ANIMAL GENETICS ANIMAL SCIENCE

Transcription

1 ANIMAL SCIENCE 8406 ANIMAL GENETICS Genetics is the science of heredity* and variation. It is the scientific discipline that deals with the differences and similarities among related individuals. All animals have a predetermined genotype that they inherit from their parents. However, an animal s genotype can be manipulated by breeding and more advanced scientific techniques (for example, genetic engineering and cloning). For many years, managers of agricultural systems have manipulated the genetic makeup of animals to improve productivity and increase efficiency. Successful manipulation of the genetic composition of animals requires an understanding of some fundamental principles of genetics. MENDELIAN GENETICS A famous monk named Gregor Mendel is recognized as the father of genetics. Mendel was not scientifically trained, and he developed his theories in the 1850s and 1860s without any knowledge of cell biology or the science of inheritance. In later years, genes, chromosomes, and DNA (deoxyribonucleic acid) were discovered and people began to understand how and why Mendel s theories worked. Mendel proposed three principles to describe the transfer of genetic material from one generation to the next. 1. The Principle of Dominance: In a heterozygote, one allele may conceal the presence of another. 2. The Principle of Segregation: In a heterozygote, two different alleles segregate from each other during the formation of gametes. 3. The Principle of Independent Assortment: The alleles of different genes segregate, or assort, independently of each other. Later studies have shown that there are some important exceptions to Mendel s third principle regarding independent assortment but otherwise, these principles are recognized as being the basis of inheritance. * Underlined words are defined in the Glossary of Terms

2 Mendel s experiments dealt with the relationships between an organism s genotype and its phenotype. The genotype is the genetic composition of an organism while the phenotype is the observable characteristics of that organism. Phenotypic characteristics of animals (called traits) can be measured or observed (e.g., coat color, eye color, and height). Two organisms may appear to be similar, but they can have different genotypes. Similarly, two animals may have the same genotype, but will appear to be different from each other if they have been exposed to different environmental conditions throughout their lives. The relationship between phenotype and genotype is expressed as the equation P = G + E where P = phenotype, G = genotype, and E = environment. If two individuals with identical genotypes are exposed to the same environmental conditions (e.g., nutrition, climate, and stress levels), their phenotypes (measurable and observable characteristics) should be the same. For example, consider identical twin lambs that have the same genotype for all the genes that code for growth. Given the same environmental conditions, both lambs should have identical birth weights, weaning weights, and market weights. To understand Mendel s principles and the relationships between phenotype and genotype, it is necessary to understand what makes up the genetic material of animals and how this is transferred from one generation to the next. GENETIC MATERIAL The body is made up of millions of cells that can only be seen by a microscope. Although cells are minute, they have a very complicated structure with many parts that have specialized roles. For the study of genetics, the most important part of the cell is the nucleus. The nucleus contains chromosomes (see Figure 1 on page 1) that are dark-staining rod-like or rounded bodies visible under the microscope. They occur in pairs in body cells, and the number of chromosomes in each cell is constant for individual species, although it differs among species. Chromosomes are made up of tightly coiled strands of DNA. DNA is a complex molecule composed of deoxyribose, phosphoric acid, and four bases. Individual genes are located in a fixed position (known as the loci) on the strands of DNA (see Figure 2). A gene is made up of a specific functional sequence of nucleotides, which code for specific proteins. This means that the information written in the nucleotide sequence of a gene is in a code form. A specific protein is produced when the appropriate apparatus of the cell reads this code. This is the role of most genes. The collection of genes that an organism has is called its genome

