Chapter 10 Genetics.
Genetics is a science that studies heredity and variation. Heredity is defined as the resemblance among individuals related by descent; variation is the occurrence of differences among individuals of the same species. Genetic material is contained on the chromosomes of all cells, and the chromosomes are found in the nucleus of cells. Reproduction is the process of getting genetic material from the male in the form of sperm to the female egg.Before reading this chapter, you should review Chapter 4. In Chapter 4, the function of the cell is discussed.
OBJECTIVES
After completing this chapter, you should be able to:
* Define a gene, an allele, and a chromosome
* Describe DNA
* Explain the difference between phenotypic and genotypic expression
* Give the correct number of chromosomes for horses and mules
* Discuss basic inheritance
* Distinguish between single- and multiple-gene inheritance
* Distinguish among recessive dominance, codominance, and partial dominance
* Describe how DNA codes for proteins that make up the body and function in the body
* Compare qualitative traits to quantitative traits
* Discuss the relationship between genetics and environment
* Explain how genetics determines coat color
* Name five genetic diseases or abnormalities
KEY TERMS
additive genes
albino
alleles
chromosomes
deoxyribonucleic acid (DNA)
dominant allele
gametes
genes
genome
genotype
heterozygous
homozygous
jack
karyotype
messenger ribonucleic acid (mRNA)
nonadditive gene
nucleotides
phenotypes
qualitative traits
quantitative traits
recessive allele
BASIC GENETICS
Genes are the basic unit of inheritance. They are carried on the chromosomes of all body cells. In the gametes--eggs or sperm--genes pass inherited traits to the next generation. Different forms of the same gene--at the same location on the chromosome--are called alleles. Genes contain the "blueprint" or code that determines how an animal will look and interact with its environment.
Genes are made of deoxyribonucleic acid (DNA). Resemblances and differences among related individuals are primarily due to genes. Genes cause the production of enzymes that control chemical reactions throughout the body, thus affecting body development and function. For normal body development and function, genes must occur in pairs. Genes are a part of the chromosomes that reside in the nucleus of all body cells. Chromosomes in the nucleus of a particular cell contain the same genetic information as the chromosomes in every cell of the body. So, the chromosomes in the cells of a horse's ear are the same as the chromosomes in its heart. The genes on the chromosomes, however, have specific functions in specific body tissues.
The number of chromosomes an animal possesses varies from species to species, but is consistent for a species--horses have 64 chromosomes, while humans have 46. In the normal cell of a horse or any mammal, chromosomes occur in distinct pairs. Horses have 32 chromosome pairs, for a total of 64 chromosomes.
Genomes
The complete set of "instructions" for making an organism is called its genome. The genome contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism. Found in every nucleus of the many trillions of cells in an animal, the genome consists of tightly coiled threads of DNA and associated protein molecules, organized into distinct, physically separate microscopic structures called chromosomes.
The horse genome is organized into 64 chromosomes in 32 pairs. All genes are arranged linearly along the chromosomes. The nucleus of most horse cells contains two sets of chromosomes, one set from each of its parents. Each set has 31 single chromosomes, or autosomes, and an X or Y sex chromosome. A normal female will have a pair of X chromosomes; a male will have an X and Y pair. Chromosomes contain roughly equal parts of protein and DNA. DNA molecules are among the largest molecules now known.
During mitosis (cell division), pairs condense and are visible with a light microscope. A karyotype analysis involves blocking cells in mitosis and staining the condensed chromosomes with dye. The dye-stained regions of the chromosomes are rich in adenine and thymine, producing a dark band. A karyotype is a photograph of the entire set of chromosomes, cut up and arranged in pairs. Thus the normal karyotype for a mare is 64, XX; for a stallion it is 64, XY. Figure 10-1 shows a karyotype for horses.
If unwound and tied together, the strands of DNA would stretch more than 5 feet long but would be only 50 trillionths of an inch wide. For every organism--from simple bacteria to remarkably complex horses--the components of these slender threads encode all the information necessary for building and maintaining life. Understanding how DNA performs this function requires some knowledge of its structure and organization.
Structure of DNA
In animals and humans, a DNA molecule consists of two strands wrapped around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen-containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, each composed of one sugar, one phosphate, and a nitrogenous base (Figure 10-2).
Four different bases are present in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Chromosomal DNA contains an average of 150 million bases. The particular order of the bases arranged along the sugar-phosphate backbone is called the DNA sequence. This sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.
The two DNA strands are held together by weak bonds between the bases on each strand, forming base pairs. Genome size is usually stated as the total number of base pairs. The horse genome contains over 3 billion base pairs.
