Incomplete dominance

SIGNIFICANCE: In most allele pairs, one allele is dominant and the other recessive; however, other relationships can occur. In incomplete dominance, one allele can only partly dominate or mask the other. Some very important human genes, such as the genes for pigmentation and height, show incomplete dominance of alleles.

Incomplete vs. Complete Dominance

Diploid organisms have two copies of each gene locus and thus two alleles at each locus. Each locus can have either a (two of the same alleles, such as AA, aa, or a⁺a) or a heterozygous genotype (two different alleles, such as Aa or a⁺a). The phenotype of an organism that is homozygous for a particular gene is usually easy to predict. If a pea plant has two tall alleles of the height locus, the plant is tall; if a plant has two dwarf alleles of the height locus, it is small. The phenotype of a heterozygous individual may be harder to predict. In most circumstances, one of the alleles (the dominant) is able to mask or cover the other (the recessive). The phenotype is determined by the dominant allele, so a heterozygous pea plant, with one tall and one dwarf allele, will be tall. When Gregor Mendel delivered the results of his pea-plant experiments before the Natural Sciences Society in 1865 and published them in 1866, he reported one dominant and one recessive allele for each gene he had studied. Later researchers, starting with Carl Correns in the early 1900s, discovered alleles that did not follow this pattern.

When a red snapdragon or four-o’clock plant is crossed with a white snapdragon or four-o’clock, the offspring are neither red nor white. Instead, the progeny of this cross are pink. Similarly, when a chinchilla (gray) rabbit is crossed with an albino rabbit, the progeny are neither chinchilla nor albino but an intermediate shade called light chinchilla. This phenomenon is known as incomplete dominance, partial dominance, or semidominance.

If the flower-color locus of peas is compared with the flower-color locus of snapdragons, the differences and similarities can be seen. The two alleles in peas can be designated W for the purple allele and w for the white allele. Peas that are WW are purple, and peas that are ww are white. Heterozygous peas are Ww and appear purple. In other words, as long as one dominant allele is present, enough purple pigment is made to make the plant’s flower color phenotype purple. In snapdragons, R is the red allele and r is the white allele. Homozygous RR plants have red flowers and rr plants have white flowers. The heterozygous Rr plants have the same kind of red pigment as the RR plants but not enough to make the color red. Instead, the less pigmented red flower is designated as pink. Because neither allele shows complete dominance, other symbols are sometimes used. The red allele might be called C^R or C₁, while the white allele might be called C^W or C₂.

The Enzymatic Mechanism of Incomplete Dominance

To understand why occurs, metabolic pathways and the role of enzymes must be understood. Enzymes are proteins that are able to increase the rate of chemical reactions in cells without the enzymes themselves being altered. Thus, an enzyme can be used over and over again to speed up a particular reaction. Each different chemical reaction in a cell needs its own enzyme. Each enzyme is composed of one or more polypeptides, each of which is coded by a gene. Looking again at flower color in peas, the W allele codes for an enzyme in the for production of purple pigment. Whenever a W allele is present, this enzyme is also present. The w allele has been changed (mutated) in some way so that it no longer codes for a functional enzyme. Thus ww plants have no functional enzyme and cannot produce any purple pigment. Since many biochemicals such as fibrous polysaccharides and proteins found in plants are opaque white, the color of a ww flower is white by default. In a Ww plant, there is only one copy of the allele for a functional enzyme. Since enzymes can be used over and over again, one copy of the functional allele produces sufficient enzyme to make enough pigment for the flower to appear purple. In snapdragons the R allele, like the W allele, codes for a functional enzyme, while the r allele does not. The difference is in the enzyme coded by the R allele. The snapdragon enzyme is not very efficient, which leads to a deficiency in the amount of red pigment. Flowers with the reduced amount of red pigment appear pink.

