University of Kentucky College of Agriculture, ASC-220 Food and Environment Cooperative Extension Service Basic Genetics Debra K. Aaron, Animal and Food Sciences

enetics is the science of heredity. It Gseeks to explain differences and simi- larities exhibited by related individuals. The application of genetics to improvement is known as animal breeding. The objective of this fact sheet is to pro- vide a refresher course on basic genetics and to show how knowledge of genetics can be used to improve sheep production. Basic Principles of Inheritance Genes are the basic units of inheritance and, as such, form the link between generations. Genes are located on mi- croscopic, threadlike structures called chromosomes. Chromosomes, and thus, genes, occur in pairs in the nucleus of all body cells. Each gene pair occupies a specific location on a particular chromo- some pair. Each species has a character- istic number of chromosomes, and all normal individuals of that species will have the same number of chromosomes. one member of a gene pair. Which gene In sheep, every body cell contains 27 pairs of a pair goes to an egg or sperm is simply Sheep in the Future for a total of 54 chromosomes. Each of the a matter of chance. Geneticists now have a virtual DNA chromosomes carries many genes. Genes map of almost the entire sheep ge- When the egg and sperm unite at nome. This map will enable them to are like beads on the various chromo- fertilization, the genes of a pair recom- pinpoint genetic controls for eco- some strings. bine in the offspring. In this process, the nomically important production traits. Chemically, chromosomes are made sex cells are in effect paired in the body Although most unique genes are not up of DNA (deoxyribose nucleic acid), and cells of the offspring. Thus, half of an yet identified, the sequence of nucleo- genes are specific portions of the DNA tides (the building blocks of DNA) is individual’s genes come at random from known for a short distance at more than molecule. Each biochemical reaction the sire and half come at random from 50,000 places along the sheep genome. that occurs within an animal’s body is the dam. This chance segregation of Identification of the variations in these controlled by a particular gene or set genes in the production of sex cells and sequences (called single nucleotide of genes. Specifically, genes control the their recombination at fertilization is the polymorphisms—SNPs—and pro- synthesis of proteins, which, in turn, nounced “snips”) is the next step toward main cause of genetic differences among identifying actual genes and their roles operate as enzymes in controlling cell related individuals. in controlling specific sheep production development and function. traits. Using a genomic tool called the Although genes occur in pairs and Inheritance of Ovine SNP50 BeadChip (Ilumina, Inc.), both members of a pair act cooperatively Color: An Example researchers are able to characterize to produce effects in an animal, genes are small genetic differences that produce A simple example involving color a wide range of economically impor- transmitted from parent to offspring not tant traits in sheep, such as improved in pairs, but singularly. During the forma- inheritance in sheep illustrates the seg- growth rate, fertility, parasite resistance tion of the sex cells, the egg and sperm, regation of a gene pair in sex cells and the and quality. This information has the genes of a pair separate or move apart. subsequent recombination of the genes in the potential to change the way sheep This separation is known assegregation . the offspring. For illustrative purposes, are selected and bred in the future. As a result, the sex cells each contain only assume a single pair of genes determines this trait.

