Alleles and Their Interactions
In many cases, alleles do not show the simple relationships between dominance and recessiveness that we have described. In others, a single allele may have multiple phenotypic effects. Existing alleles can give rise to new alleles by mutation, so there can be many alleles for a single character.
New alleles arise by mutation
Different alleles of a gene exist because genes are subject to mutations, which are rare, stable and inherited changes in the genetic material. In other words, an allele can mutate to become a different allele. Mutation is a random process; different copies of the same allele may be changed in different ways. One particular allele of a gene may be defined as the wild type because it is present in most individuals in nature (“the wild”) and gives rise to an expected trait or phenotype. Other alleles of that gene, often called mutant alleles, may produce a different phenotype. The wild-type and mutant alleles reside at the same locus and are inherited according to the rules set forth by Mendel. A genetic locus with a wild-type allele that is present less than 99 percent of the time (the rest of the alleles being mutant) is said to be polymorphic(from the Greek poly “many” and morph “form”).
Many genes have multiple alleles
Because of random mutations, a group of individuals may have more than two alleles of a given gene. (Any one individual has only two alleles, of course — one from its mother genes have alleles that are not dominant or recessive to one another. Instead, the heterozygotes show an intermediate phenotype — at first glance, like that predicted by the old blending theory of inheritance. For example, if a true-breeding red snapdragon is crossed with a true-breeding white one, all the F1 flowers are pink. That this phenomenon can still be explained in terms of Mendelian genetics, rather than blending, is readily demonstrated by a further cross. The blending theory predicts that if one of the pink F1 snapdragons is crossed with a true-breeding white one, all the offspring should be a still lighter pink. In fact, approximately 1⁄2 of the offspring are white, and 1⁄2 are the same shade of pink as the F1 parent. When the F1 pink snapdragons are allowed to self-pollinate, the resulting F2 plants are distributed in a ratio of 1 red : 2 pink : 1 white. Clearly the hereditary particles — the genes — have not blended; they are readily sorted out in the F2.
We can understand these results in terms of the Mendelian laws of inheritance. All we need to do is recognize that the heterozygotes show a phenotype intermediate between those of the two homozygotes. In such cases, the gene is said to be governed by incomplete dominance. Incomplete dominance is common in nature. In fact, Mendel’s paper was unusual in that all seven of the examples he described are characterized by complete dominance.
In codominance, both alleles are expressed
Sometimes the two alleles at a locus produce two different phenotypes that both appear in heterozygotes. An example of this phenomenon, called codominance, is seen in the ABO blood group system in humans. Early attempts at blood transfusion frequently killed the patient. Around 1900, the Austrian scientist Karl Landsteiner mixed blood cells and serum (blood from which cells have been removed) from different individuals. He found that only certain combinations of blood are compatible. In other combinations, the red blood cells from one individual form clumps in the presence of serum from the other individual. This discovery led to our ability to administer compatible blood transfusions that do not kill the recipient. Clumps form in incompatible transfusions because specific proteins in the serum, called antibodies, react with foreign, or “nonself,” cells. The antibodies react with proteins on the surface of nonself cells, called antigens. Blood compatibility is determined by a set of three alleles (IA, IB, and iO) at one locus, which determine the antigens on the surface of red blood cells. Different combinations of these alleles in different people produce four different blood types, or phenotypes: A, B, AB, and O. The AB phenotype found in individuals of IAIB genotype is an example of codominance—these individuals produce cell surface antigens of both the Aand B types.
Some alleles have multiple phenotypic effects
Mendel’s principles were further extended when it was discovered that a single allele can result in more than one phenotype. When a single allele has more than one distinguishable phenotypic effect, we say that the allele is pleiotropic. A familiar example of pleiotropy involves the allele responsible for the coloration pattern (light body, darker extremities) of Siamese cats, discussed later in this chapter. The same allele is also responsible for the characteristic crossed eyes of Siamese cats. Although these effects appear to be unrelated, both result from the same protein producedunder the influence of the allele.
Thus far we have treated the phenotype of an organism, with respect to a given character, as a simple result of the alleles of a single gene. In many cases, however, several genes interact to determine a phenotype. To complicate things further, the physical environment may interact with the genetic constitution of an individual in determining the phenotype.