Phenotypic Ratios

Phenotypic ratios in the progeny from controlled crosses are also different than for simple Mendelian traits. For Mendelian traits, crossing two heterozygous individuals will produce the following results: Ww × Ww → ¼WW + ½Ww + ¼ ww. Since both WW and Ww look the same, the ¼WW and the ½Ww can be added together to give ¾ purple. In other words, when two heterozygotes are crossed, the most common result is to have ¾ of the progeny look like the dominant and ¼ look like the recessive—the standard 3:1 ratio. With incomplete dominance, each genotype has its own phenotype, so when two heterozygotes are crossed (for example, Rr × Rr), ¼ of the progeny will be RR and look like the dominant (in this case red), ¼ will be rrand look like the recessive (in this case white), but ½ will be Rr and have an intermediate appearance (in this case pink)—a 1:2:1 ratio.

Codominance

One type of inheritance that can be confused with incomplete dominance is codominance. In codominance, both alleles in a are expressed simultaneously. Good examples are the A and B alleles of the human ABO blood system. ABO refers to chemicals, in this case short chains of sugars called antigens, that can be found on the surfaces of cells. Blood classified as A has A antigens on the surface, B blood has B antigens, and AB blood has both A and B antigens. (O blood has neither A nor B antigens on the surface.)

Genetically, individuals that are homozygous for the A allele, IAIA, have A antigens on their cells and are classified as type A. Those homozygous for the B allele, IBIB, have B antigens and are classified as type B. Heterozygotes for these alleles, IAIB, have both A and B antigens and are classified as type AB. This is called because both alleles are able to produce enzymes that function. When both enzymes are present, as in the heterozygous IAIB individual, both antigens will be formed. The progeny ratios are the same for codominance and incomplete dominance, because each genotype has its own phenotype.

Whether an allele is called completely dominant, incompletely dominant, or codominant often depends on how the observer looks at the phenotype. Consider two alleles of the hemoglobin gene: HA (which codes for normal hemoglobin) and HS (which codes for sickle-cell hemoglobin). To the casual observer, both HAHA homozygotes and HAHS heterozygotes have normal-appearing blood. Only the HSHS shows the sickling of blood cells that is characteristic of the disease. Thus HA is dominant to HS. Another observer, however, may note that under conditions of oxygen deprivation, the blood of heterozygotes does sickle. This looks like incomplete dominance. The phenotype is intermediate between never sickling, as seen in the normal homozygote, and frequently sickling, as seen in the HSHS homozygote. A third way of observing, however, would be to look at the hemoglobin itself. In normal homozygotes, all hemoglobin is normal. In HSHS homozygotes, all hemoglobin is abnormal. In the heterozygote, both normal and abnormal hemoglobin is present; thus, the alleles are codominant.

Incomplete Dominance and Polygenes

In humans and many other organisms, single characteristics are often under the genetic control of several genes. Many times these genes function in an additive manner so that a characteristic such as height is not determined by a single height gene with just two possible alternatives, as in tall and dwarf peas. There can be any number of these genes that determine the expression of a single characteristic, and very often the alleles of these genes show incomplete dominance.

Suppose one gene with an incompletely dominant allele determined height. Three genotypes of height could exist: HH, which codes for the maximum height possible (100 percent above the minimum height), Hh, which codes for 50 percent above the minimum height, and hh, which codes for the minimum height. If two height genes existed, there would be five possible heights: AABB (maximum height); AaBB or AABb (75 percent above minimum); AAbb, AaBb, or aaBB (50 percent above minimum); Aabb or aaBb (25 percent above minimum); and aabb (minimum). If there were five genes involved in height, there would be aabbccddee individuals with minimum height; Aabbccddee, aaBbccddee, and other individuals having genotypes with only one of the incompletely dominant alleles at 10 percent above the minimum; AAbbccddee, aaBbccDdee, and other individuals with two incompletely dominant alleles at 20 percent above the minimum; all the way up to AABBCCDDEE individuals that show the maximum (100 percent above the minimum) height. The greater the number of genes with incompletely dominant alleles that affect a phenotype, the more the distribution of phenotypes begins to look like a continuous distribution. Human skin, hair, and eye pigmentation phenotypes are also determined by the additive effects of several genes with incompletely dominant alleles.