3 In somatic cells (body cells), chromosomes occur in pairs, known as homologous pairs or homologous chromosomes. As a result, genes also occur in pairs. Somatic cells are referred to as diploid, or 2n. Gametes (reproductive cells) do not have paired chromosomes and are referred to as haploid, or n (see Figure 3). Cell Division Cells must divide and increase in number so that animals can grow. A new cell is formed when one cell divides. Mitosis and meiosis are the two processes by which cells divide. Mitosis is the type of cell division in which the genetic material in the parent cell is duplicated and then divides into two separate cells with identical genetic material. Both cells are therefore diploid cells (2n) with a complete set of chromosomes identical to those in the parent cell. Meiosis is the process of cell division that occurs in reproductive cells. In this type of division, the chromosome number is halved from the diploid number (2n) to the haploid number (n). If gametes were diploid cells, the number of chromosomes per cell would double with each generation. Meiosis ensures that gametes receive only one-half the number of chromosomes that are present in parent cells. Fertilization Gametes from one individual join with another gamete from an individual of the opposite sex during the process of fertilization. All animals originate from a single cell, the female s ovum or egg, which is fertilized by the male spermatozoa (sperm) and becomes a zygote. The zygote, which develops into a new animal, will have a full set of chromosomes, with one-half of them coming from the male parent and onehalf coming from the female parent. When discussing different generations in genetics, the first generation is referred to as the parent or P generation. Their offspring are referred to as the first filial or F 1 generation. When individuals from the F 1 generation are mated with each other, their offspring are referred to as the F 2 generation

4 THE PRINCIPLE OF DOMINANCE In animals, chromosomes are paired and therefore genes are also paired. These paired genes code for the same trait, but they are not identical. They can have different forms, known as alleles. For example, sheep and cattle can be polled or horned. One gene codes for this trait and the two possible forms (alleles) of the gene are polled or horned. A capital letter is used to denote the dominant form of the gene (P) and a small letter is used to denote the recessive form of the gene (p). In this example, the polled allele is dominant and therefore denoted by P, while the horned allele is recessive and denoted by p. Genes are paired; therefore an animal can have three different combinations of the two alleles, PP, Pp or pp. When both genes in a pair take the same form (PP or pp), the animal is referred to as being homozygous for that trait. An animal with a PP genotype is referred to as homozygous dominant and an animal with the pp genotype is homozygous recessive. If one gene in the pair is the dominant allele (P) and the other gene is the recessive allele (p), then the animal is referred to as being heterozygous for that trait and its genotype is denoted as Pp. If the animal has the combination PP, it will be polled. If it has the combination pp, it will be horned. If it is a heterozygote, then genotypically it will have both traits but phenotypically it will be polled because the polled allele (P) is the dominant form of the gene (see Figure 4). Mendel s principle of dominance states that in a heterozygote, one allele may conceal the presence of another. In this example, the polled allele is concealing the horned allele and, therefore, is referred to as the dominant allele

5 THE PRINCIPLE OF SEGREGATION When animals reproduce, they only pass on one-half of their genetic material to their offspring because gametes, or reproductive cells, only have one chromosome from each pair. The offspring will only receive one of the paired alleles from each parent. This principle explains some of the differences that are observed in successive generations of animals and can be used to predict the probability of different combinations of alleles occurring in offspring. As discussed earlier, three kinds of individuals are possible when discussing a pair of genes: homozygous dominant (PP), homozygous recessive (pp), and heterozygous (Pp). Considering these three types of individuals, six combinations of the various genotypes are possible: PP x PP (both parents are homozygous polled), PP x Pp (one homozygous polled parent and one heterozygous polled parent), PP x pp (one homozygous polled and one homozygous horned parent), Pp x Pp (both parents are heterozygous polled), Pp x pp (one heterozygous polled parent and one homozygous horned parent), and pp x pp (both parents are homozygous horned). Predicting the Phenotype of Offspring A punnett square can be used to determine the probable results in breeding livestock. The male genotype is normally indicated at the top, and the female genotype is indicated in the vertical margin. When crossing homozygous dominant parents (PP x PP), all offspring will be homozygous dominant polled individuals. Each of them can only produce a gamete with the dominant gene for the polled trait. The situation is the same when crossing homozygous recessive parents (pp x pp). All offspring will be horned. When crossing a homozygous dominant parent with a homozygous recessive parent (PP x pp), all offspring would be heterozygous and polled. This cross is illustrated in Figure 5. When crossing a heterozygous parent with a homozygous dominant parent (Pp x PP), the expected offspring would occur in a 1:1 ratio of homozygous dominant to heterozygous individuals. Phenotypically, all offspring would be polled. When crossing a heterozygous parent with a homozygous recessive parent (Pp x pp), offspring would occur in a genotypic ratio of 1:1 for heterozygous to homozygous recessive. About one-half of the offspring would be polled