Some genes alter the effects of other genes
Epistasisoccurs when the phenotypic expression of one gene is affected by another gene. For example, several genes determine coat color in mice. The wild-type color is agouti, a grayish pattern resulting from bands on the individual hairs. The dominant allele B determines that the hairs will have bands and thus that the color will be agouti, whereas the homozygous recessive genotype bb results in unbanded hairs, and the mouse appears black. A second locus, on another chromosome, affects an early step in the formation of hair pigments. The dominant allele A at this locus allows normal color development, but aa blocks all pigment production. Thus, aa mice are all-white albinos, irrespective of their genotype at the B locus. If a mouse with genotype AABB (and thus the agouti phenotype) is crossed with an albino of genotype aabb, the F1 mice are AaBb and have the agouti phenotype. If the F1 mice are crossed with each other to produce an F2 generation, then epistasis will result in an expected phenotypic ratio of 9 agouti:3 black:4 albino. (Can you show why? The underlying ratio is the usual 9:3:3:1 for a dihybrid cross with unlinked genes, but look closely at each genotype, and watch out for epistasis.) In another form of epistasis, two genes are mutually dependent: The expression of each depends on the alleles of the other. The epistatic action of such complementary genesmay be explained as follows: Suppose gene A codes for enzyme A in the metabolic pathway for purple pigment in flowers, and gene B codes for enzyme B: In order for the pigment to be produced, both reactions must take place. The recessive alleles a and b code for nonfunctional enzymes. If a plant is homozygous for either a or b, the corresponding reaction will not occur, no purple pigment will form and the flowers will be white.
Hybrid vigor results from new gene combinations and interactions
If Mendel’s paper was the most important event in genetics in the nineteenth century, perhaps an equally important paper in applied genetics was published early in the twentieth century by G. H. Shull, titled “The composition of a field of maize.” Farmers growing crops have known for centuries that mating among close relatives (known as inbreeding) can result in offspring of lower quality than those from matings between unrelated individuals. The reason for this is that close relatives tend to have the same recessive alleles, some of which may be harmful, as we saw in our discussion of human pedigrees above. In fact, it has long been known that if one crosses two true-breeding, homozygous genetic strains of a plant or animal, the result is offspring that are phenotypically much stronger, larger and in general more “vigorous” than either of the parents. Shull began his experiment with two of the thousands of existing varieties of corn (maize). Both varieties produced about 20 bushels of corn per acre. But when he crossed them, the yield of their offspring was an astonishing 80 bushels per acre. This phenomenon is known as heterosis(short for heterozygosis), or hybrid vigor. The cultivation of hybrid corn spread rapidly in the United States and all over the world, quadrupling grain production. The practice of hybridization has spread to many other crops and animals used in agriculture. The actual mechanism by which heterosis works is not known. A widely accepted hypothesis is overdominance in which the heterozygous condition in certain important genes is superior to either homozygote.
The environment affects gene action
The phenotype of an individual does not result from its genotype alone. Genotype and environment interact to determine the phenotype of an organism. Environmental variables such as light, temperature, and nutrition can affect the translation of a genotype into a phenotype. A familiar example of this phenomenon involves the Siamese cat. This handsome animal normally has darker fur on its ears, nose, paws and tail than on the rest of its body. These darkened extremities normally have a lower temperature than the rest of the body. Afew simple experiments show that the Siamese cat has a genotype that results in dark fur, but only at temperatures below the general body temperature. If some dark fur is removed from the tail and the cat is kept at higher than usual temperatures, the new fur that grows in is light. Conversely, removal of light fur from the back, followed by local chilling of the area, causes the spot to fill in with dark fur. Two parameters describe the effects of genes and environment on the phenotype:
Penetranceis the proportion of individuals in a group with a given genotype that actually show the expected phenotype.
Expressivityis the degree to which a genotype is expressed in an individual.
For an example of environmental effects on expressivity, consider how Siamese cats kept indoors or outdoors in different climates might look.
Most complex phenotypes are determined by multiple genes and environment
The differences between individual organisms in simple characters, such as those that Mendel studied in peas, are discrete and qualitative. For example, the individuals in a population of peas are either short or tall. For most complex characters, however, such as height in humans, the phenotype varies more or less continuously over a range. Some people are short, others are tall, and many are in between the two extremes. Such variation within a population is called quantitative, or continuous, variation. In most cases, quantitative variation is due to two factors: multiple genes, each with multiple alleles, and environmental influences on the expression of these genes. Geneticists call the genes that together determine a complex character quantitative trait loci. Identifying these loci is a major challenge and an important one. For example, the amount of grain that a variety of rice produces in a growing season is determined by many interacting genetic factors. Crop plant breeders have worked hard to decipher these fac-tors in order to breed higher-yielding rice strains. In a similar way, human characteristics such as disease susceptibility and behavior are caused in part by quantitative trait loci.
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