Each time a cell divides into two daughter cells, its full genome is duplicated; for horses and other complex organisms, this duplication occurs in the nucleus. During cell division, the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Strict base-pairing rules are adhered to--adenine will pair only with thymine (an A-T pair) and cytosine only with guanine (a C-G pair). Each daughter cell receives one old and one new DNA strand. The cell's adherence to these base-pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.
[FIGURE 10-1 OMITTED]
[FIGURE 10-2 OMITTED]
How the Code Works
Each DNA molecule contains many genes. A gene is a specific sequence of nucleotide bases, whose sequences carry the information required for constructing the proteins that provide the structural components of cells and tissues as well as the enzymes for essential biochemical reactions.
Within the gene, each specific sequence of three DNA bases (a codon) directs the cell's protein-synthesizing machinery to add specific amino acids. For example, the base sequence ATG codes for the amino acid methionine. Since three bases code for one amino acid, the protein coded by an average-sized gene (3,000 base pairs) will contain 1,000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.
The protein-coding instructions from the genes are transmitted indirectly through messenger ribonucleic acid (mRNA). This mRNA is moved from the nucleus to the cellular cytoplasm, where it serves as the template for protein synthesis. The cell's protein-synthesizing machinery then translates the code into a string of amino acids that will constitute a specific protein molecule (Figure 10-3).
Fundamentals of Inheritance
Chromosomes and gene numbers change during gamete (sex cell) formation (Figure 10-4). Gametes are the eggs produced by sexually mature mares and the sperm cells produced by sexually mature stallions. During gamete formation in a horse, the 32 chromosome pairs of a cell duplicate. Then, one of the four members associated with each of the duplicated chromosome pairs is randomly transferred to one of four forming gametes. The newly formed gamete now contains only one member of each original chromosome pair. This splitting of chromosome pairs causes a random transfer of each member into a forming gamete.
[FIGURE 10-3 OMITTED]
When an egg and sperm unite at fertilization, each carries only one member of each of its original chromosome pairs. The joining of a particular egg and sperm cell occurs at random. At fertilization, the chromosome number is restored to its original value. The new cell, the zygote that develops into a fetus, has one member of each chromosome pair from its sire and the other member from its dam. The resulting offspring will be genetically different from either parent due to the union of randomly matched gametes. Since horses have 64 chromosome pairs, the possible number of distinct assortments of genes in forming gametes is infinite. The possible number of genetically different horses is much larger than the total number of horses being raised on the nation's farms.
[FIGURE 10-4 OMITTED]
How Sex Is Determined
Chromosomes also determine the sex of a horse. The most common system of sex determination is the XY system of mammals. In this system, females carry XX chromosomes and males carry XY chromosomes. When females produce eggs, every egg possesses one X chromosome. When males produce sperm, half the sperm carry the X chromosome and half the Y chromosome. When the eggs and sperm unite, half the zygotes will be XX (female) and the other half will be XY (male). On average, in a normal population, half of the offspring are males and half are females. Figure 10-5 illustrates how sex is determined with the XY system.
Chromosomes and Genes
The random transfer of chromosomes and their genes to form gametes is called random segregation. Random segregation is the major cause of genetic differences among related individuals. These differences in genetic makeup are often referred to as genetic variation. Traits showing a great deal of genetic variation have a better chance of responding to selective breeding. If a large amount of genetic variation is present in a population, some animals will carry many favorable genes while others will have more undesirable genes for a given trait. If individuals with favorable genes can be identified and bred, the likelihood that their offspring will possess those favorable genetic traits increases. The specific genes that reside in the gene pairs which control a trait comprise the animal's genotype.
[FIGURE 10-5 OMITTED]
Alleles. Every cell contains a duplicate set of genes. Each set is derived from the single gene sets contributed at conception by both the mother and the father. The gene sets contain similar, but not necessarily identical, information. For example, both sets contain a gene determining hair structure, but one set may contain the instructions for straight hair and the other for curly hair. The alternative forms of each gene are called alleles.
If both alleles are identical, the animal is said to be homozygous at that gene; if the alleles are dissimilar, the animal is said to be heterozygous at that gene. Information about homozygosity or heterozygosity for various genes can be inferred from information about parents and/or progeny and can be used for predicting the outcome of matings. For most alleles of horse coat colors, one cannot tell by looking at an animal whether it is homozygous at each coat color gene, so zygosity information is not critical for purposes of identification. Sometimes, however, information about coat colors of parents may be used as an indication of incorrect parentage or erroneous identification, so some familiarity with genetic relationships may be useful.
