Allelic Interactions - dominant and recessive alleles, codominant alleles, lethal alleles, penetrance or expressivity, multiple alleles

                                                ALLELIC RELATIONSHIP

 Dominant and Recessive Alleles:

 Whenever one of a pair of alleles can come to phenotypic expression only in a homozygous genotype, we call that allele a recessive factor. The allele which can phenotypically express itself in the heterozygote as well as in the homozygote is called a dominant factor. Upper- and lower-case letters are commonly used to designate dominant and recessive alleles respectively. Usually, the genetic symbol corresponds to the first letter in the name of the abnormal (or mutant) trait.

 (a) Carriers:

 Recessive alleles (such as the one for albinism) are often deleterious to those who possess them in duplicate (homozygous recessive genotype). A heterozygote may appear just as normal as the homozygous dominant genotype. A heterozygous individual who possesses a deleterious recessive allele hidden from phenotypic expression by the dominant normal allele is called a carrier. Most of the deleterious alleles harbored by a population are found in carrier individuals.

 (b) Wild Type Symbolism:

 A different system for symbolizing dominant and recessive alleles is widely used in numerous organisms from higher plants and animals to the bacteria and viruses. Different genetics texts favor either one or the other system. In the author's opinion, every student should become familiar with both kinds of allelic representation and be able to work genetic problems regardless of the symbolic system used. Throughout the remainder of this book the student will find both systems used extensively. Where one phenotype is obviously of much more common occurrence in the population than its alternative phenotype, the former is usually referred to as wild type. The phenotype which is rarely observed is called the mutant type. In this system, the symbol + is used to indicate the normal allele for wild type. The base letter for the gene usually is taken from the name of the mutant or abnormal trait. If the mutant gene is recessive the symbol would be a lowercase letter(s) corresponding to the initial letter(s) in the name of the trait. Its normal (wild type) dominant allele would have the same lowercase letter but with a + as a superscript.

Remember that the case of the symbol indicates the dominance or recessiveness of the mutant allele to which the superscript + for wild type must be referred. After the allelic relationships have been defined, the symbol + by itself may be used for wild type and the letter alone may designate the mutant type.

 Codominant Alleles:

 Alleles which lack dominant and recessive relationships may be called intermediate alleles or codominant alleles. This means that each allele is capable of some degree of expression when in the heterozygous condition. Hence the heterozygous genotype gives rise to a phenotype distinctly different from either of the homozygous genotypes. Usually, the heterozygous phenotype resulting from codominance is intermediate in character between those produced by the homozygous genotypes, hence the erroneous concept of "blending". The phenotype may appear to be a "blend" in heterozygotes but the alleles maintain their individual identities and will segregate from each other in the formation of gametes.

 (a) Symbolism for Codominant Alleles:

For codominant (or intermediate) alleles, all uppercase base symbols with different superscripts should be used. The uppercase letters call attention to the fact that each allele can express itself to some degree even when in the presence of its alternative allele (heterozygous).

 Lethal Alleles:

 The phenotypic manifestation of some genes is the death of the individual either in the prenatal or postnatal period prior to maturity. Such factors are called lethal genes. A fully dominant lethal allele (i.e. one which kills in both the homozygous and heterozygous conditions) occasionally arises by mutation from a normal allele. Individuals with a dominant lethal die before they can leave progeny. Therefore, the mutant dominant lethal is removed from the population in the same generation in which it arose. Recessive lethal’s kill only when homozygous and may be of two kinds: (1) one which has no obvious phenotypic effect in heterozygotes, and (2) one which exhibits a distinctive phenotype when heterozygous.

 Penetrance and Expressivity:

 Differences in environmental conditions or in genetic backgrounds may cause individuals which are genetically identical at a particular locus to exhibit different phenotypes. The percentage of individuals with a particular gene combination which exhibits the corresponding character to any degree represents the penetrance of the trait.

 Multiple Alleles:

 The genetic systems proposed thus far have been limited to a single pair of alleles. The maximum number of alleles at a gene locus which any individual possesses is two, one on each of the homologous chromosomes. But since a gene can be changed to alternative forms by the process of mutation, a large number of alleles is theoretically possible in a population of individuals. Whenever more than two alleles are identified at a gene locus, we have a multiple allelic series.

 (a) Symbolism for Multiple Alleles:

 The dominance hierarchy should be defined at the beginning of each problem involving multiple alleles. A capital letter is commonly used to designate the allele which is dominant to all others in the series. The corresponding lowercase letter designates the allele which is recessive to all others in the series. Other alleles, intermediate in their degree of dominance between these two extremes, are usually assigned the lowercase letter with some suitable superscript.