6 If two heterozygous parents are crossed (Pp x Pp), one can expect a phenotypic ratio of 3:1 (polled to horned) in the offspring. Genotypically, there is a possibility of one homozygous dominant polled and one homozygous recessive horned individual as well as two heterozygous polled offspring. The horned individual produced from the cross will not carry a gene for the polled trait, even though both parents were polled. This cross is illustrated in Figure 6. Considering Multiple Traits Most commonly, there are multiple traits that need to be considered when mating animals. With two pairs of heterozygous genes, each affecting a different trait, a phenotypic ratio of 9:3:3:1 would be expected (see Figure 7). For example, consider cattle that can be horned or polled and white or red faced with horns and red coloring being the recessive traits. If two heterozygous individuals are mated, the expected offspring would (phenotypically) include nine polled white faced, three polled colored faced, three horned white faced, and one horned colored faced offspring

7 THE LAW OF INDEPENDENT ASSORTMENT When considering multiple traits, Mendel hypothesized that genes for different traits are separated and distributed to gametes independently of one another. Therefore, when considering polled and white faced traits, Mendel assumed that there was no relationship between how they were distributed to the next generation. Advances in genetics have shown that crossing over can occur between genes for different traits. However, this is not normal. In most cases, genes do assort independently. OTHER CONCEPTS IN GENETICS Partial Dominance Sometimes a heterozygote has a phenotype different from either of its homozygous counterparts. This will occur when an allele is only partially dominant. Partially dominant alleles for color are seen in several species of flowers. The homozygous flowers will be red or white and the heterozygous flowers will be pink. Sheep exhibit incomplete dominance in the trait for eye color. When a pure, brown-eyed sheep is crossed with a pure, green-eyed sheep, blue-eyed offspring are produced. Codominance Another exception to the normal principle of dominance is codominance. This occurs when a heterozygote exhibits traits found in both associated homozygotes. An example of codominance is the coat color of roan horses. The two allelic genes involved are R for red color and W for white color. If the genotype of a horse is RR, it will be red, and if its genotype is WW, it will be white. However, if its genotype is RW, the horse will be roan, or a mixture of red and white. This is different to partial dominance because when you look closely, a roan animal actually has both red and white hairs, rather than pink hairs. Under these circumstances, neither allele is dominant and neither is recessive, therefore each allele is denoted by a capital letter. Epistasis It is possible for more than one gene to control a single trait. This type of interaction between two nonallelic genes is referred to as epistasis. When two or more genes influence a trait, an allele of one of them may have an overriding effect on the phenotype. When an allele has such an overriding effect, it is said to be epistatic to the other genes that are involved. Comb shape in chickens is an example of an epistatic relationship. Domestic chickens can have four different types of comb shape: rose, pea, walnut, and single (see Figure 8)

8 Comb shape is influenced by two independently assorting genes, R and P, each with two alleles. Wyandotte chickens with rose combs have the genotype RR pp, while Brahma chickens with pea combs have the genotype rr PP. The F 1 hybrids between these two varieties are therefore Rr Pp; phenotypically they have walnut combs. If those hybrid birds are intercrossed with each other, all four types of combs appear in the progeny in a ratio of 9:3:3:1 for walnut:rose:pea:single. This is shown in Figure 9 Mutations and Other Chromosomal Abnormalities Genes have the capability of duplicating themselves. However, sometimes a mistake is made in duplication and a new gene called a mutation results. This new gene will cause a change in the code sent by the gene to the protein formation process. Some mutations may cause defects in animals while others may be beneficial. Mutations are responsible for variations in coat color, size, shape, behavior, and other traits in several species of animals. The beneficial mutations are helpful to animal breeders in improving domestic animals. Sometimes changes in chromosome number (nondisjunction), chromosome breakage (translocation or deletion), and the rearrangement of genes on a chromosome (inversion) can occur during meiosis. These changes are reflected in the phenotypes of animals. Some of these chromosomal aberrations or changes will result in abnormalities while others are lethal and result in the death of the animal. Death may occur shortly after fertilization, during prenatal development, or even after birth. Sex-Linked Traits Some traits are said to be sex-linked because they are carried on the X or Y chromosomes, which are involved in determining the sex of animals. The female mammal has the genotype XX in the sex chromosome pair, while the male has an XY genotype. The X chromosome is larger and longer than the Y chromosome. Therefore, a portion of the X chromosome does not pair with genes on the Y chromosome. Additionally, a certain portion of the Y chromosome does not link with the X chromosome. The traits on this portion of the Y chromosome are transmitted only from fathers to sons. Sex-linked traits are often recessive and are covered up in the female mammal by dominant genes

