The following points highlight the seven main exceptions to the principle of dominance and paired factors. The exceptions are: 1. Incomplete Dominance 2. Co-Dominance 3. Multiple Alleles 4. Test Cross 5. Pleiotropy 6. Polygenic Inheritance 7. Complementary Genes.
Exception # 1. Incomplete Dominance:
It is a phenomenon in which phenotype of the F1 hybrid offspring does not resemble any of the parent, but it is intermediate between the expression of two alleles in their homozygous state.
It is not the blending inheritance because parental characters reappear in F2-generation. Here, the phenotypic ratio deviates from Mendel’s monohybrid ratio.
For example. In Snapdragon (Dog flower, Antirrhinum majus) and Four O’clock plant (Mirabilis jalapd), there are two types of pure breeding plants, red flowered and white flowered.
On crossing the two, F1-plants or hybrids possess pink flowers. And on selfing them, F2-generation has 1 red: (RR): 2 pink (Rr): 1 (rr) white flowered.
Pink flower colour is due to incomplete dominance of red flower trait over white flower trait.
Exception # 2. Co-Dominance:
It is the phenomenon in which two alleles express themselves independently when present together in an organism. In other words, it is the phenomenon in which offspring shows resemblance to both the parents.
For example, ABO blood group in humans.
ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene.
The gene I has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar, while allele i does not produce any sugar.
In humans, each person possesses any two of these three I gene alleles. IA and IB are completely dominant over i. When IB and i are present, only IB expresses (because i does not have any sugar).
Same is the case with IA and i. But when IA and IB are present together, they both express their own types of sugars, this is due to co-dominance. Therefore, the red blood cells have both A and B types of sugars. Since, there are three different types of alleles; there can be six different combinations.
Hence, a total of six different genotypes of the human ABO blood types present are given below in the table:
Exception # 3. Multiple Alleles:
These are multiple forms of a Mendelian factor or gene, which occur on the same gene, distributed in different organisms in the gene pool. The ABO blood grouping is a good example of multiple alleles. In this case, more than two, i.e., three alleles are present governing the same character. Multiple alleles can be found only when population studies are made.
Exception # 4. Test Cross:
This is a method devised by Mendel to determine the genotype of an organism. In this cross, the organism with an unknown dominant genotype is crossed with the recessive parent, instead of self-crossing.
For example, in a monohybrid cross between violet colour flower (W) and white colour flower (w), the F1 -hybrid was a violet colour flower.
If all the 1-progeny are of violet colour, then the dominant flower is homozygous and if the progenies are in 1: 1 ratio, then the dominant flower is heterozygous.
Exception # 5. Pleiotropy:
It is the phenomenon in which a single gene product may produce multiple or more than one phenotypic effect.
In garden pea, such a phenomenon has been observed in the following characters:
(a) Starch synthesis, size of starch grains and the shape of seeds is controlled by one gene.
(b) Flower colour and seed coat colour are also found to be controlled by the same gene.
A pleiotropic gene often has a more evident effect on one trait called the major effect and less evident effects on other traits called secondary effects. When a number of related changes are caused by a pleiotropic gene, the phenomenon is called syndrome.
Some examples of pleiotropy are:
i. Sickle-cell anaemia (an autosomal hereditary disorder) and. Phenylketonuria (PKU) is a defect in a single gene that codes for phenylalanine enzyme which results in multiple phenotypes with PKU, including mental retardation, eczema and pigment defects.
ii. In Drosophila white eye mutation leads to depigmentation in many other parts of the body, giving a pleiotropic effect.
iii. In transgenic organisms, the introduced gene can produce different effects depending on the where the gene has introgressed.
Exception # 6. Polygenic Inheritance:
It was given by Galton in 1833 and suggested that many instances of continues variations is heritable. In this case, traits are controlled by three or more genes and the graded phenotypes are due to the additive or cumulative effect of all the different genes of the trait, e.g., human skin colour, height and intelligence. Polygene is a gene where one dominant allele controls only a unit or partial quantitative expression of a trait.
It also takes into account the influence of environment and is called quantitative inheritance, as the character/phenotype can be quantified, like amount of pigment, intelligence in human beings and milk yield in animals have been found to be determined by many genes and their effects have been cumulative. These traits are called polygenic traits.
Let us take an example of Human Skin Colour to understand the phenomenon of polygenic inheritance:
Skin colour in humans is caused by a pigment called melanin. The quantity of melanin is due to three pairs of polygenes (A, B and C). If it is black or very dark (AABBCC) and white or very light (aabbcc) individuals marry each other the offspring or individuals of F1-generation show intermediate colour often called mulatto (AaBbCc).
A total of eight allele combinations is possible in the gametes forming 27 distinct genotypes distributed into 7 phenotypes i.e., 1 very dark, 6 dark, 15 fairly dark, 20 intermediate, 15 fairly light, 6 light and 1 very light. Therefore, a histogram prepared form the frequencies of various phenotypes shows a bell-shape curve (Fig. 5.7)
Exception # 7. Complementary Genes:
These genes complement the effect of each other to produce a phenotype so, they are called complementary genes.
For example, there are two varieties of sweet pea (Lathyrus odoratus), controlled independently by two different genes.
A cross between a purple (PPCC) and white (ppcc) flowered plants produces an F2. Here, the ratio between purple flowered and white flowered is 9: 7, which is different than Mendelian ratio, i.e., 9: 3: 3: 1.
