Complete dominance (Mendelian) In complete dominance, the effect of one allele in a heterozygous genotype completely masks the effect of the other. The allele that masks are considered
dominant to the other allele, and the masked allele is considered
recessive. When we only look at one trait determined by one pair of genes, we call it
monohybrid inheritance. If the crossing is done between parents (P-generation, F0-generation) who are homozygote dominant and homozygote recessive, the offspring (F1-generation) will always have the heterozygote genotype and always present the phenotype associated with the dominant gene. However, if the F1-generation is further crossed with the F1-generation (heterozygote crossed with heterozygote) the offspring (F2-generation) will present the phenotype associated with the dominant gene ¾ times. Consider now the cross between parents (P-generation) of genotypes homozygote dominant and recessive, respectively. The offspring (F1-generation) will always heterozygous and present the phenotype associated with the dominant allele variant.) occurs when the phenotype of the heterozygous genotype is distinct from and often intermediate to the phenotypes of the homozygous genotypes. The phenotypic result often appears as a blended form of characteristics in the heterozygous state. For example, the
snapdragon flower color is homozygous for either red or white. When the red homozygous flower is paired with the white homozygous flower, the result yields a pink snapdragon flower. The pink snapdragon is the result of incomplete dominance. A similar type of incomplete dominance is found in the
four o'clock plant wherein pink color is produced when true-bred parents of white and red flowers are crossed. In
quantitative genetics, where phenotypes are measured and treated numerically, if a heterozygote's phenotype is exactly between (numerically) that of the two homozygotes, the phenotype is said to exhibit
no dominance at all, i.e. dominance exists only when the heterozygote's phenotype measure lies closer to one homozygote than the other. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Pink:White).
Co-dominance (non-Mendelian) in humans show co-dominance, but the O type is recessive to A and B. Co-dominance occurs when the contributions of both alleles are visible in the phenotype and neither allele masks another. For example, in the
ABO blood group system, chemical modifications to a
glycoprotein (the H antigen) on the surfaces of blood cells are controlled by three alleles, two of which are co-dominant to each other (
IA,
IB) and dominant over the recessive
i at the
ABO locus. The
IA and
IB alleles produce different modifications. The enzyme coded for by
IA adds an N-acetylgalactosamine to a membrane-bound H antigen. The
IB enzyme adds a galactose. The
i allele produces no modification. Thus the
IA and
IB alleles are each dominant to
i (
IAIA and
IAi individuals both have type A blood, and
IBIB and
IBi individuals both have type B blood), but
IAIB individuals have both modifications on their blood cells and thus have type AB blood, so the
IA and
IB alleles are said to be co-dominant. Another example occurs at the locus for the
beta-globin component of
hemoglobin, where the three molecular phenotypes of
HbA/HbA,
HbA/HbS, and
HbS/HbS are all distinguishable by
protein electrophoresis. (The medical condition produced by the heterozygous genotype is called
sickle-cell trait and is a milder condition distinguishable from
sickle-cell anemia, thus the alleles show
incomplete dominance concerning anemia, see above). For most gene loci at the molecular level, both alleles are expressed co-dominantly, because both are
transcribed into
RNA. . A white bull (WW) mates with a red cow (RR), and their offspring exhibit co-dominance expressing white and red hairs. Co-dominance, where allelic products co-exist in the phenotype, is different from incomplete dominance, where the quantitative interaction of allele products produces an intermediate phenotype. For example, in co-dominance, a red homozygous flower and a white homozygous flower will produce offspring that have red and white spots. When plants of the F1 generation are self-pollinated, the phenotypic and genotypic ratio of the F2 generation will be 1:2:1 (Red:Spotted:White). These ratios are the same as those for incomplete dominance. Again, this classical terminology is inappropriate – in reality, such cases should not be said to exhibit dominance at all. ==Relationship to other genetic concepts==