Dominance. Both sets of genes function simultaneously in the cell. Often when the gene pair is heterozygous, one allele is visibly expressed but the other is not. The expressed allele in a heterozygous pair is known as the dominant allele; the unexpressed one is the recessive allele. The term dominant is given an allele only to describe its relationship to related alleles and does not indicate any kind of physical or temperamental strength of the allele or the animal possessing it. Likewise, possession of a recessive allele does not connote weakness.
For simplicity in constructing models, geneticists symbolize genes by letters such as A, B, and so on. The dominant allele of a gene is symbolized by a capital letter, for example R, and the recessive by a lowercase letter--r.
For a given gene pair, the two genes can be alike or different. A homozygous gene pair has two identical genes, while a heterozygous gene pair has different genes. Gene action can be grouped into two categories: nonadditive and additive.
Nonadditive gene action: When nonadditive gene pairs control a trait, members of the gene pairs will not be equally expressed.
Additive gene action: Additive genes are those in which members of a gene pair have equal ability to be expressed. Expression of the gene pair is the sum of the individual effects of the genes in the pair.
Heredity versus Environment
All traits of horses are not controlled by just one gene pair. In fact, very few economically important traits are controlled by a single or even just a few gene pairs. Traits are controlled by possibly hundreds of gene pairs. Consequently, traits are generally grouped into two categories: qualitative and quantitative.
Qualitative Traits. Qualitative traits have four distinguishing characteristics:
1. Qualitative traits are controlled by a single or a few gene pairs.
2. Phenotypes (the trait characteristics we can see) of qualitative traits can be broken into distinct categories in which every member in that category looks the same.
3. The environment has little effect on the expression of the gene pair(s) controlling a qualitative trait. For example, coat color stays the same regardless of the environment.
4. The genotype of an individual for a qualitative trait can be determined (identifying the genes that occupy the gene pair[s]) with reasonable accuracy.
Qualitative traits show three types of gene action:
1. Dominance
2. Codominance
3. Partial dominance
An example of dominant inheritance is combined immunodeficiency. Blood type is an example of codominance, and coat color is an example of partial dominance. Quantitative Traits. Quantitative traits are dissimilar in their attributes when compared to qualitative traits. Characteristics of quantitative traits include:
1. Quantitative traits are controlled by possibly hundreds or thousands of gene pairs located on several different chromosome pairs. Some gene pairs will contain additive genes, others will contain nonadditive genes. Most economically important traits are quantitative traits.
2. The environment does affect expression of the gene pairs controlling quantitative traits.
3. Phenotypes of quantitative traits cannot be classified into distinct categories, since they range from one extreme to another. It is impossible to accurately determine how many gene pairs are controlling a quantitative trait, so an exact gene type can never be determined.
These factors make it difficult to identify individuals that have superior genotypes for quantitative traits.
Table 10-1 shows how much of some important traits in horses are due to genetics and how much are due to environment.
Genotypic Expression. With all traits, the individual's phenotype is the sum of effects of the genotypic and environmental effects (phenotype + genotype + environmental effects). Since qualitative traits are usually not affected by the environment, the phenotype of a qualitative trait is a good indicator of the genotype. Environmental effects do influence the phenotypic expression of a quantitative trait. An individual with an inferior genotype can rank higher phenotypically than individuals with superior genotypes because of favorable environmental effects. To reduce environmental effects, all animals must be treated the same. Usually an individual's phenotype, compared to an average for a similar group, is a good indicator or estimate of his or her genotype, or genotypic value.
Genetic evaluation programs often estimate the transmitting ability of an individual. Estimated transmitting ability is equal to one-half of an individual's estimated breeding value. The estimate of transmitting ability is the contribution a stallion or mare is expected to make to the genotypic value of their offspring.
Horse Improvement Programs
Most genetic progress for quantitative traits in livestock, including horses, has been made by selection based on phenotypes or on estimates of breeding values derived from phenotype, with no knowledge of the number of genes affecting the trait or the effects of individual genes. The basic principle of a horse improvement program is to identify superior animals within each breed. The major criterion for selecting superior animals is form (conformation) as related to function (performance), with the primary emphasis on the athletic ability of the horse. Horse improvement programs are designed to help improve the quality of bred horses, identify superior horses, and establish markets.
The process can vary, but typically three evaluators assess the yearling and 2-year-old classes. A consensus evaluation score is given to each horse. Horses in performance classes receive an evaluation score based on a combination of performance and conformation as outlined in their breed-specific class guidelines. Horses are scored up to 20 points in each of five categories:
1. Front limbs
2. Hind limbs
3. Head, neck, body, and balance
4. Athletic movement
5. Type
Standardized scorecards or forms are used to keep scoring consistent.
