What is it called when one allele is not completely dominant over another allele?

Abstract

Incomplete dominance results from a cross in which each parental contribution is genetically unique and gives rise to progeny whose phenotype is intermediate. Incomplete dominance is also referred to as semi-dominance and partial dominance. Mendel described dominance but not incomplete dominance. Had he worked with Mirabilis jalapa rather than pea he would have pondered the cross of a plant with red flowers by one with white flowers. All progeny were pink flowered, an intermediate phenotype. An individual who is heterozygous for a gene has two different alleles, but it is not always sufficient to produce an intermediate phenotype. Some genotypes that cause disease if both alleles are recessive show no phenotype as heterozygotes. But, if each parent contributes the mutated form of the gene, the resulting child will be homozygous, recessive, and sick. Other diseases manifest as a phenotype that is intermediate to the parents. Incomplete dominance can also result from the effect of one gene that masks the phenotype of another. Many color pathways are complicated due to these epistatic interactions.

Phenotypic variation

Homozygote horses with PSSM1 are more severely affected than heterozygotes, revealing incomplete dominance of the R309H mutation and explaining some of the variation in phenotypic severity between horses.27 Nonetheless, there are other factors that influence phenotype which can include the breed. For example, affected Quarter Horses and Belgian draught horses differ metabolically: compared with normal animals, Quarter Horses with PSSM cleared glucose from plasma more rapidly after its intravenous injection (Fig. 7.15) and had reduced peak plasma glucose concentrations after oral carbohydrate intake.187,188 Insulin sensitivity was not however detected in Belgian draught horses with PSSM.189 Differences between breeds likely reflect the genetic background: for example, some phenotypic variation might result from the underlying fiber-type profiles of the major muscle groups. The influence of modifying genes is also revealed by evidence showing that PSSM1-affected horses that carry an additional mutation in RYR1 (associated with MH – see above) – are more severely affected.157 The influence of diet on the phenotype is discussed in the treatment and management sections above.

Qualitative Traits

Although the presence of shell in the fruit is controlled by a gene exhibiting incomplete dominance, the thickness of the shell was purported to be modified by minor genes or under polygenic control (Van der Vossen, 1974). Okwuagwu (1988) and Okwuagwu & Okolo (1992, 1994) postulated maternal inheritance of kernel size with the involvement of a kernel-inhibiting factor. Fertile Ps do occur occasionally, and fertile P × fertile P crosses can produce highly fertile P progenies, suggesting perhaps fertility/sterility and absence of shell in the P are different genetic control (Wonki-Appiah, 1987). A number of other qualitative traits have also been reported from observations and genetic analyses to be simply inherited (Hartley, 1988), such as crown disease, leaf form (idolatrica = fused pinnae), fruit shape (mantled), and fruit color (nigrescens = black unripe, virescens = green unripe, albescence = whitish unripe; Fig. 2.6). Oil quality traits (e.g., high oleic, high stearic, low lipase) are also likely to be controlled by one or few major genes. Although the Dumpy trait appeared to be simply inherited, observations on segregating progenies did not confirm it (Soh et al., 1981). Population 12 from MPOB's Nigerian prospection has also a dwarf stature (Rajanaidu et al., 2000). Its inheritance has yet to be elucidated but is likely to have some major genes involved, likewise for long bunch stalk. The mode of inheritance for tolerance to Fusarium wilt has been postulated to be controlled by a few major genes (De Franqueville & Renard, 1990). The partial resistance trait in MPOB's Zaire × Cameroon and in CIRAD's advanced breeding parents are also likely to be controlled by a few major genes.

5 Growth Habit

Erect, intermediate, and prostrate growth habits were found in ratios that indicated a single gene with incomplete dominance (Ladizinsky, 1979b). Based on that study, the prostrate type was designated as gh gh and the erect type as Gh Gh. This gene has significance for lentil breeders who are attempting to develop upright and lodging-resistant cultivars. A single recessive gene designated as tnl by Vandenberg and Slinkard (1990) reportedly controls the presence or absence of tendrils on the ends of the leaves. This trait also has significance for lentil breeding in that good tendril activity is important for canopy formation and lodging resistance by the plants with tendrils being able to intertwine and provide mutual support.

