Gregor Mendel's laws such as Law of Segregation and Law of Independent Assortment describe the basic genetic mechanisms. However, further studies by scientists have showed that the laws need to be expanded to account for more complex patterns of inheritance. Mendel's laws fail to adequately explain some phenomenon like mitochondrial gene inheritance and inheritance due to linkage between genes on the same chromosome and variation in gene expression and gene interactions can produce results other than phenotypic ratio predicted by Mendel's laws. Following are some phenomenons that are not explained by Mendel's laws:

Codominance and Incomplete dominance
Codominance is a phenomenon in which both the different alleles of a heterozygote are fully expressed . Most common example showing co-dominance is the ABO blood group system. A person having "A" allele and "B" allele will have a blood type "AB" because both "A" and "B" alleles are codominant with each other. A heterozygote, in which the dominant allele is only partially expressed, usually in an offspring with an intermediate phenotype, is called incomplete dominance. Technically, incomplete dominance can be termed as lack of dominance. Example for incomplete dominance can be found in Snapdragon plant. Pureline phenotypes of red (RR) and white (rr) give rise to Rr plant with pink flowers. In incomplete dominance the heterozygous plant carrying both alleles Rr will not be able to produce sufficient red pigment. This is because the dominant allele, which is responsible for the production of red pigments, is only partially expressed and therefore the flower has an appearance of pink. Similar situations exist in humans also. A child has higher probability of having a wavy hair if one parent of the child has curly hair and the other straight hair. This is because wavy hair is the intermediate between curly and straight hair.

Penetrance and Expressivity
The terms penetrance and expressivity are used to describe degrees of gene expression. Penetrance is the frequency or rate of a particular trait or condition to occur and is typically expressed as a percentage. Genetic penetrance provides estimation of the likelihood of expression of a particular disease causing gene that will result in disease. If only some individuals actually show the associated traits of a genetic disease while others do not even though they carry the same disease causing genes, then the disease can be said to have incomplete penetrance. Only some individuals with a mutation in the BRCA1 or BRCA2 gene will develop cancer during their lifetime but others do not and hence this case can be taken as an example for incomplete penetrance. In the case of every person having a disease causing gene, develops the associated trait or condition, then the disease is said to show hundred percent or complete penetrance. Huntington's disease is an example for complete penetrance. Expressivity refers to the extent of manifestation of the effects by an expressed gene in an organism. In other words, it refers to instances where a phenotype is expressed to a different degree between different individuals but all having same genotype. Marfan disease is an example of this phenomenon. In individuals suffering from Marfan disease, only some have long fingers and toes but others would have a full blown disease with defects of heart valves and aorta.

This is a situation where a single gene controls or influences multiple phenotypic traits or the phenotype expresses many symptoms with different subsets in various people. Such conditions are difficult to identify as genetic disorders as individual members of the family may express different subsets of the symptoms which may resemble different disorders altogether. Examples include autosomal dominant disorder- porphyria variegate, in which different combinations of the symptoms in family appear to be different unrelated diseases.

This the phenomenon in which a phenotype appears to be inherited but is actually caused by environment. Phenocopy can produce symptoms that either resemble a common hereditary disease or occur in certain members of a family there by mimicking inheritance pattern. As an example, frequent colds in an underweight person may match the symptoms of cystic fibrosis but may actually be suffering from malnutrition.

It is a situation in which one gene suppresses or influences the expression of another gene. One of the better known examples of epistasis was reported by Bateson and Punnett in 1905. They crossed two strains of inbred white sweet peas and obtained an F1 progeny that all had purple flowers. Another well know example is Bombay phenotype. It is a result of two interacting genes, "I" and "H".
Although Mendel's laws were basic in nature they initiated the era of genetics. Scientist after Mendel built upon his laws and as a outcome, organised genetics as a branch of science with tremendous commercial and scientific implications.

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