Objectives of the performance evaluation in an improvement program include the evaluation of athletic ability, temperament, and trainability of young horses. The evaluation covers simple maneuvers consistent with basic training of young horses in each breed, including such maneuvers as walking, trotting, cantering, changing leads, changing direction, moving off the leg, halting, and backing. A horse should excel if it remains under control; moves in a light, relaxed, rhythmic style; and executes transitions correctly and quietly. The horse should appear to be a willing participant, enjoying the activity.
A horse improvement program tries to identify horses with potential to excel in a wide range of performances. Training potential for specialized activities such as pleasure, trail, reining, pack horse, dressage, or jumping is not part of the evaluation.
Until all genes have been sequenced and identified, and scientists have learned to manipulate individual genes, genetic improvement will continue to rely on record keeping, observation, judgment, and selection.
COAT COLOR
A system for classifying horse coat colors and markings is important in any horse identification program. To have accurate and uniform application of the terminology for color classes, the system should stress recognition of basic, definable characteristics and should minimize the importance of subtleties that cannot be clearly defined. A scheme of coat-color classification based on recognition of the effects of the alleles of seven genes provides the tools necessary to define most of the common colors seen in horses.
Though an animal may show the dominant allele of a gene, it is not possible to determine by looking at the animal whether the second allele is dominant or recessive. A recessive allele may be masked by a dominant allele, which leads to the expression "hidden recessive." Dominant alleles are never hidden by their related recessive alleles. Table 10-2 lists the seven genes and their action in the coat color of horses.
The W and G Genes
The W (white) gene and the G (gray) gene represent alleles whose actions can obscure the actions of the other coat-color genes. If either the W allele of the W gene or the G allele of the G gene is present, the other coat-color genes cannot be determined by superficial examination.
A horse with the dominant allele W typically lacks pigment in its skin and hair at birth. The skin is pink, the eyes brown (sometimes blue), and the hair white. Such a horse is termed white, or sometimes called albino. The W allele is only rarely encountered. All nonwhite horses are ww.
In horses, gray is controlled by the dominant G allele. A horse with a G allele will be born any color but gray and will gradually become white, or white with red or black flecks, as it ages. Earliest indications of change to gray can be seen by careful scrutiny of the head of a young foal. Often the first evidence of the gray hairs is seen around the eyes. In intermediate stages of the graying process, the horse will have a mixture of white and dark hairs. In contrast to white (W) horses, gray horses are born pigmented and go through lightening stages, but always contain pigment in their skin and eyes at all stages of coloration change.
A gray horse will be either GG or Gg. It is not possible to tell by looking at the horse whether it is homozygous for G. All nongray horses will be gg. For homozygous recessive colors, both alleles are written in the notation for color assignment, since a horse showing a color or pattern produced by recessives is by definition homozygous for the recessive alleles; for example, aa allows black hair to be uniformly distributed over the body.
Because gray is produced by a dominant gene, at least one parent of a gray horse must be gray. If a gray horse does not have a gray parent, then the purported parentage should be considered incorrect and seriously reevaluated.
The E Gene
The first step for defining the coat color of a horse that is neither gray nor white is to decide if the animal has any black pigmented hairs. These hairs may be found in a distinctive pattern on the points (such as legs, mane, and tail), or black hair may be the only hair color (except for white markings) over the entire body. If a horse has black hair in either of these patterns, then the animal has an allele of the E gene, which contains the instructions for placing black pigment in hair. The alternative allele to E is e. The e allele allows black pigment in the skin but not in the hair. The pigment conditioned by the e allele makes the hair appear red.
If an animal has no black-pigmented hair, it has the genetic formula ee. Basically, an ee animal will be some shade of red ranging from liver chestnut, to dark chestnut, to chestnut, or sorrel. Manes and tails may be lighter (flaxen), darker (not black), or the same color as the body. These pigment variations of red cannot yet be explained by simple genetic schemes. Shades of red are not consistently defined by breeds or regions of the country, so use of specific terms for the shades of red can be confusing.
Since the red animal is not gray and not white, its genetic formula is ww, gg, ee. When two red horses are bred (ww, gg, ee x ww, gg, ee), the offspring should also be red (ww, gg, ee). If the offspring has black pigment (E) or is gray (G) or white (W), the assumed parentage is incorrect.
The A Gene
The gene that controls the distribution pattern of black hair is known as A. The A allele in combination with E will confine the black hair to the points to produce a bay. Various shades of bay, from dark bay or brown through mahogany bay, blood bay, copper bay, and light bay, exist. The genetics of these variations has not been defined. Any bay horse will include A and E in its genetic formula as well as ww and gg.
