In this article we will discuss about the Mendel’s Principles. Also learn about its major limitations.

Actually, the basic principles of heredity was formulated by Mendel in 1866 but under- standability and confirmation of Mendel’s prin­ciples of heredity was clear to us after the rediscovery of Mendel’s laws in 1900 by three geneticists : Correns, de Vries and Tschermark independently.

Mendel put forward the basic law and principles of heredity from his experi­ment on pea-plant by applying the laws of probability. Probability is the ratio of a speci­fied event to total events. Limits of probability are from 0 if an event never occurs, to 1 if it always occurs. The laws of probability apply to the genetic mechanism as well as to other pro­cesses in which uncertainty exists.

The conclu­sions that Mendel drew from his experiment are as follows:

1. An individual receives from each parent a “unit factor” (nowadays, the gene) that can determine a trait of the individual.

2. When genes from the two parents cause different effects the individual will resem­ble one parent but not the other (Principle of Dominance).

3. In the formation of gametes, the parental gene for each characteristics segre­gates from the maternal gene for that same characteristic (Principle of Segregation).

4. The distribution of segregated genes in the gametes is random with respect to one another (Principle of Independent Assortment).

Although the Mendelian principles of heredity and the laws of probability apply in many cases—including human genetics— there are several exceptions to these princi­ples. Now the question is where could we apply the Mendelian principles in human heredity? Traits with a simple pattern of inheritance may sometimes be traced by applying Mendelian principle accurately enough to justify predictions concerning the likelihood of their expression in future chil­dren, if related individuals or those with a fam­ily history of such traits marry.

The first step in such an analysis is to determine whether the trait in question is behaving as a dominant or a recessive trait. McKusick (1978) lists 2,811 traits associated with specific genes and some of those impor­tant traits are listed in Table 3.1.

Unfortunately, most of the human traits are often affected by many genes and some are associated with the differential action of certain specific genes and have been identified as dom­inants or recessives in family group. Only recessive genes are difficult to follow because they may remain hidden by their dominant alle­les generation after generation.

In the absence of data to indicate which individuals are carri­ers, we may apply probability as the best tool for determining the likelihood of expression of a given recessive gene in a certain family.

Human Traits

All the principles of heredity which were advocated by Mendel have many exceptions and for that it is not always possible to apply the Mendelian principles for determining the inheritance pattern of all the traits or genes in human.

Limitations of Mendelian Principles:

The major limitations of Mendel’s principles are due to the following facts:

1. Dominance did not occur in every case of contrasting characters.

2. Blending type of inheritance may occur in many crosses.

3. Sometimes two dominants come together to form co-dominance e.g. in blood group inheritance (MN group).

4. Non-allelic gene interaction like epistasis, inhibiting factors, presence of comple­mentary gene and additive factors etc.

5. Linkages between different genes.

6. Non-disjunctions (non-separation of homologous chromosomes) of chromo­somes during the time of formation of gametes (i.e. during Meiosis).

7. Multiple factor (gene), that may or may not be allelic, for a single trait (e.g. skin color, height and weight of human) and their inheritance.

8. Pleiotropic effect of a gene i.e. a gene having influences on more than one trait.

9. Allelism i.e. the presence of a series of alternative genes at a given locus in a chromosome.

10. Differential penetrance power (complete or incomplete) and expressivity of a given gene in different time.

11. Presence of a lethal gene, i.e. an allele of a gene that renders inviable an organism.

12. Differential environmental influences on the genetic effect of a particular pheno- type.

13. Sex-linked genes i.e. the genes present in the ‘X’ or ‘Y’ chromosomes.

14. Position effects of the gene, i.e. a single gene may have differential activity or expression in different positions in a single chromosome.

15. Atavism, i.e. a gene comes to expression after a long period of non-expression because of the presence of different mask­ing effects.

16. Presence of phenocopy, i.e. the similarity between the phenotype of a particular genotype trait with the phenotype of another genotype. This phenomenon occurs due to the environmental influ­ences or hormonal influences.

Therefore, it appears that, due to so many limitations regarding the application of Mendelian principles to determine the inheri­tance of traits, Mendelian principle in itself tells us nothing in particular about the inheri­tance of traits in man. Actually the Mendelian principle is a general principle that applies to the gene (the unit of inheritance) not to the traits and is probably at the bottom of all inher­itance.

This is because chromosome mecha­nism of the germs cells in meiosis provides the biological basis of Mendelian principles of seg­regation and independent assortment. For example, in case of humans—who have 23 pairs of chromosomes and the probabilities that a gamete produced by an individual in the population will have any specific combination of chromosomes is which is in the order of one in eight million.

This corroborates the Mendelian segregation and independent assortment.

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