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Babcock Institute for International Dairy Research and Development Dairy Essentials University of Wisconsin-Madison 14) BASIC GENETIC CONCEPTS Michel A. Wattiaux Babcock Institute WHAT IS GENETICS? Genetics is a science that studies the variation and transmission of features or traits from one generation to the next. In this definition, the word variation refers to genetic variation; that is, the range of possible values for a trait as it is influenced by heredity. Heredity is the transmission of traits from the parents to the offspring via genetic material. This transmission takes place at the time of fertilization in reproduction—when the bull’s semen unites with the cow’s ovum (egg) to produce a calf with a unique genetic makeup. Only identical twins have an identical genetic makeup because they come from one fertilized ovum that has separated into two embryos during the early phase of development. WHAT IS ENVIRONMENT? Environment is often thought of as an animal's physical surroundings—light, temperature, ventilation and other parameters that may contribute to the physical comfort of an animal. However, in genetics, the word environment has a more general meaning. The environment is the combination of all factors, except the genetic ones, that may affect the expression of genes. For example, a cow’s milk production is affected by age at calving, season of calving, nutrition and many other factors. Thus cows with similar or even identical genetic makeups will produce different amounts of milk when they are subjected to different environments. For example, the lactation performance of a pair of identical twins will vary drastically if the two calves are separated at birth and sent to different countries. However, there may be a great difference in milk yield between these twins when they are placed on two separate farms in the same area, each having different management levels. GENOTYPE AND PHENOTYPE The genotype of an animal represents the gene or the set of genes responsible for a particular trait. In a more general sense, the genotype describes the entire set of genes inherited by an individual. In contrast, the phenotype is the value taken by a trait; in other words, it is what can be observed or measured. For example, the phenotype may be an individual cow's milk production, the percentage of fat in the milk or a classification score for conformation. There is an important difference between genotype and phenotype. The genotype is essentially a fixed characteristic of the organism; it remains constant throughout life and is unchanged by environmental factors. When only one or a few genes are responsible for a trait, the phenotype usually remains unchanged throughout life (e.g., hair color). In this case, the phenotype gives a good indication of the genetic composition of an individual. However, for some traits, the phenotype changes continually throughout the life of the individual in response to environmental factors. In this case, the phenotype is not a 240 Agriculture Hall, 1450 Linden Dr., Madison, WI 53706 USA, phone: 608-265-4169, [email protected] 53 Dairy Essentials - Reproduction and Genetic Selection reliable indicator of the genotype. This usually occurs when many genes are involved in the expression of a trait such as milk yield. As a result, the milk yield of a cow is often expressed as follows: Phenotypic milk yield = G + E, where: G is the genetic merit of the cow for milk yield (the effect of the genes); E refers to the effect of the cow’s management and environment. THE GENETIC MATERIAL The genetic material is located in the nucleus of each cell in the body. Except for the reproductive cells (spermatozoa and ova) and a few other exceptions (red blood cells), cells contain two copies of an animal’s complete genetic material. When cells divide, the genetic material organizes itself in a series of long threadlike structures called chromosomes (Figure 1). In body cells, each chromosome has a counterpart that has the same length and shape (except for the two chromosomes that determine the sex) and contains genetic information for the same trait. These two chromosomes are two members of a chromosome pair, one derived from the sire, one from the dam. The number of chromosome pairs is typical of a species and it is usually abbreviated as the letter "n." For example, in humans n = 23, in swine n = 19 and in cows n = 30. Thus cells in the bodies of humans, swine and cows contain 2n = 46, 38 and 60 chromosomes, respectively. Genes are located along the chromosomes. A gene is the basic functional unit of heredity; that is, it contains the genetic information that is responsible for the expression of a particular trait. The entire length of a chromosome can be divided into thousands of these functional units, each responsible for a particular trait. A gene is composed of material called deoxyribonucleic acid or DNA. The function of the DNA is to carry the 54 Figure 1: Chromosomes magnified thousands of times information necessary for the synthesis of proteins. As proteins are synthesized and DNA replicates itself, the number of cells in the body increases (growth) and cells may specialize into specific functions (development) in which some genes are turned on and others turned off. For example, cells of the skin (a specialized tissue) contain all the genetic material needed to recreate an