Mendel’s Principles of Inheritance

The principles of inheritance that arose from Mendel's careful experiments on the pea plants are the:

Law of dominance

Law of segregation and

Law of independent assortment.

LAW OF DOMINANCE 

Mendels this law states that, in a heterozygous condition, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect.

Complete dominance

In complete dominance, one of the alleles for a particular character completely dominates over the other and masks its phenotypic expression in the heterozygous condition. In this type of dominance relation, both homozygous dominant and heterozygous dominant have the same phenotype.

Example

Mendel crossed a true-breed purple flower plant with a true-bred white color flower plant. He observed that all the offspring in the  F1 generation were purple color flowering plants. He supposed the purple color was “dominant.” 

Symbols used to represent Mendel’s Laws

Alleles for particular characters were represented by the upper case letters (Dominant) and lower case letters (recessive). For example, the representation for the height would be:

• Dominant homozygous (tall):  TT
• Dominant heterozygous (tall):  Tt.
• Recessive trait (dwarf) :  tt

 Along with the letter presentation a + sign was also use for Wild type phenotype.

LAW OF SEGREGATION

Law of segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the diploid cells.

This law is applicable to the genes present on the same locus (alleles). Alleles are located on the homologous chromosomes. This law is also called as “law of Purity”.

Explanation

According to law of dominance or law of purity of gametes, in a heterozygous condition the dominance and recessive allele remain together without mixing with each other. The allele separate or segregates from each other during gametogenesis, so that each gamete receives only one allele, either it is dominant or recessive.

Law of segregation can be easily demonstrated in a monohybrid cross.

LAW OF INDEPENDENT ASORTMENT

Law of independent assortment states that two or more genes assort independently that is, each pair of alleles segregates independently of any other pair of alleles during gamete formation. This law applies only to genes (allele pairs) located on different chromosomes (non-homologous) or, alternatively, to genes that are very far apart on the same chromosome. This principle can be easily demonstrated in a dihybrid cross.

Dihybrid crosses

 It is a cross between the two individual that are hybrid (heterozygous) for two characters e.g. seed shape and seed color (Round Yellow Seed – RrYy)            

How genes get separated?

Independet assortment of genes occurs because of the presence of two different genes i.e. gene for seed color and gene color either on non-homologous chromosomes or due to crossing over (exchange of chromosomal segments in meiosis) in case the genes are present on homologous chromosomes. 

Limitations of Mendelian Principles:

A.      Mendelian principles cannot explain incomplete dominance, co-dominance, over-dominance, and other complex inheritance patterns.

B.      Mendelian principles can only be applied to diploid organisms.

C.      Genes on the same chromosomes could not be assorted independently until crossing over.

D.     X linked inheritance patterns may also vary from the Mendel’s inheritance patterns

Mendel’s predictions:

Mendel predicted that:

a.       Characters are controlled by some agents. He called these agents as factors or elements.

b.      Each parent has a pair of these factors for one character.

c.       Each parent inherits only one of these factors to their offspring (Assort independently during gamete formation).

d.      Factors retain their individuality from generation to generation.

 

Mendelian factors were later named as gene in 1900 by Johnson.