Incomplete Dominance and Sex Linkage

In many organisms, sex is determined by the presence of a particular combination of sex chromosomes. Human females, for example, have two of the same kind of sex chromosomes, called X chromosomes, so that all normal human females have the XX genotype. Human males have two different sex chromosomes; thus, all normal human males have the XY genotype. The same situation is also seen in the fruit fly Drosophila melanogaster. When genes with incompletely dominant alleles are located on the X chromosome, only the female with her two X chromosomes can show incomplete dominance. The apricot (wa) and white (w) alleles of the eye color gene in D. melanogaster are on the X chromosome, and wa is incompletely dominant to w. Male flies can have either of two genotypes, aY or wY, and appear apricot or white, respectively. Females have three possible genotypes: wawa, waw, and ww. The first is apricot and the third is white, but the second genotype, waw, is an intermediate shade often called light apricot.

In birds and other organisms in which the male has two of the same kind of sex chromosomes and the female has the two different sex chromosomes, only the male can show incomplete dominance. A type of codominance can also be seen in genes that are sex linked. In domestic cats, an orange gene exists on the X chromosome. The alleles are orange (X^O) and not orange (X⁺). Male cats can be either black (or any color other than orange, depending on other genes that influence coat color) when they are X⁺Y, or they can be orange (or light orange) when they are X^OY. Females show those same colors when they are homozygous (X⁺X⁺ or X^OX^O) but show a tortoiseshell (or calico) pattern of both orange and not-orange hairs when they are X⁺X^O.

Key Terms

• alleleone of the alternative forms of a gene
• codominancethe simultaneous expression of two different (heterozygous) alleles for a trait
• complete dominanceexpression of an allele for a trait in an individual that is heterozygous for that trait, determining the phenotype of the individual
• heterozygoushaving two different alleles at a gene locus, often symbolized Aa or a⁺a
• homozygoushaving two of the same alleles at a gene locus, often symbolized AA, aa, or a⁺a⁺
• phenotypethe expression of a genotype, as observed in the outward appearance or biochemical characteristics of an organism
• recessive traita genetically determined trait that is expressed only if an organism receives the gene for the trait from both parents

Bibliography

Alexander, Denis R. The Language of Genetics. West Conshohocken: Templeton, 2011. Digital file. Templeton Science and Religion.

Birchler, James A. "Heterosis: The Genetic Basis of Hybrid Vigour." Nature Plants 1.3 (2015): n. pag. Nature Plants. Nature Publishing, 3 Mar. 2015. Web.20 Jan. 2016.

Grant, V. Genetics of Flowering Plants. New York: Columbia UP, 1975. Print.

Korf, Bruce R., and Mira B. Irons. Human Genetics and Genomics. Chichester: Wiley, 2013. Print.

Lewis, Ricki. Human Genetics: Concepts and Applications. 11th ed. Dubuque: McGraw, 2014. Print.

Mielke, James H., Lyle W. Konigsberg, and John Relethford. Human Biological Variation. 2nd ed. New York: Oxford UP, 2011. Print.

Nolte, D. J. “The Eye-Pigmentary System of Drosophila.” Heredity 13 (1959): 233–241. Print.

Pritchard, D. J., and Bruce R. Korf. Medical Genetics at a Glance. 3rd ed. Chichester: Wiley, 2013. Print.

Ringo, John. “Genes, Environment, and Interactions.” Fundamental Genetics. New York: Cambridge UP, 2004. Print.

Searle, A. G. Comparative Genetics of Coat Color in Mammals. New York: Academic, 1968. Print.

Snustad, D. Peter, and Michael J. Simmons. “Incomplete and Complete Dominance.” Principles of Genetics. 6th ed. Hoboken: Wiley, 2012. Print.

Strom, Susan, et al. "Clarifying Mendelian vs Non-Mendelian Inheritance." Genetics, vol. 227, no. 3, July 2024, doi.org/10.1093/genetics/iyae078. Accessed 4 Sept. 2024.

Thomas, Alison. "Extensions to Monohybrid Inheritance." Introducing Genetics. New York: Garland, 2015. 21–40. Print.

Yoshida, A. “Biochemical Genetics of the Human Blood Group ABO System.” American Journal of Genetics 34.1 (1982): 1–14. Print.