Genetic Interactions Help
The phenotype is a result of many gene products expressed in a given environment. The environment includes not only external factors such as temperature and the amount or quality of light, but also internal factors such as hormones and enzymes. One important type of protein gene product is enzymes. Enzymes perform catalytic functions, causing the splitting or union of various molecules during metabolism. Metabolism is the sum of all the physical and chemical processes by which cells are produced and maintained and by which energy is made available for the uses of the organism. These biochemical reactions occur as stepwise conversions of one substance into another, each step being mediated by a specific enzyme. All of the steps that transforma precursor substance to its end product constitute a biosynthetic pathway.
Several genes are usually required to specify the enzymes involved in even the simplest pathways. Each metabolite (A, B, C) is produced by the catalytic action of different enzymes (ex) specified by different wild-type genes (gx+). One common example of genetic interaction occurs whenever two or more genes specify enzymes that catalyze steps in a common pathway. If substanceCis essential for the production of a normal phenotype, and the recessive mutant alleles g1, g2, and g3 produce defective enzymes, then a mutant (abnormal) phenotype would result from a genotype that is homozygous recessive at any of the three loci. If g3 contains a mutation, the conversion of B to C does not occur and substance B tends to accumulate in excessive quantity; if g2 contains a mutation, substance B will not be produced and substance Awill accumulate. Thus, gene mutations are said to produce "metabolic blocks." An organism with amutation in only g2 could produce a normal phenotype if it were given either substance B or C, but an organism with a mutation in g3 has a specific requirement for C. Thus, gene g+3 becomes dependent upon gene g+2 for its expression as a normal phenotype. If the genotype is homozygous for the recessive g2 allele, then the pathway ends with substance A. Neither g+3 nor its recessive allele g3 has any effect on the phenotype. Thus, the genotype g2g2 can hide or mask the phenotypic expression of alleles at the g3 locus. It is said that g2 is epistatic to g3, i.e., it masks any expression that may or may not occur from g+3 alleles. A gene or locus that suppresses or maskes the action of a gene at another locus is termed epistatic. The term "epistasis" refers to any type of gene interaction that involves the masking of one of the gene effects. Dominance involves intra-allelic gene suppression, or the masking effect that one allele has upon the expression of another allele at the same locus. Epistasis involves interallelic gene suppression, or the masking effect that one gene locus has upon the expression of another. The classical phenotypic ratio of 9 : 3 : 3 : 1 observed in the progeny of dihybrid parents becomes modified by epistasis into ratios that are various combinations of the 9 : 3 : 3 : 1 groupings.
EXAMPLE 4.1 A particularly illuminating example of gene interaction occurs in white clover. Some strains have a high cyanide content; others have a low cyanide content. Crosses between two strains with low cyanide have produced an F1 with a high concentration of cyanide in their leaves. The F2 shows a ratio of 9 high cyanide : 7 low cyanide. Cyanide is known to be produced from the substrate cyanogenic glucoside by enzymatic catalysis. One strain of clover has the enzyme but not the substrate. The other strain makes substrate but is unable to convert it to cyanide. The pathway may be diagrammed as follows where Gx produces an enzyme and gx results in a metabolic block.
Tests on leaf extracts have been made for cyanide content before and after the addition of either glucoside or the enzyme e2.
If the leaves are phenotypically classified on the basis of cyanide content of extract alone, a ratio of 9 : 7 results. If the phenotypic classification is based either on extract plus glucoside or on extract plus e2, a ratio of 12 : 4 is produced. If all of these tests form the basis of phenotypic classification, the classical 9 : 3 : 3 : 1 ratio emerges.
Genetic interactions can also occur during embryogenesis in developmental pathways as well as other complex biological processes.
Interactions with Three or More Factors
The progeny from trihybrid parents are expected in the phenotypic ratio 27 : 9 : 9 : 9 : 3 : 3 : 1. This classical ratio can also be modified whenever two or all three of the loci interact. Interactions involving four or more loci are also possible. Most genes probably depend to some extent upon other genes in the total genotype. The total phenotype depends upon interactions of the total genotype with the environment.
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