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United Suffolk Sheep Association |
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December/January 2003/2004 The Expression of Genes How genotype affects phenotype
Larry Arehart
The first article in this series discussed setting goals in your sheep breeding program in order to achieve a desirable outcome (Suffolk News August/September Issue). This article will be directed to the ways genes are expressed to determine the phenotype of an individual. These concepts are very involved and can be difficult to understand for most breeders unless they have had some background study in animal genetics. However, these concepts influence any selection program and help explain why progress in a breeding program is not as fast as we might wish. Genotype determines phenotype. However, since animals have two copies (alleles) of each gene (one from their mother and one from their father) and since many genes may be involved in determining a single phenotypic trait, it is gene expression that actually determines phenotype. The affect of gene expression on phenotype is generally thought of as being either additive or non-additive. Additive gene action occurs when the phenotypic effect of one gene adds to the phenotypic effect of another gene. The other gene may be the other allele of the first gene or may be some other gene in genome of the animal. When the gene expression is non-additive, the phenotypic expression of one gene doesn't necessarily add to the phenotypic expression of another gene. NON-ADDITIVE GENE EXPRESSION Non-additive gene expression may exhibit: (a) dominance, (b) recessivity, (c) no dominance (lack of dominance), (d) over dominance, and (e) epistasis. Dominant gene expression is observed when a gene exerts its effect on the phenotype regardless of the genotype of its companion allele. An excellent example of this in Suffolk sheep is wool color (black vs. white). It has been determined that black wool occurs only when the two recessive are present, (ww - one w allele coming from the dam and one from the sire). If big W appears in the genotype then the sheep is white (WW or Ww). Another example of dominant gene expression with which we are all familiar today is resistance to scrapie. A sheep with a single R allele is resistant to scrapie (RR or QR) and only if two sensitive (“recessive”) alleles are present is the sheep susceptible. It is important to also understand that color in Suffolk sheep is also influenced by mutation of skin cells. When black sheep cut themselves or rub their skin they might destroy or change the skin cell genotype by a mutation in the skin cells. This would cause black fiber in the region of the damage. However, these types of color changes are not transferred to the next generation. No dominance or lack of dominance is defined as the inability of a gene to express itself over its allele. An example of this appears in Shorthorn cattle. When Rr appears in the genotype of Shorthorn cattle, roan color appears in the hair coat. What is happening is that both red hair (R) and the white hair (r) are expressing themselves at the same time. Thus the phenotype of the offspring is a combination of red and white hairs appearing roan. It is likely that there are lack of dominance gene expressions occurring in sheep, but clear examples have not been identified. Over dominance is the interaction between genes which are alleles and results in heterozygous individuals (animals containing different alleles of a given gene) being superior to either type of homozygous individual (animals in which both copies of a given gene are the same allele). For example, three different genotypes such as AA, AB, BB may be observed. When A and B occur together (as in genotype AB) they produce a product that is different than those produced in AA or BB animals, perhaps because of some interaction between the products of the different alleles. This type of gene expression is thought to be responsible for hybrid vigor, which is a result of crossbreeding and outcrossing and which will be discussed in later articles. Epistasis is another form of nonadditive gene action. It involves two or more pairs of genes which are not alleles. Unlike over dominance, which involves interactions between alleles, epistasis involves interactions between two entirely different pairs of genes. The genes may be on the same or different chromosomes. In epistatic interactions one gene may control the degree to which another gene is expressed. This kind of gene expression is thought to have a tremendous impact on the expression of economic traits such as rate of gain, efficiency of gain and carcass characteristics and quality. It may be of significant importance for some traits in specific families. Over dominance and epistasis include the interaction of many genes (perhaps thousands) and play a big roll in achieving increased performance in offspring. Taking advantage of these two types of gene expression is best accomplished in line breeding programs (breeding animals more closely related than the average of the population).
ADDITIVE GENE ACTION In this type of inheritance, there is no sharp distinction between genotypes, but there are many gradations between the two extremes. An example of how additive genes express themselves may be illustrated by imagining a large glass cylinder of clear water on a desk top. The water would represent genes on the chromosomes that are neutral (have no expression). If a red pill is added to the water it begins to turn pink. If another pill is added it turns light red, add another pill and it turns red. The point is as the neutral genes on the chromosomes are replaced with genes that have additive expression and the phenotype of the individual changes. Thus, each pill added changes the water color in a linear manner. The same would be true when replacing neutral alleles on the chromosomes with additive alleles. Additive gene action affects many important traits in sheep. Examples include growth rate, milk production, conformation, carcass quantity traits and carcass quality. Remember non-additive gene action many also affect these traits. Those traits affected by additive gene action are moderately to highly heritable and will be affected very little by outcrossing and inbreeding. It is important to realize that this type of gene action influences many of the traits a breeder is interested in selecting for in a breeding program. Improvement in phenotype can be made fairly rapidly if this additive gene action is involved.
SUMMARY Genes express themselves in many different ways. Most of the traits a breeder is selecting for involve many, many genes (perhaps hundreds or even thousands). It is a breeder’s goal to line them up in just the right combination to get the desired phenotype. Since most of the alleles for which a breeder will select can be expected to segregate independently at mating time, this is a real tough chore for breeders and progress is slow. The next article will discuss tools to measure and take advantage of the ways the genes are expressed.
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