The alternative a allele does not restrict the distribution of black hair. Thus, in the presence of the E allele of the E gene, a uniformly black horse is produced. In most breeds the a allele is rare, so black horses are infrequently seen. In many black horses, the hair fades in the sun, especially around the muzzle and flanks; such animals may be called brown. The term brown can be used for several genetic combinations--various reds, bays and dark bays, as well as some blacks.
Neither A nor a affects either the pigment or its distribution in red (ee) horses. So, an examination of coat color cannot determine which alleles of the A gene a red horse has.
The C Gene
An allele of the C gene, known as C, causes pigment dilution. Fully pigmented horses are CC. Heterozygous horses (CC) have red pigment diluted to yellow, but black pigment is not affected. A bay (E, A) becomes a buckskin by dilution of the red color body to yellow without affecting the black color of the mane and tail. The genetic formula for a buckskin is ww, gg, A, E, CC. A red horse (ee) becomes a palomino (Figure 10-6) by dilution of the red pigment in the body to yellow, with mane and tail further diluted to flaxen. The genetic formula for a palomino is ww, gg, ee, CC.
A genetically black horse (E, aa) can carry the dilution allele without expressing it, because CC affects only red pigment.
In the homozygous condition, C completely dilutes any coat color to a very pale cream with pink skin and blue eyes. Such horses are often called cremello (also perlino or albino). Typically, such horses are the product of the mating of two dilute-colored animals such as palominos or buckskins. Cremello may be difficult to distinguish from white.
[FIGURE 10-6 OMITTED]
The D Gene
The D gene determines a second kind of dilution of coat color. Its effects can be confused with those of C, but several important differences in the effects of D and C on color exist. First, D dilutes both black and red pigment on the body but does not dilute either pigment in the points. Red body color is diluted to a pinkish-red, yellowish-red or yellow; black body color is diluted to a mouse-gray. Second, in addition to pigment dilution, a predominant characteristic of the D allele is the presence of a particular pattern that includes dark points, dorsal stripe, shoulder stripe, and leg barring. Third, homozygosity for D does not produce extreme dilution to cream, as does C.
This pigment dilution pattern is called dun. In an otherwise red horse, the D allele produces a pinkish-red horse with darkened points known as a red dun or claybank dun (ww, gg, ee, CC, D). In an otherwise bay animal, the D allele produces a yellow or yellow-red animal with black points known as a buckskin dun (ww, gg, E, A, CC, D). An otherwise black animal with the D dilution allele is a mouse-gray color with black points known as a mouse dun or grulla. (ww, gg, E, aa, CC, D).
The effect of D and C can be easily confused in A, E horses, so care must be taken in identification. An animal can have both the C and D dilutions, a situation that may be difficult to distinguish except by breeding tests. D is found in only a few breeds of horse, and probably in the United States would be seen only in stock horse breeds, as well as in some ponies.
The TO Gene
Several different white spotting patterns exist in horses, but so far only that of the tobiano has been clearly shown to be conditioned by a single gene. Tobiano spotting, symbolized by TO, is a variable restricted pattern of white hair with underlying pink skin that can occur with any coat color. The pattern is present at birth and stable throughout life. Generally, white extends across the back in an apparent top-to-bottom distribution on the body. The white areas may merge to form an extensive white pattern of generally smooth outline. The legs are white, but the head is usually dark except for a facial marking pattern. Tobianos can now be screened for their potential to be true breeders for the tobiano pattern.
Assignment of Coat Color by Genetic Formula
Defining the coat color of a horse is a stepwise process. The first step is to determine if either G or W is present. If yes, then the animal is gray or white, and this is the end of the identification task.
If the horse is neither gray nor white, then assignment of alleles of the other genes can be made to define the color. First, one must decide if the horse has E. If E, then it must be decided whether the horse has A. If the animal does not have E, then a decision about A cannot be made. If none of the colors is diluted and if no spotting pattern is present, these decisions about E and A will define the colors bay, black, and red.
If dilution of the basic colors to yellow, light red, mouse gray, or cream is present, then further definition can be made with addition of the C and D alleles to the basic formula containing W, G, E, and A. In the absence of white spotting, these decisions will define the colors palomino, buckskin, cremello, red dun, buckskin dun, and mouse dun.
If a white spotting pattern that meets the definition of tobiano is present, TO can be assigned to the genetic formula.
The outcome of decisions about the genes W, G, E, A, C, D, and TO results in the assignment of alleles for each gene. Each assignment should be carefully reviewed to consider if the chosen alleles are likely to be found in the breed of horse being identified. Some of the genetic formulas and the color definitions that can be assigned by this process are shown in Table 10-3.