Cooperative Extension Service | Agriculture and Natural Resources | Family and Consumer Sciences | 4-H Youth Development | Community and Economic Development The two genes involved are a gene for Table 1. Possible gene combinations for the w gene is also ½ or 0.5. Using the first white wool, symbolized by W, and a gene wool color example probability rule, the chance of a lamb for black wool, symbolized by w. Each Genotype Phenotype from this kind of mating being black- sheep will have two genes for the color WW White-wooled wooled (ww) is ½ × ½ = ¼. Likewise, the Ww White-wooled trait. The two genes will be located on ww Black-wooled chance of a lamb from this kind of mating one of the 27 chromosome pairs, with being homozygous white-wooled (WW) one gene on one member of the chro- is also one quarter. mosome pair and the other gene at the A lamb with the genotype Ww may same location on the other member of Probabilities be produced in one of two ways. A lamb the chromosome pair. There are three will be heterozygous white-wooled if the The next step is to illustrate how genes of possible gene combinations (genotypes) sperm carries the W gene and the egg a pair recombine in the offspring when the for the two genes being considered. These carries the w gene (which, using the first egg and sperm unite at fertilization. With are shown in Table 1. probability rule, will occur ½ × ½ = ¼ of the the three genotypes possible for a trait Phenotype refers to the appearance or time) or if the sperm carries the w gene controlled by a single pair of genes (Table measured performance of an individual and the egg carries the W gene (which will 1), it is possible to have six different kinds of for a trait. Only two phenotypes, white- also occur ¼ of the time). Finally, using the matings (Table 2). Because the reproductive wooled and black-wooled, are distin- second probability rule, the chance of a process ensures one gene from each gene guishable in this example. lamb being Ww from this type of mating pair will be transmitted to each sex cell (egg Notice the phenotypes of sheep with is the sum of the chances of each event or sperm) and because chance determines genotypes WW and Ww are indistin- occurring separately, ¼ + ¼ = ½. the union of egg and sperm at fertilization, guishable. The gene for white wool, W, On the average, the mating of two the expected offspring genotypic ratios is said to be a dominant gene because heterozygous white-wooled sheep is can be found using simple probabilities. it masks or covers up the effect of the expected to produce one WW (white- For example, consider the mating of two gene for black wool, w, which is called wooled) to two Ww (white-wooled) to heterozygous (Ww) white-wooled sheep. a recessive gene. Because the W gene is one ww (black-wooled) offspring when What is the chance, or probability, of get- dominant over the w gene, black-wooled four lambs are produced. Although ting offspring of the three possible geno- sheep must have the phenotype ww. both parents are white-wooled pheno- types, WW, Ww and ww? During reproduction, each parent typically, there is a one-in-four chance Two basic probability rules are used to transmits either a W or a w gene to that a mating of this type will produce answer this question. First, the chance two the offspring. A sheep with genotype a black-wooled (ww) offspring. Thus, independent events will occur together is WW can only transmit a W gene to its heterozygous individuals do not breed the product (multiplication) of the chance, offspring. Similarly, a sheep of genotype true. White-wooled sheep are known to or likelihood, of each separate event. Sec- ww can only transmit a w gene to its carry the black wool gene if they have at ond, the chance of occurrence of one or offspring. Individuals with either of these least one black-wooled lamb or if one of the other of two mutually exclusive events two genotypes are said to be homozygous their parents is black-wooled. (that is, events that cannot occur together) for this pair of genes because both genes All possible crosses involving the is the sum (addition) of the chances of of the pair are alike. three genotypes (WW, Ww and ww), each event occurring separately. A sheep with genotype Ww can with the expected lamb genotypes and Half of the sex cells produced by transmit either a W or a w gene to its phenotypes for each, are illustrated in heterozygous white-wooled sheep (Ww) offspring. Individuals with genotype Ww Table 2. The expected offspring ratios will contain the gene for white wool (W) are said to be heterozygous for this pair of for each mating type can be obtained by and the other half will contain the gene genes because the two genes of the pair using the same procedure as was used to for black wool (w). In the mating of two are different. Heterozygous individuals obtain the offspring ratios for the mat- heterozygous white-wooled sheep, the are sometimes referred to as “carriers” ing between heterozygous (Ww × Ww) chance of the sperm carrying the w gene because their genotypes carry the re- white-wooled sheep. is ½ and the chance of the egg carrying cessive gene. On the average, a sheep of genotype Ww will transmit the gene for white wool (W) to half its offspring and Table 2. Possible matings and expected genotypic and phenotypic ratios for the wool the gene for black wool (w) to the other color example half. As a result, the chance, or probabil- Mating Type Expected Ratios Among Offspring ity, an individual with genotype Ww will WW (white-wooled) × WW (white-wooled) All WW (white-wooled) transmit the w gene to any one offspring WW(white-wooled) × Ww (white-wooled) 1 WW to 1 Ww (all white-wooled) is ½ or 0.5. WW (white-wooled × ww (black-wooled) All Ww (white-wooled) Ww (white-wooled) × Ww (white-wooled) 1 WW (white-wooled) to 2 Ww (white- wooled) to 1 ww (black-wooled) Ww (white-wooled) × ww (black-wooled) 1 Ww (white-wooled) to 1 ww (black-wooled) ww (black-wooled) × ww (black-wooled) All ww (black-wooled)

