Moreover, modules of epistatic pleiotropic effects within the GP map match the degree and in the correlation of the effects of pleiotropic mutations. The replicative capacity (fitness) of each sample was measured by. The human ABO blood groups are an example of multiple alleles, and the relationship between phenotype and genotype is depicted in the. For example, the case K = 1 (interaction with one other locus) is modelled by creating .. (c) Correlation between epistasis and beneficial effect.
There is thus pleiotropy and epistasis in the GP map Hansen ; Phillips The way pleiotropic and epistatic gene effects are organized within the GP map is expected to play a capital role in the capacity of living organisms to adapt and evolve Wagner and Altenberg ; Hansen ; Wagner et al.
In particular, the modular clustering of pleiotropic effects among sets of traits allows phenotypic modules to respond to selection independently from each other, potentially increasing the evolvability of the organism Wagner and Altenberg ; Wagner et al. Trait genetic integration i. Indeed, when several traits are affected by a common set of pleiotropic loci, as within a module, they become less evolutionary independent because changes in allelic frequencies at those loci will affect all traits, and changes favorable to one trait may be detrimental to the other traits Otto This would be the case if, for instance two traits are genetically correlated while selection acts to change only one of them, while keeping the other constant.
Evolution may break such pleiotropic constraints, or build them up, if variation in pleiotropy exists at the underlying genes. Trait integration is often deduced from trait phenotypic or genetic correlations within a population fig.
Genetic correlations among traits are function of the correlation among pleiotropic mutational effects at the underlying genes fig.
Variation in genetic covariation among traits thus depends on variation in the pleiotropic degree and in the correlation of the effects of pleiotropic mutations. Which of these two properties of pleiotropic mutations is more likely to vary and to contribute to trait correlations in natural systems is mostly unknown.
The key to understanding the difference between the three types is to look at the phenotype of the individuals with heterozygous alleles, then classify the relationship accordingly.
Multiple Alleles So far we have only examined traits controlled by two alleles. This is easy to visualize because diploid organisms can only possess two alleles.
Within a population, however, more than two alleles can exist although any given individual only has two alleles. The human ABO blood groups are an example of multiple alleles, and the relationship between phenotype and genotype is depicted in the figure above.
There are four possible phenotypic blood types for this particular gene: A, B, AB, and O. The letters refer to two specific carbohydrate molecules on the surface of red blood cells.
Note, these are not the same molecules as the MN blood groups. Individuals can have the A carbohydrate blood type Athe B carbohydrate blood type Bboth the A and B carbohydrates blood type ABor neither carbohydrate blood type O. The ABO blood groups are formed by various combinations of three different alleles; IA codes for carbohydrate AIB codes for carbohydrate Band i codes for the lack of any carbohydrate.
As shown in this figure, individuals with one or two IA alleles will have blood type A, those with one or two IB alleles will have blood type B, those with both the IA and IB alleles will have blood type AB, and those with the genotype ii will have blood type O. Click image to enlarge Figure. Multiple alleles for the ABO blood groups.
Impact of epistasis and pleiotropy on evolutionary adaptation
Click image to enlarge The Punnett square can be used to predict the genotype frequencies resulting from multiple allele crosses. However, one cannot be certain of an individual's genotype if they are blood type A or B because there are two possible genotypes for each of these blood types. Therefore, many cross problems that examine blood types are similar to test crosses; that is, the parental genotype is uncertain.
A few examples will aid in your understanding. At the following Web sites, find the correct answer to the multiple-choice monohybrid cross questions. Work out each problem. After viewing the correct answer, close the Monohybrid Cross Problem Set window to return to this page.
Pleiotropy So far we have only considered genes that affect a single phenotypic character. This actually is a rare situation because it is more common that one gene can have multiple effects pleiotropy. For example, albino individuals lack pigment in their skin and hair, and also have crossed eyes at a higher frequency than pigmented individuals see photograph. This occurs because the gene that causes albinism can also cause defects in the nerve connections between the eyes and the brain.
These two traits are not always linked, again showing the complexity of genetic interactions in determining phenotypes. Pleiotropy in individuals with albinism. Click image to enlarge Mendel also recognized this effect. He observed that pea plants with red flowers had red coloration where the leaf joined the stem, but that their seed coats were gray in color.
Plants with white flowers had no coloration at the leaf-stem juncture and displayed white seed coats. These combinations were always found together, leading Mendel to conclude that they were likely controlled by the same hereditary unit i. Epistasis Sometimes a gene at one location on a chromosome can affect the expression of a gene at a second location epistasis. A good example of epistasis is the genetic interactions that produce coat color in horses and other mammals. In horses, brown coat color B is dominant over tan b.
Gene expression is dependent on a second gene that controls the deposition of pigment in hair.
The dominant gene C codes for the presence of pigment in hair, whereas the recessive gene c codes for the absence of pigment. If a horse is homozygous recessive for the second gene ccit will have a white coat regardless of the genetically programmed coat color B gene because pigment is not deposited in the hair.
There was a problem providing the content you requested
The figure above demonstrates this scenario. Several of the white horses have genotypes for brown or tan coat color in the first gene, but are completely white because they are homozygous recessive for the gene controlling pigment deposition. An example of epistasis. Click image to enlarge At the following Web sites, find the correct answer to the multiple-choice dihybrid cross questions.
When haplotypes adapt at high mutation rates, more epistatic pairs of substitutions are observed on the line of descent than expected. The highest fitness is attained in landscapes with an intermediate amount of ruggedness that balance the higher fitness potential of interacting genes with their concomitant decreased evolvability.
Our findings imply that the synergism between loci that interact epistatically is crucial for evolving genetic modules with high fitness, while too much ruggedness stalls the adaptive process. Introduction As a population adapts to its environment, it accumulates mutations that increase the chance for the long-term success of the lineage or lineages it represents. Instead, deleterious mutations play an important role as stepping stones of adaptive evolution that allow a population to traverse fitness valleys.
In this respect, the impact of the sign i. If we move from the single gene level to networks of genes, the situation becomes even more complex.
To understand the evolution of such systems, we have to take into account the interaction between loci, and furthermore abandon the limit where mutations on different loci fix sequentially.