9 The expression of certain genes, which are carried on the regular body chromosomes of animals, is also affected by the sex of the animal. The sex of an animal may determine whether a gene is dominant or recessive. Scurs in polled European cattle appear to be inherited in this way. Lactation in cattle and egg laying in poultry may be affected by many genes, but are expressed only in one sex. In poultry, the male has the genotype XX, while the female has a Xw genotype. An example of a sex-linked trait in poultry is the barring of Barred Plymouth Rock chickens. If barred hens are mated to nonbarred males, all the barred chicks from this cross are males, and the nonbarred chicks are females. Traits such as cock-feathering or hen-feathering are limited in their phenotypic expression to only one sex. GENETIC SELECTION Permanent improvements in domestic animals can be made by genetic selection. The two kinds of genetic selection are natural and artificial. Natural selection occurs in wild animals while artificial selection is planned and controlled by humans. Animals that exhibit desirable traits are selected and mated. Similarly, animals that exhibit undesirable traits are not allowed to reproduce or are culled from the herd. The goal of selection is to increase the frequency of animals with optimal levels of performance while culling aims to reduce the frequency of individuals with poorer performance. Genetic improvement is a slow process and can take several generations to see an improvement in a trait. Artificial insemination and embryo transfer are breeding methods that are commonly used to decrease the time taken to improve a trait. Traits are passed from parents to offspring, but some traits are more heritable than other traits. That is, the genotype of an individual will be expressed more strongly and environment will be less influential for particular traits (see Table 1). Several genes influence some traits. For example, rate of growth is a trait that is influenced by appetite, energy expenditure, feed efficiency, and body composition. Table 1. Heritability of various traits in sheep, swine, and cattle (%). Sheep Swine Cattle Weaning weight Postweaning gain efficiency Post weaning rate of gain Feed efficiency Loin eye area Breeding systems aim to improve a single trait or multiple traits. Single trait selection is the result of selection policies aimed at improving one trait in a breeding program with little or no regard for improvement in other (associated) traits. In comparison, multiple trait selection aims to simultaneously improve a number of traits. Theoretically, multiple trait selection should result in a faster rate of gain toward a specific objective. Most domestic species now have a recognized system in place that allows breeders to estimate the genetic merit of individuals. In the United States, cattle, sheep, goat, and swine breeders use expected progeny differences (EPDs)

10 EPDs are used to compare animals from the same species and breed. For example, the owners of a breeding bull will advertise him as having certain expected progeny differences (EPDs). The bull might have a birth weight EPD of 3.1, a weaning weight EPD of 9.1, and a yearling weight EPD of These numbers alone do not provide the buyer with much information. To use these values effectively, the buyer also needs to know the breed averages, the accuracy of the EPDs, and who estimated the EPDs. A high EPD is not necessarily good; it depends on the trait being considered and the breeding objectives of the buyer. MODERN GENETICS Gregor Mendel s theories have been used for years to improve herds of animals and to increase the frequency of expression of desirable traits. In recent years, these methods of improvement have been superceded by genetic manipulation. A substantial amount of research has focused on direct manipulation of genes and DNA. Gene Transfer The science of genetic engineering is a promising new field of animal science. Genetic engineering basically refers to transferring a gene from one individual to another. Scientists are able to code genes for desirable compounds and insert them into other cells, such as microorganisms. These microorganisms produce these desirable compounds on a large scale. This area of genetic manipulation makes important contributions to domesticated animals in relation to immunology, vaccines, aging, and cancer. Researchers have successfully transferred the gene for rabbit hemoglobin into mice embryos. As a result, the genetically engineered mice are able to manufacture the rabbit hemoglobin. Once a gene is successfully introduced, it is transmitted to succeeding generations. Research has shown that injections of growth hormones into lactating cows result in significant increases in milk production. Suppose an additional gene or genes for growth could be introduced into an embryo. The mature cow would then increase production of milk without hormone injections. The implications for introducing superior production, conformation, and disease-resistant traits into domestic animals through gene transfer holds considerable promise in the genetic improvement of animals. Cloning Embryonic cloning of animals involves the chemical or surgical splitting of developing embryos shortly after fertilization and, consequently, developing two identical individuals. The separated embryos are allowed to culture, or grow, to a more advanced embryonic stage before they are implanted into the uterus of a recipient mother for full development. This procedure has been performed successfully in several species of animals. Nuclear Transfer Nuclear transfer is another method that has a great deal of potential for producing identical high-producing individuals. It involves the microsurgical collection of cells from the outer layer of a developing embryo and transferring them into an unfertilized ovum that has had its own nucleus removed. The cells that develop successfully become identical individuals