2 Yearling Grease Fleece Table 3. Expected genotypic and pheno- Types of Traits typic ratios for the yearling grease fleece Qualitative Traits Weight: An Example weight example (AaBb × AaBb) Color inheritance (white versus The following simple, hypothetical ex- Yearling Grease ample illustrates the concept of quantita- Genotypes Fleece Weight (lb) black wool) has been used to illustrate 1 AABB 16 basic genetic principles involved in the tive inheritance. Suppose yearling grease fleece weight in sheep is influenced by 2 AABb 14 transmission of characteristics from 2 AaBB 14 parent to offspring because only a single two pairs of genes (one pair at the A 1 AAbb 12 pair of genes is involved. In addition to locus and the other pair at the B locus) 4 AaBb 12 with additive [AaBb = ½ (AABB + aabb)] 1 aaBB 12 wool color, one or only a few pairs of 2 Aabb 10 genes determine several other traits in and equal (A = B) gene effects. Also, envi- ronmental effects are assumed to be the 2 aaBb 10 sheep. The presence or absence of horns 1 aabb 8 and some genetic defects, such as parrot same for all genotypes. Phenotypic values mouth, inverted eyelids (entropion) and are assigned to genotypes to reflect these cryptorchidism, are also examples of assumptions. traits in which one pair of genes has the Suppose individuals with genotype Genetic Improvement major influence. In thesequalitative traits, aabb have yearling grease fleece weights through Selection non-genetic or environmental factors of 8 pounds, and every A or B gene have very little influence on phenotypic contributes 2 pounds to an individual’s Genetic improvement results from expression. Also, sharp distinctions usu- yearling grease fleece weight. Thus, -in selection and is possible because dif- ally exist between phenotypes. dividuals with genotype AABB would ferent genotypes tend to determine have yearling grease fleece weights of different phenotypes. However, genetic Quantitative Traits 16 pounds. If individuals with genotype improvement of quantitative traits is AaBb (12 lb) are mated together, the ex- more complicated than that of qualita- Most traits of economic importance pected genotypic and phenotypic ratios tive traits because genotypic expression in sheep production are controlled by among the offspring produced are shown in quantitative traits is often limited or many pairs of genes, the exact number in Table 3. exaggerated by environmental effects. of which is not known. In addition, there Notice individuals that were mated Thus, not all of the phenotypic differ- is often no sharp distinction between have yearling grease fleece weights of 12 ences exhibited by parents are transmit- phenotypes. To complicate matters pounds, and the average yearling grease ted to their offspring. further, environmental factors such as fleece weight of the lambs produced is nutrition, climate and disease may have also 12 pounds. However, considerable Heritability a large effect on phenotypic expression of variation exists among the offspring Heritability these traits. Birth weight, 60-day weaning refers to the proportion of (grease fleece weights range from 8 to 16 weight, yearling weight, carcass fat depth, phenotypic differences among animals lb). grease fleece weight and milk yield are all for a specific trait that is transmitted examples of quantitative traits in sheep. to offspring. The higher the heritabil- A Word of Caution ity for a trait, the more rapid the rate Inheritance of a Note that this illustration is oversim- of genetic improvement is for that trait. plified. Most quantitative traits, such Heritability values range from 0 to 100 Quantitative Trait as yearling grease fleece weight, are percent. Reproductive traits are lowly Inheritance of a quantitative trait is influenced by other types of gene action heritable (0 to 20%), growth traits are low more complex than that of a qualitative such as dominance and epistasis (genes to moderately heritable (10 to 40%) and trait, such as wool color, because of the in- at one locus influence the action of genes fleece and carcass traits are moderate to creased number of genes involved. Thou- at another locus), in addition to the ad- highly heritable (25 to 55%). Estimates sands of gene combinations are possible ditive gene action illustrated here. Also, of 25 percent for postweaning gain and in any animal. In quantitative traits, genes many pairs of genes influence yearling 40 percent for yearling weight indicate may act additively, and individual gene grease fleece weight in sheep, and the selection should be fairly effective for effects are often small. When genes act contribution of the different gene pairs these traits. However, it is important to additively, their effects can be compared may vary. Furthermore, breed, as well as realize that traits vary in heritability and, to adding block upon block in construct- environmental factors such as nutrition, as a result, rates of genetic improvement ing a building. Although the principles of season and climate, influence yearling will vary, depending on the specific trait segregation and recombination still hold, grease fleece weight. involved. it is currently still impossible to identify All of these factors tend to compli- individual genes and determine specific cate the genotypic expression of the genotypic ratios in quantitative traits. individual. In reality, the phenotype of a quantitative trait may not reflect the genotype directly because of environ- mental and non-additive genetic effects.