GENETIC ABNORMALITIES
Defects in DNA can result in the failure to form essential proteins or in the formation of abnormal proteins, which may in turn cause death or disease in the horse. These defects may be caused by abnormalities in a single gene, the cumulative effect of a group of abnormal genes, or some chromosomal abnormality. Table 10-4 lists and describes some of the genetic abnormalities or diseases in horses.
MULES
No discussion of genetics and horses would be complete without a mention of the mule. In a way, mules are themselves a genetic abnormality--they have an uneven number of chromosomes. A mule is the offspring of a male donkey (jack) and a female horse (mare). A mule is much like the horse in size and body shape but has the shorter, thicker head, long ears, and the braying voice of the donkey. It also lacks, as does the donkey, the horse's calluses, or "chestnuts," on the hind legs. The reverse cross--between a stallion and a female donkey (called a jennet or jenny)--is a hinny, sometimes also called a jennet. A hinny is similar to the mule in appearance but is smaller and more horselike, with shorter ears and a longer head. It has the stripe or other color patterns of the donkey. Hinnies are more difficult to produce than mules. Although they may display normal sex drives, mules are generally considered infertile. Rarely, a female mule or hinny may come into heat and produce a foal. Horses have 64 chromosomes (32 pairs); donkeys have 62 (31 pairs). Mules and hinnies have 63 chromosomes.
Mules are not the only hybrids of horses. Horses and Grant's zebras (Equus burchelli) have been successfully crossed, producing a zorse. These show the coat color and markings of the horse and the zebra. Like mules, zorses are infertile. Horses have 64 chromosomes, and zebras have 66 chromosomes.
First Cloning of Mules Dr. Gordon Woods, a University of Idaho (Moscow) professor of animal and veterinary science, began working on an equine cloning project in 1998. Eventually the research team working on the project also included Dr. Kenneth L. White, Utah State University professor of animal science, and Dr. Dirk Vanderwall, a University of Idaho assistant professor of animal and veterinary science. [ILLUSTRATION OMITTED] For three years, from 1998 to 2000, the team worked without apparent success, after transferring the nuclei from the mule cells into 134 horse eggs and implanting them into mares. In 2001, the team began to focus on the calcium levels in the fluid surrounding the eggs during the cloning procedure. The researchers continued to adjust the calcium levels in the fluid surrounding the egg during the cloning procedure, resulting in more success and eventually leading to the birth of the first clone of a hybrid animal and the first cloned equine species. The baby mule, Idaho Gem, was born May 4, 2003, to a surrogate mare, Syringa. The foal's DNA comes from a fetal cell culture first established in 1998 at the University of Idaho, making it a full sibling of a champion racing mule owned by Idaho businessman and mule enthusiast Don Jacklin of Post Falls, Idaho. In 2003 two more healthy mule clones, Utah Pioneer and Idaho Star, were born--one on June 9 and the other July 27. In 2005, Idaho Gem and Idaho Star were transported to trainers to prepare them for racing in 2006. On June 4, 2006, Idaho Gem finished third in the Winnemucca Mule Race. This was the first showdown between cloned and natural-born mules. After his first six races, Idaho Gem had collected two firsts, two seconds, a third, and a fourth.
SUMMARY
Genes are the basic unit of inheritance. They are composed of DNA and arranged along the length of the chromosomes. Foals receive one-half of their genetic material from the stallion and one-half from the mare. The genetic makeup of a horse is called its genotype. How the genetic information is expressed in terms of the physical appearance of the horse is called the phenotype. Most traits of a horse are controlled by many genes. The genetic makeup of a horse and the environment interact. Harmful genetic material passed to a foal may produce disease or death.
REVIEW
Success in any career requires knowledge. Test your knowledge of this chapter by answering these questions or solving these problems.
True or False
1. The complete set of instructions for making an organism is called its genome.
2. The possible number of distinct assortments of genes in forming gametes is not over 100.
3. Differences in genetic makeup are often referred to as genetic variation.
4. The environment has little effect on the expression of the gene pair(s) controlling a qualitative trait.
5. Alanine is one of the four bases in DNA.
6. Some genes actually dilute the coat color of horses.
Short Answer
7. How many pairs of chromosomes does a horse have?
8. In horses, which sex carries the XY chromosomes and which sex carries the XX chromosomes?
9. What is the difference between a homozygous gene pair and a heterozygous gene pair?
10. List the three types of gene action that qualitative traits show.
11. Name five economically important traits.
12. Every cell contains a duplicate set of genes. What is the alternative form of each gene called?
13. What do the W and G genes express in horse coat color?
14. List five genetic diseases or abnormalities.
15. How is a dominant and recessive allele of a gene indicated when making models? Critical Thinking/Discussion
16. Briefly describe the structure of DNA.
17. What is a gamete?
18. Explain an animal's genotype.
19. Discuss the genetic control of coat color in horses.
20. Discuss the role of genetics and environment in the expression of economically important traits in the horse.
21. Compare qualitative to quantitative traits.
STUDENT ACTIVITIES
1. In a report or presentation, use color photographs, videos, or real horses to describe the genetics of the coat color.
