University of Kentucky College of Agriculture, ASC-220 Food and Environment Cooperative Extension Service Basic Sheep 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 livestock 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 Wool 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 meat 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 as segregation. 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.