth keeping while in other years there may be a surplus to choose from It is most important to give the right weight to a particular trait, considering the purposes for which the horse being bred and will be used. For example, a breeder for endurance purposes "I assign high priority to straightness of legs. If a low score in straightness of legs would be considered by the breeder to override high scores in all other traits, then this attribute must be given a suitable weighting to make up a high proportion of the total possible score. If a perfect score is 1500 then 'straightness of legs' might be assigned a possible value of 500. The heritability of a trait must also he taken into account. The body weight or height of a horse may be influenced by environmental factors such as illness or poor nutrition. Less than ideal maintenance methods for stock may be the reason for lower scores in some criteria and the answer in such a case is to change farm practices rather than change matings. § TOP §

MINIMUM CULLING LEVEL

When the breeder finds that the goals of a breeding program are close to being reached, a stricter selection system can be introduced. Minimum Culling Level selection is effective in a high quality herd. The number of traits selected for can be reduced as there is less variation among individuals and only a small percentage of the crop is destined to be retained. All-or-none traits such as colour can be dealt with using this method. Where a trait shows continuous variation (such as height or temperament), selection can be achieved by setting minimum levels below which all animals are culled. As an example, the grading chart for Index Selection can be adapted for the Minimum Culling Level method by retaining the traits Temperament, Movement and Conformation. Progeny which fad to score at least 200, 200 and 450 respectively for these traits would be culled. By this stage, it is assumed that all under-sized, plain-headed, crooked-legged horses with undesirable pedigrees have been eliminated from the herd. The disadvantage of this method is that there is no provision for priority rating of the traits. Because it is an 'advanced' selection method, it is understood that each animal possesses the basic degree of excellence. Selection by this method is hard-core culling. § TOP §

FASHION

The dictates of fashion can be the undoing of a breeding program. Studs producing well-conformed, well-performed horses can find that a different family has become the flavour of the month. Charismatic stallions have a way of casting a long shadow over established bloodlines. In a bid to attract the fashion buyers, the new line might be introduced into a long-term program without any real justification even as a complementary outcross. Selection of an entire generation can thus be wasted. A breeder who has embarked on a defined selection program should resist the temptation to alter the goals of the system in response to the fads of the show ring. It is a breeder's duty to the breed to be consistent. § TOP §

READING:

William E. Jones. 1982, Genetics and Horse Breeding. Philadelphia: Lea and Febiger.
Maxine Singer and Paul Berg. 1991. Genes and Genomes: A Changing Perspective. Mill Valley, California: University Science Books. § TOP §