2. Obtain prepared microscope slides and view the chromosomes in the nucleus of some animal cells.
3. Draw or make a three-dimensional representation of DNA.
4. Using the Internet and/or other research methods, develop a report or presentation about any work that is being done to map the genome of horses.
5. To understand the randomness of the genetic process, have five people roll a pair of dice 12 times. Make a table that tracks how many times each person rolls two 1s, two 2s, two 3s, two 4s, two 5s, and two 6s.
6. Develop a report on the chromosome numbers of horses, donkeys, and mules. Which chromosome is missing in the mule when compared to the chromosomes of a horse? Why are mules generally sterile?
ADDITIONAL RESOURCES
Books
American Youth Horse Council. (2004). Horse industry handbook: A guide to equine care and management. Lexington, KY: Author.
Asimov, I. (1962). The genetic code. New York: New American Library.
Bowling, A. T., & Ruvinsky, A. (2000). The genetics of the horse. Oxon, UK: CAB International.
Frandson, R. D., Fails, A. D., & Wilke, W. L. (2003). Anatomy and physiology of farm animals (6th ed). Philadelphia: Lippincott Williams & Wilkins.
Hafez, E. S. E. (2000). Reproduction in farm animals (7th ed.). Philadelphia: Lippincott Williams & Wilkins.
Kahn, C. M. (Ed.). (2005). The Merck veterinary manual (9th ed.). Whitehouse Station, NJ: Merck & Co.
McKinnon, A. O., & Voss, J. L. (1993). Equine reproduction. Philadelphia: Lea & Febiger.
Internet
Internet sites represent a vast resource of information, but remember that the URLs (uniform resource locator) for World Wide Web sites can change without notice. Using one of the search engines on the Internet such as Yahoo!, Google, or About.com, find more information by searching for these words or phrases:
chromosomes
dominance
environment
genes
genotype
heredity
horse coat color
horse genetics
horse improvement
program
karyotype
phenotype
qualitative traits
quantitative traits
selection
Table A-18 in the appendix also provides a listing of some useful Internet sites that can serve as a starting point for further exploration.
TABLE 10-1 Influence of Genetics and Environment on Some Traits in
Horses
Due to Genetics Due to Environment
Trait (Percent) (Percent)
Height at withers 45 to 50 50 to 55
Body weight 25 to 30 70 to 75
Body length 35 to 40 60 to 65
Heart girth circumference 20 to 25 75 to 80
Cannon bone circumference 20 to 25 75 to 80
Pulling power 20 to 30 70 to 80
Running speed 35 to 40 60 to 65
Walking speed 40 to 45 55 to 60
Trotting speed 35 to 45 55 to 65
Movement 40 to 50 50 to 60
Temperament 25 to 30 70 to 75
Reproductive traits 10 to 15 85 to 90
TABLE 10-2 Action of Horse Coat-Color Genes
Gene Alleles (1) Observed Coat Color
W W WW: Lethal.
w Ww: Horse is typically pigmented in skin, hair,
and eyes and appears to be white.
ww: Horse is fully pigmented.
G G GG: Horse shows progressive silvering with age
to white or flea-bitten but is born any nongray
color. Pigment is always present in skin and
eyes at all
g
stages of silvering.
Gg: Same as GG.
gg: Horse does not show progressive silvering
with age.
E E EE: Horse has ability to form black pigment in
skin and hair. Black pigment in hair may be
e either in a points pattern or distributed overall.
Ee: Same as EE.
ee: Horse has black pigment in skin, but hair
pigment appears red.
A A AA: If horse has black hair (E), then that
black hair is in a points pattern.
a Gene A has no effect on red (ee) pigment.
Aa: Same as AA.
aa: If horse has black hair (E), then that black
hair is uniformly distributed over body and
joints. Gene A has no effect on red (ee) pigment.
C C CC: Horse is fully pigmented.
Ccr (2) CCcr: Red pigment is diluted to yellow; black
pigment is unaffected.
CcrCcr: Both red and black pigments are diluted
to pale cream. Skin and eye color are also
diluted.
D D DD: Horse shows a diluted body color to a
pinkish-red, yellow-red, yellow, or mouse-gray
d color and has dark points including dorsal
stripe, shoulder stripe, and leg barring.
