Dominant and Recessive Alleles Help
Dominant and Recessive Alleles
Allelic forms of many genes are expressed by coding for the synthesis of a protein, which, in turn, affects the phenotype of the organism. If a distinctive phenotype is associated with an allele (a) only when its alternative allele (A) is absent from the genotype, the allele is called recessive. A dominant allele (A) is observed phenotypically in the heterozygote as well as in the homozygote. In some cases, dominance and recessiveness might be conceived as the presence or absence of a trait, protein, or gene product; however, there is no general mechanism that applies to all cases of dominance in molecular or cellular terms. Dominance is not a causal property inherent in the trait or allele itself, but rather it is a relationship between pairs of alleles. Other forms of allelic relationships, such as codominance or incomplete dominance, are more common in nature than complete dominance and are discussed later in this chapter. It is important to remember that the frequency of any allele in a natural population of individuals is ultimately determined by evolutionary processes such as natural selection, not by its relationship to other alleles (i.e., dominance).
EXAMPLE 2.5 Lack of pigment deposition in the human body is an abnormal recessive trait called ''albinism.'' Using A and a to represent the dominant (pigment-producing) allele and the recessive (albino) allele, we can describe respectively the three genotypes and two phenotypes that are possible:
Recessive alleles (such as the one for albinism) may be deleterious to those individuals 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 view by the dominant normal allele is called a carrier. Most of the deleterious alleles harbored by a population are found in carrier individuals.
Wild-Type And Mutant Alleles
An allele that is very common in a population is referred to as wild type. Alleles that are less common are referred to as mutant alleles. An organism exhibiting the phenotype associated with the wild-type allele is termed a wildtype organism; an organism exhibiting the phenotype associated with the rare allele is termed a mutant. Often, wild-type alleles are dominant and mutant alleles are recessive; however, this is not always true and should never be assumed. Genetic crosses or tests, such as those described in this chapter (e.g., the Testcross), are always required to determine the relationships between alleles.
Different systems for symbolizing genotypes (dominant and recessive alleles) and phenotypes (wild type and mutant) are used by geneticists for numerous organisms, from plants and animals to yeast, bacteria, and viruses. Students should become familiar with the different types of symbolic representations and be able to work genetic problems regardless of the symbolic system used. The base letter for the gene usually is taken from the name of the mutant or abnormal trait and can be a single letter (e.g., a), an abbreviation (e.g., cdc, for cell division cycle), or the first few letters of the gene name (e.g., pro, for a gene involved in proline amino acid biosynthesis). Generally, dominant alleles are denoted with an uppercase letter (e.g., A), first letter upper case (e.g., Pb), or all letters upper case (e.g., HIS4), and a recessive allele is denoted with all lowercase letters (e.g., a, pb, his4). However, there are other systems. Wild-type alleles in fruit flies are additionally designated with a + symbol; they can be either dominant or recessive. The case of the symbol indicates the dominance or recessiveness of the mutant allele to which the superscript þ for wild type must be referred. The wild-type allele in this system can also just be referred to as " + " with no letter designation (e.g., + / a). Also, since most organisms that we will use as examples are diploid, both alleles are represented in a genotype either as two symbols side-by-side (e.g., Aa) or two symbols with a slash in between (e.g., A / a). Also, gene symbols are usually italicized.
EXAMPLE 2.6 Ebony (black) body color in Drosophila (fruit fly) is governed by a recessive, mutant allele e, while wild-type body color is gray and is governed by a dominant allele e+. Thus, a fly with both e+ alleles (e+ / e+) or with a dominant e+ and a recessive e allele (e+ / e) will have a gray body. However, a fly with both recessive e alleles (e / e) will be black.
EXAMPLE 2.7 Lobed-shaped eyes in Drosophila are governed by a mutant allele L that is dominant, L, while wild-type (oval-shaped) eyes are governed by a recessive allele L+. Thus, a fly with both dominant L alleles (L / L+)or a fly with one of each allele (L / L+) will have lobed eyes, while a fly with both recessive alleles (L+ / L+) will have wild-type, oval-shaped eyes.
In bacteria, mutant alleles are additionally designated with a superscript minus sign after the gene symbol (e.g., proA-). In some systems, specific alleles are often designated by a number or letter following the allele symbol. For example, a specific mutant allele of the yeast gene might be designated as his4-11, suggesting that this is the eleventh allele of the fourth wild-type HIS gene (genes involved in the synthesis of histidine) to be identified. In yeast, wild-type alleles are designated by all upper-case letters.
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