individual, but the only specialized genes that are turned on in these cells are the ones responsible for the formation and color of hair. TRANSMISSION OF GENETIC MATERIAL Male or female The testes of the bull and the ovaries of the cow produce reproductive cells by a special series of cellular divisions that halve the normal number of chromosomes in a cell. The spermatozoa and the ova contain only one member of a chromosome pair. Thus the cells of cows and bulls contain 60 chromosomes (2n = 60), but the spermatozoa in the semen and the ova in the ovaries contain only 30 chromosomes (n = 30, Figure 2). The two basic principles of the transmission of a trait (e.g., sex) are as follows (Figure 2): The Babcock Institute 14 - Basic Genetic Concepts Male or female? How are chromosomes transmitted? Parents (2n) 2n=60 2n=60 Testis Ovary Spermatozoa (n=30) Ova (n=30) X Y X X Division (Meiosis) Reproductive cells (n) Spermatozoa X Y Ova X Fertilization X X Female Zygote (2n) 2n=60 X Y Male Offspring Figure 2: Chromosomes are transmitted with reproductive cells that contain only half the normal number of chromosomes for a species. Chance at the time of fertilization is responsible for specific traits inherited by the offspring (e.g., gender). 1) Separation of the paired chromosomes during the formation of reproductive cells; 2) Union of a spermatozoon with an ovum to create a new cell with a unique set of chromosomes. For 29 of the chromosome pairs, both members are visually identical. However, for one of the pairs, one member is much longer; it is called the X chromosome, and the shorter member is called the Y chromosome. All the ova carry the X chromosome, but the spermatozoa can carry either the X or the Y chromosome. During cellular division to form the reproductive cells, each member of a chromosome pair goes into a separate cell. As a result, 50% of the spermatozoa will carry the X chromosome and the other 50%, the Y chromosome. If by chance a spermatozoon carrying a Y chromosome fertilizes an ovum, the offspring will be a male. However, an offspring that receives two X chromosomes develops into a female (Figure 2). It is important to realize that it is University of Wisconsin-Madison impossible to predict the sex of an offspring at the time of mating (insemination); however, we can predict that, on the average, 50% of all the offspring will be male and 50% will be female. Qualitative traits Qualitative traits tend to fall into discrete categories. Usually just one or a few genes have a major effect on qualitative traits. Environment usually has a minor role in influencing the category into which the animal falls. In this case, the phenotype of the animal reflects its genotype. Examples of qualitative traits in dairy cattle are: • Hair color; • Hereditary defects such as dwarfism; • Presence or absence of horns; • Blood type. Quantitative traits Quantitative traits differ from qualitative traits in two important ways: 1) They are influenced by many pairs of genes; 55 Dairy Essentials - Reproduction and Genetic Selection 2) The phenotypic expression is more strongly influenced by the environment than is true for qualitative traits. Many of the economically important traits in dairy cattle are quantitative traits: • Milk yield; • Milk composition; • Conformation (also referred to as type); • Efficiency of feed conversion; • Disease resistance. The combined influence of many genes and the effect of the environment on quantitative traits make it much more difficult to determine the genotype accurately than in the case of most qualitative traits. Sometimes the animal's phenotype tells us very little about its genotype. For example, a lactation record only tells a portion of the information about the genetic merit of a cow for milk yield. What makes the genotype of a cow unique? When ova are formed, they receive one of the two members of a chromosome pair. Thus a particular chromosome in an ovum can be like either the first member or the second member of the parental chromosome pair. There are only two different kinds of ova for that particular chromosome. If instead of one 56 chromosome pair, we now consider two, what does the total number of different ova become? In other words, what is the total number of possible chromosome combinations? This situation is like flipping two coins at the same time. The number of possible combinations is: two possible values for the first coin times two possible values for the second coin = 2 x 2 = 22 = 4 different possibilities. The number of different genotypes for an ovum is four and the probability of any particular combination of chromosomes is 1/4. This is also true for the number of possible genotypes in male reproductive cells. Thus when one out of four possible kinds of spermatozoa fertilizes one out of four possible kinds of ova, the number of possible genetically different offspring is 4 x 4 = 16 (i.e., 22 x 22 ). Thus the chance for any particular genotype in the newborn offspring is 1/16. When the 30 chromosome pairs of a dairy cow are separated during the formation of the reproductive cells and then reunited at fertilization, the total number of possible chromosome combinations is 230 x 23 0 = 1,152,900,000,000,000,000, each being unique. With this number of possibilities for each mating, it is easy to understand why no two individuals are alike in a population, even when they have the same parents. The Babcock Institute