Genetics of Earliness

Earliness has been found to be recessive to late maturity (Badami, 1923, 1928). Incomplete dominance for late maturity over earliness has been reported by Patel et al. (1936) and Hassan (1964). Holbrook et al. (1989) observed the role of 4 or 5 genes with complete dominance for late maturity over earliness. Days to first flowering was conditioned by a single gene with additive gene action. Days to accumulation of 25 flowers from the appearance of the first flower were governed by 3 independent genes with complete dominance. The segregation of 13:3 for late and early maturity indicated the components were governed by dominant–recessive epistasis (Upadhyaya and Nigam, 1994). Varman and Raveendran (1996) noticed the involvement of two recessive genes that act in an additive manner to influence days to first flowering. Jogloy et al. (2011) reported a negative correlation of maturity between pod yield and harvest index, which suggested the possibility of selecting early-maturity cultivars without affecting pod yield or harvest index.

Cropped or Notched Ears

Cropped or notched ears was a deformity identified in a group of Highland cattle, affecting 45 of 46 progeny from one breeding sire. The inheritance was determined to be incomplete dominance of a single autosomal gene. Other auricular anomalies that have been reported, primarily in sheep, include polyotia (presence of more than one auricle), macrotia (enlarged auricles), synotia (fusion of auricles), and misplaced auricles (heterotopic otia). A single case report of a Friesian cross calf with epitheliogenesis imperfecta had one ear deformed from rolling of its lateral margins followed by fusion of the surfaces after being brought into close proximity.

Interactions among Alleles

In the early twentieth century, researchers sought to confirm and extend Mendel's observations of heredity in Pisum using different organisms. In addition to the observation of genetic linkage, researchers also began to encounter other examples of non-Mendelian inheritance. Non-Mendelian patterns of inheritance were identified when crosses yielded a modified version of Mendel's 3:1 phenotype ratio in the F2 generation of a monohybrid cross. In some cases the altered phenotype ratios in the F2 generation reflected different types of dominance relationships among the alleles of a gene. This is observed for phenotypes resulting from incomplete dominance and codominance. Codominance and incomplete dominance yield unique phenotypes for heterozygous offspring (Aa). Incomplete dominance results in heterozygotes with intermediate phenotypes, as in the case of snapdragons when parents with red flowers and white flowers are crossed resulting in heterozygous offspring with pink flowers. Codominance occurs when both alleles show dominance, as in the case of the AB blood type (IA IB) in humans. Furthermore, the human ABO blood groups represent another deviation from Mendelian simplicity since there are more than two alleles (A, B, and O) for this particular trait. Deviations from Mendelian inheritance are also observed in traits with phenotypes having variable penetrance and expressivity. In these cases, individuals with the same allele combination can produce different degrees of a phenotype in different individuals. An example of a trait that is incompletely penetrant is polydactyly, which is a dominant trait causing extra fingers and toes. In some cases, the dominant trait of polydactyly can skip generations due to incomplete penetrance and then reappear in future generations.

Incomplete dominance and codominance

The results of Mendel established that genes can exist in two alternate forms (i.e., one dominant and other recessive). The dominant allele expresses the same phenotypic effect in heterozygotes as in homozygotes (i.e., Aa or AA are phenotypically similar). In contrast to the complete dominance exhibited by seven traits studied by Mendel in peas, sometimes the heterozygote shows partial or incomplete dominance (also called semidominance). The phenotype of heterozygote is different from that of both the homozygotes. The inheritance of flower color in a cross of red flower (RR) and white flower (rr) in snapdragon (Antirrhinum majus) parents produced F1 heterozygote with pink flower. The F2 generation of this cross expressed both the genotypic and the phenotypic ratios which were same, that is, 1 red (AA): 2 pink (Aa): 1 white (aa). This is a modified phenotypic ratio of 3 (dominant): 1 (recessive) and is due to incomplete dominance, which produced a blended phenotype (pink type). In some case the alleles that lack a dominant and recessive relationship is called codominant alleles (i.e., similar function) and the pattern in codominance. Both the alleles are active and expressed in the F1 heterozygote (e.g., allozymes).