11 Nuclear Fusion Another innovation in genetic engineering, called nuclear fusion, involves the union of nuclei from two gametes, male or female sex cells. This fusion shows promise for the uniting of nuclei from two outstanding females, two outstanding males, or the normal outstanding male and female combination. The possibility for selecting desired traits at the cellular level holds exciting implications for the genetic improvement of domestic animals. Acknowledgements Claire Barrett, Student Technician, Department of Animal Science, Texas A&M University, researched and developed this topic. Larry Ermis, Curriculum Specialist, Instructional Materials Service, Texas A&M University, reviewed and edited this topic. Vickie Marriott, Office Software Associate, Instructional Materials Service, Texas A&M University, edited and prepared the layout and design for this topic. Christine Stetter, Artist, Instructional Materials Service, Texas A&M University, prepared the illustrations for this topic. REFERENCES Botany Online: Classic Genetics. [Online]. Available: b_online/e10/10.htm [2001, June]. Mendelian Genetics. [Online]. Available: [2001, June] Schraer, W. D. and H. J. Stoltze. Biology, The Study of Life. Prentice-Hall Inc.: USA, Snustad, D. P., M. J. Simmons, and J. B. Jenkins. Principles of Genetics. New York: John Wiley and Sons, Inc.,

12 GLOSSARY OF TERMS Allele - One of a pair of genes that occupy the same location on homologous chromosomes and affect the same trait in animals. Crossing over - Exchange of genes by homologous chromosomes during synapsis of meiosis prior to formation of the sex cells or gametes. Diploid - Refers to chromosome pairs (2n) in the body cells; two of each kind of chromosome are in each body cell as compared with one of each pair (1n) in sex cells. Dominant - Refers to genes that produce their characteristic phenotype whether its allele is identical or dissimilar. Epistasis - A form of gene interaction where one gene interferes with the phenotypic expression of another nonallelic gene or genes. Fertilization - The process in which a sperm unites with an egg to form a zygote. Fertilized - To be made pregnant; to be impregnated. Gametes - The haploid cells that fuse with other haploid cells to form zygotes; the sperm or egg cells. Genotype - The inheritable information carried by an animal. Haploid - Refers to the number of chromosomes (1n) in sex cells; sex cells contain only one of each pair of chromosomes found in body cells. Heredity - The biological process where genetic factors are transmitted from one generation to the next. Heterozygote - An organism that is heterozygous for a specific trait. Heterozygous - Having dissimilar alleles at corresponding chromosomal loci. Homologous - Having the same relative position and proportion. Homozygous - Having identical alleles at corresponding chromosomal loci. Mutation - A change or alteration in form or quality. Nucleotides - Compounds composed of a nitrogen base, a sugar, and a phosphate. Ovum - An egg; the female reproductive cell. Phenotype - The observable properties of an animal. Polled - To be without horns. Punnett square - A grid-like method used to display and predict the consequent genotype of progeny from parents with specific alleles