Dd: Same as DD.
dd: Horse has undiluted coat color.
TO TO TOTO: Horse is characterized by white spotting
pattern known as tobiano.
Legs are usually white.
to
Toto: Same as TOTO.
toto: No tobiano pattern is present.
(1) Different forms of the same gene.
(2) In this table, cr codes for a dilution factor.
TABLE 10-3 Genetic Formulas and Resulting Coat Colors
Genetic Formulas (1) Color
W White
G Gray
E, A, CC, dd, gg, ww, toto Bay
E, aa, CC, dd, gg, ww, toto Black
ee, aa, CC, dd, gg, ww, toto Red
E, A, CCcr, dd, gg, ww, toto Buckskin
ee, CCcr, dd, gg, ww, toto Palomino
CcrCcr Cremello
E, A, CC, D, gg, ww, toto Buckskin dun
E, aa, CC, D, gg, ww, toto Mouse dun
ee, CC, D, gg, ww, toto Red dun
E, A, CC, dd, gg, ww, TO Bay tobiano
ee, CC, D, gg, ww, TO Red dun tobiano
(1) Refer to Table 10-2 for a description of the action of each gene.
TABLE 10-4 Some Genetic Diseases of Horses Caused by a Single or a
Few Genes
Genetic Disease Description
Combined immunodeficiency Failure of immune system to form;
(CID) animals die of infections.
Hyperkalemic periodic Defect in the movement of sodium and
paralysis (HyPP) potassium in and out of muscle;
animals intermittently have attacks
of muscle weakness and/or tremors and
collapse.
Myotonic dystrophy Spasms occur in various muscles.
Hemophilia A Failure to produce blood-clotting
factor; bleeding into joints,
development of hematomas.
Hereditary multiple exostosis Bony lumps develop on various bones
throughout the body.
Parrot mouth Lower jaw is shorter than upper jaw;
incisor teeth improperly aligned.
Lethal white foal syndrome Failure to form certain types of
nerves in the intestinal tract; foals
die of colic several days after birth.
Laryngeal hemiplegia Paralysis of the muscles that move the
cartilages in the larynx; causes noise
in the throat with exercise.
Cerebellar ataxia Degeneration of specific cells in the
cerebellum of the brain; causes in
coordination.
Gonadal dysgenesis Animals tend to be small and weak at
birth; show disorders of the
reproductive system; mares are sterile.
Hydrocephalus Accumulation of fluid within
compartments of the brain; results in
crushing of brain.
Neonatal isoerythrolysis (NI) Hemolytic disease of the newborn;
foal's red blood cells destroyed by
antibody in mare's colostrum; results
in anemia and sometimes death.
Umbilical hernias Opening in the body wall at the navel
does not close normally; intestines
may drop through opening.
Inguinal hernias Openings through which testicles
descend allow intestines to escape
into the scrotum; may cause colic.
Connective tissue disease Skin is hyperelastic--easily stretched
and injured.
Epitheliogenesis imperfecta Skin fails to form over parts of the
body or in the mouth.
Cataracts Cloudiness of the lens in the eye;
result in blindness.
Genetic Disease Comments
Combined immunodeficiency Disease of Arabian and part-Arabian
(CID) horses; transmitted as autosomal
recessive; mutation of a single gene.
Hyperkalemic periodic Disease of quarter horses;
paralysis (HyPP) transmitted as autosomal
dominant; involves one gene.
Myotonic dystrophy
Hemophilia A Disease of Thoroughbreds, quarter
horses, Arabians, and Standard-
breds; transmitted as X-linked.
Hereditary multiple exostosis
Parrot mouth
Lethal white foal syndrome Affects some offspring produced
by mating two overo paint horses;
several genes involved.
Laryngeal hemiplegia
Cerebellar ataxia
Gonadal dysgenesis Presence of a single X chromosome
in a female; caused by failure
of the X chromosome to separate
after duplication.
Hydrocephalus
Neonatal isoerythrolysis (NI) Genetic makeup of animal
predisposes to disease; underlying
basis is incompatibility in blood
type between the mare, the foal,
and the foal's sire.
Umbilical hernias
Inguinal hernias
Connective tissue disease
Epitheliogenesis imperfecta
Cataracts
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| |
| Author: | Parker, Rick |
|---|---|
| Publication: | Equine Science, 3rd ed. |
| Date: | Jan 1, 2008 |
| Words: | 7192 |
| Previous Article: | Chapter 9 Determining age, height, and weight of horses. |
| Next Article: | Chapter 11 Reproduction and breeding. |
| Topics: | |