2.2 Phenotypic variation in plants

The phenotype of a plant is the collective expression of the genotype in conjunction with the impacts of the environment on a plant’s observable characteristics. However, plants of the same genotype interacting with different environments may manifest as different phenotypes; on the other hand, plants may have the same phenotype but different genotypes. The latter may occur in situations where:

A gene or genes display incomplete dominance, for example, if a plant is heterozygous for a gene and as a result, its phenotype is intermediate to the two homozygous classes (see Appendix D for dominance, heterozygosity).

A gene or genes associated with a phenotype are changed by the presence of modifiers, suppressors, or other regulators produced by other genes (epistasis, epistatic interaction) such that the phenotype is altered (see Chapter 10, Section 3.1.3).

A certain combination of environmental conditions affects the relative expression of a gene or genes, or a combination of these factors affects the relative expressivity of a gene or genes, thus altering the degree to which the genotype manifests a phenotype.

Phenotypic variation is usually assessed in replicated field trials under a range of environmental conditions that are representative of where the crop is grown.

2.4.3 Mendelian Patterns of Inheritance in Humans

In modern genetics, Mendelian principles have been extended to more complex inherited traits than Mendel described. His peas have a relatively simple genetic basis—each character is determined by only one gene with two versions (or alleles), of which one is completely dominant. In fact, the relationship between genotype and phenotype for the majority of heritable traits is very complicated, but the laws of segregation and independent assortment can still be applied.

Recall from previous sections that pea characters, such as flower color, seed color, and seed shape, are completely controlled by their respective dominant alleles: P, Y, and R. In the case of flower color, Mendel’s heterozygous F1 offspring (Pp) all had purple flowers because the P allele shows a completely dominant effect over the recessive p allele. However, some heterozygous genotypes may cause incomplete dominance, meaning that the appearance of those heterozygous individuals has an intermediate phenotype between the phenotypes of the parent generation. Along with complete and incomplete dominance patterns of inheritance, there is co-dominance, in which a heterozygous individual expresses both phenotypes of the two alleles. Furthermore, the dominant effect of one allele on a phenotype is reflected by the mechanisms/pathways from genotype to phenotype, which does not imply the ability of one allele to mute another at the DNA level or the abundance of that allele in a population.

The relationship between genotype and phenotype may also be explained by multiple gene alleles (instead of only the two reported by Mendel); pleiotropy (i.e., one gene affects several phenotypic traits); epistasis (i.e., a gene at one locus interferes with the expression of a second gene at a different locus); and environment (e.g., nutrition affects human height and sun exposure affects skin color). Some characters, such as human height and skin color, result from an additive effect of two or more genes in a continuous fashion. These characters are said to be polygenic. Environment influences these polygenic traits, which are thus known as multifactorial.

Mendel’s discovery of the patterns of individual gene transmission from parents to offspring led to Mendelian models, which were developed and broadly used to explain the inheritance patterns of epistasis and polygenic characters. In the case of human inheritance, family pedigree analysis shows evidence that supports Mendelian patterns, with some following those patterns recessively (i.e., inherited from the heterozygous genotype) or dominantly (i.e., inherited from the dominant allele). Besides these simple Mendelian disorders, humans are more prone to diseases that have a multifactorial basis. For example, heart disease, diabetes, cancer, and many others diseases are the result of multiple genes, interactions between genes, and interactions between genes and the environment. Using technologies developed in recent years, studies have been conducted on the genetic and environmental components of multifactorial human traits. These will be discussed in later chapters.