13 Recessive - Refers to genes that produce their characteristic phenotype only when they are paired with an identical allele. Sex-linked - Refers to genes carried on the nonhomologous portion of the X chromosome; allelic genes that have their locus on the sex chromosomes, usually the X chromosome. Somatic - Of or pertaining to the body. Spermatozoa - The male germ cell responsible for fertilizing the female ovum; sperm. Zygote - A fertilized ovum. SELECTED STUDENT ACTIVITIES SHORT ANSWER/LISTING: Answer the following questions or statements in the space provided or on additional paper if necessary. 1. What is the difference between an animal s phenotype and genotype? 2. What is the difference between mitosis and meiosis? 3. What is a zygote? 4. What is meant by dominant and recessive genes? Give an example of each

14 5. Explain the difference between homozygous and heterozygous genes. 6. In swine, the gene for drooping ears (D) is dominant while the gene for erect ears (d) is recessive. If two crossbred swine with drooping ears that are heterozygous for the trait are mated (Dd x Dd), what would be the expected outcome? a. Number of homozygous drooped ears? b. Number of homozygous erect ears? c. Number of heterozygous drooped ears? 7. In horned Hereford cattle, the white-faced gene is dominant (W) while the horned characteristic is recessive (p). Angus cattle have the dominant polled trait (P). If two black baldies (Hereford x Angus) cattle are mated (WwPp x WwPp), what would be the expected outcome? a. Number of white-faced, polled offspring? b. Number of white-faced, horned offspring? c. Number of colored-faced, polled offspring? d. Number of colored-faced, horned offspring? 8. What is epistasis? 9. What is an EPD, and how can livestock breeders use it to improve their animals? 10. What is the difference between cloning and nuclear transfer?

15 MULTIPLE CHOICE: Place the letter of the correct answer in the space provided at the left of each number. 11. Which of the following statements was not one of Mendel s theories on the laws of genetic interaction. a. In a heterozygote, one allele may conceal the presence of another. b. In a heterozygote, two different alleles segregate from each other during the formation of gametes. c. Gametes have only one-half the genetic material of normal body cells. d. The alleles of different genes segregate, or assort, independently of each other. 12. A goat breeder wants to mate two animals that are both heterozygous for coat color (Bb) and horn shape (Cc). Therefore, the cross will be BbCc x BbCc. BB animals are brown and bb animals are white, while CC animals have curling horns and cc animals have straight horns. What is the expected ratio of progeny? a. Nine brown with curling horns, three white with straight horns, three brown with straight horns, and one white with curling horns. b. Nine brown with curling horns, three white with curling horns, three brown with straight horns, and one white with straight horns. c. Nine white with curling horns, three brown with straight horns, three white with curling horns, and one brown with straight horns. d. Nine brown with straight horns, three brown with curling horns, three white with straight horns, and one white with curling horns. 13. Which of the following statements is true? a. An example of codominance is when a homozygous brown-eyed sheep is mated with a homozygous green-eyed sheep to give blue-eyed progeny. b. Each allele of genes that exhibit partial dominance is denoted with capital letters. c. Roan coloring is an example of codominance. d. Epistasis is an interaction between two alleles of a specific gene. TRUE OR FALSE: Circle the T if the statement is true or F if it is false. T F 14. Genes are located in fixed positions on DNA. T F 15. Meiosis ensures that gametes receive only one-half the number of chromosomes that are present in parent cells. T F 16. Genes always have a dominant and a recessive allele for a specific trait. T F 17. Offspring can receive two alleles from one parent and none from the other. T F 18. An animal that is homozygous dominant for a specific trait would be denoted as Pp. T F 19. Some traits are said to be sex-linked because they are associated with the X and Y chromosomes. T F 20. Genetic selection only occurs when animal breeders control the individuals to be mated (artificial selection)

16 ADVANCED ACTIVITIES 1. A great deal of debate over genetic engineering exists in the scientific world. Using newspapers, magazines, the Internet, and any other available resources, present one argument for and one argument against genetic engineering being used in agriculture. 2. When cells divide during the process of mitosis, genetic material (DNA) is copied. Research how new DNA strands are copied off old DNA strands. Draw a diagram to illustrate how mrna and trna are involved in the process. 3. Create a punnett square that considers three traits and determine the expected phenotypic ratio of the progeny. ALL RIGHTS RESERVED Reproduction prohibited without written permission. Instructional Materials Service Texas A&M University 2588 TAMUS College Station, Texas