3 Breeding Values breed. Suppose the ram in the previous Summary example (weaning weight EBV = 12 lb; Every animal has a breeding value for EPD = 6 lb) is compared to a ram having Understanding the basic principles of each trait. A breeding value represents a weaning weight EBV of 4 pounds (EPD inheritance is necessary if producers are the total additive effect for all genes = 2 lb). Assuming these rams are mated to use genetic tools for sheep improve- influencing the phenotype for the trait. to an equal set of ewes, their offspring ment. Although the basic principles hold In other words, a breeding value is a are expected to differ by an average of 4 for all traits, the inheritance and genetic measure of gene transmitting ability. Un- pounds (6–2 lb). The advantage goes to improvement of a quantitative trait, such fortunately, an animal’s breeding value the first ram because he has the higher as yearling grease fleece weight, weaning cannot be observed directly. estimated breeding value. weight or yearling weight, is more com- Breeding values can be estimated as When available, performance of plex than that of a qualitative trait, such the difference between the individual relatives (progeny, siblings and ancestors) as wool color. Traditional selection, based animal’s performance and the average of can be used to estimate genetic merit on estimation of genetic merit using the its contemporary group (selection differen- of an animal. For example, information individual’s phenotype and performance tial) multiplied by the proportion of the from the individual, paternal half-sibs, information on relatives, has resulted in difference expected to be transmitted to maternal half-sibs and progeny can be improved sheep production. This has the offspring(heritablity for the trait). Be- combined into a single estimate of the been enhanced by breed-wide genetic cause an estimated breeding value (EBV) individual’s EBV for a specific trait. evaluations conducted by the National is a prediction of what an individual is Including information on relatives, in Sheep Improvement Program and Lamb- expected to transmit to its offspring for addition to the individual’s own perfor- Plan. In the future, genetic improvement a particular trait, those animals with the mance, will increase the accuracy of EBV in economically important traits may be highest EBVs are expected to produce estimation, especially for lowly heritable accelerated using genomic tools for direct the most desirable progeny. As a result, traits. When heritability of a trait is mod- selection of genetic differences underly- frequency of desirable genes in the flock erately high, the individual’s own record ing phenotypes. or breed is increased. is more important and the improvement Because an animal contributes only in accuracy is less. half its genes to its offspring, an indi- Presently, the National Sheep Im- vidual transmits only half its breeding provement Program (NSIP), in con- value. For example, if a ram has an EBV junction with Australia’s LambPlan, of 12 pounds for weaning weight, on calculates EBVs for maternal, growth the average he is expected to transmit 6 and wool traits using information on the pounds of that to his progeny. In other individual, ancestors, collateral relatives words, if this ram is mated to a random and progeny. These EBVs can be used to sample of ewes, the resulting progeny are make genetic comparisons among ani- expected to be 6 pounds above average mals evaluated within a breed. Currently, for weaning weight. This value (one half 20 breeds participate in NSIP/LambPlan. of the EBV) is referred to as the expected Border Leicester, Dorper/White Dorper, progeny difference (EPD). Dorset, Hampshire, Katahdin, Polypay, Estimated breeding values are most Rambouillet, Shropshire, Sufflok and useful for comparing genetic merit be- Texel are some of the breeds that are tween two individuals within the same included.

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Educational programs of Kentucky Cooperative Extension serve all people regardless of race, color, age, sex, religion, disability, or national origin. Issued in furtherance of Co- operative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Nancy M. Cox, Director, Land Grant Programs, University of Kentucky College of Agriculture, Food and Environment, Lexington, and Kentucky State University, Frankfort. Copyright © 2014 for materials developed by University of Kentucky Cooperative Extension. This publication may be reproduced in portions or its entirety for educational or nonprofit purposes only. Permitted users shall give credit to the author(s) and include this copyright notice. Publications are also available on the World Wide Web at www.ca.uky.edu. Issued 12-2014