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According to Mendel’s law phenotypical characteristics would be determined by pair of factors (alleles) that separate independently in gametes. What are the main types of inheritances that are exceptions to Mendel’s rules?

There are many types of inheritance that do not follow the mendelian pattern. Notable among them are: multiple alleles, gene interactions (complementary genes, epistasis and quantitative, or polygenic, inheritance), linkage with or without crossing over and sex-linked inheritance.

Pleiotropy, lacking of dominance and lethal genes do not fit as variations of inheritance since genes can have these behavior and in the same time obey mendelian laws.

Mutations and aneuploidies are abnormalities that also altere the mendelian pattern of inheritance as well the mitochondrial inheritance (passage of mitochondrial DNA from the mother through the cytoplasm of the egg cell to the offspring).

What is the genetic condition in which the heterozygous individual has different phenotype from the homozygous individuals?

This conditions is called lack of dominance and it can happen in two manners: incomplete dominance or codominance.

In incomplete dominance the heterozygous presents an intermediate phenotype between the two types of homozygous, as in sickle cell anemia in which the heterozygous produces a part of sick red blood cells and another part of normal red blood cells. Codominance occurs, for example, in the genetic determination of the MN blood group system, in which the heterozygous has a phenotype totally different from the homozygous, not being an intermediate form.

What is pleiotropy?

Pleiotropy (or pliotropy) is the phenomenon in which a single gene conditions several different phenotypical traits.

Some phenotypical traits may be sensitive to pleiotropic effects (for example, inhibition) of other genes, even when conditioned by a pair of alleles in simple dominance. In these cases a mix situation of pleiotropy and gene interaction is characterized.