Recessive Genes and Traits
In diploid organisms, the attribute of a specified character (or phenotype) is said to be recessive if it is masked when in the presence of a dominant allele. A recessive phenotype is expressed only in homozygotes (or, for X-linked traits, also in hemizygotes). Strictly speaking, it is the phenotype that is recessive (or dominant), not the allele; however, the term "recessive gene" is widely, if loosely used.
Gregor Mendel recognized the dominant and recessive aspects of the traits he studied in garden peas in the mid nineteenth century. Those traits that become latent in the process of the hybridization he described as recessive, whereas those that constituted the character of the hybrid were seen as dominant. Mendel studied seven characters of peas, each with two recognizable traits; for example, round or wrinkled seeds, dwarf or tall stems, and several others. Starting with pure strains for each trait, he created hybrid crosses (called F1) and subsequently crosses of the hybrids to one another (called F2). For each character, one of the traits disappeared in the F1, but reappeared in the F2 generation in about one quarter of the plants. He described the agent responsible for each trait as a "factor"; that factor (now called a gene) was responsible for the phenomenon of a recessive trait that could be hidden but not destroyed. The term atavism, though rarely used today, refers to this reappearance of a character after several generations.
In experimental organisms, the study of two characters simultaneously is called a dihybrid cross. An organism with the recessive phenotype for both is called a double recessive organism.
The concept of dominance and recessivity is an operational one, and the distinction is not absolute. Moving from Mendel's peas to examples from human genetics, a trait may be clinically undetectable in heterozygotes (also called carriers), and thus defined as recessive at that level of the clinical phenotype.At the cellular, biochemical or molecular level, the trait might be seen as recessive, incompletely recessive, or even dominant, depending on the aspect being studied. Consider the example of the mutation in the beta-globin gene that, when homozygous, causes the disease sickle cell anemia. The heterozygous individual is not anemic under normal circumstances, therefore the mutant gene would be considered recessive. Carriers, however, may demonstrate some sickle-shaped red blood cells under conditions of reduced oxygen tension, though not to the extent seen in the homozygotes, so at the cellular level, the trait is incompletely recessive. At the molecular level, both normal and sickle hemoglobin can be found within the cells, and the trait could be described as co-dominant at this level.
In family studies, autosomal recessive traits have certain distinctive inheritance characteristics. Traits that are lethal or deleterious are usually rare, and the inheritance patterns described apply to such rare alleles. First, a recessive trait often appears sporadically, with no evidence of it in either parent or collateral relatives. Those parents, however, are then obligate carriers of the recessive allele, and any of their subsequent offspring have a one in four chance of being affected by the disease or trait in question. Second, the more rare the recessive gene, the more likely the parents are to be related to one another. Consanguinity in the parents' relationship often alerts a clinician to consider a recessive basis for a diagnosis.
Recessive traits may also result from genes carried on the X chromosome. In this case, hemizygous males reveal the recessive phenotype due to the lack of any dominant allele to mask it.
At a molecular level, recessive traits are frequently the result of mutations in enzymes rather than in structural genes, and are more likely to result from the absence of a gene product that an altered one. The reason for this is that a trait will be recessive if the alternate allele provides enough product for full cellular function. Enzymes are typically produced in huge excess by cells, so that a 50% reduction is often not deleterious. This characteristic is a rule-of-thumb, but certainly not absolute.
Genetic selection acts on the phenotype of an individual. Carriers of recessive traits, by definition, are not detectable phenotypically, will not be selected against, and can, therefore, harbor and transmit their recessive alleles throughout generations.
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