Mendelian Genetics : Biology Blog

                                    Mendelian Genetics

Genetics is the branch of biology dealing in inheritance and variation of characters generation to generation.         

Gregor Johann Mendel -" The Father Of genetics "was born in 1822 in Heinzendorf, which was a part of Czechoslovakia. He was the inventor of Mendel's law,  a naturalist, a priest who began his genetic experiments on garden pea in 1856 in the garden at the monastery.

Selection of pea plant:  There are some reasons for choosing garden pea (Pisum sativum) for experiments by Mendel like –

1. Pea has many separate contrasting characters.
2. The life span of the pea plant is very short.
3. Flowers show self-pollination.
4. It is easy to artificially cross-pollinate the pea flowers, therefore, the hybrids produced were fertile.

Mendel’s Experiments

• Gregor Johann Mendel - the father of genetics proposed some laws of inheritance.
•          He used garden pea as his sample.
•          Large sampling size gave reliability to his unshakable data.
•          Garden pea plants possessed certain completely opposite traits. Example − tall and dwarf plants

         He worked on the following seven traits of garden pea: 

Working method:

Mendel’s achievement was due to his accurate planning and course of action  –

• He studied only one character at a time.
• He used all available techniques to avoid cross-pollination by undesirable pollen grains.
• He applied mathematics and statistics to analyze the results obtained by him.

Mendel’s Laws of Inheritance

Based on his experiments, Mendel proposed three laws or principles of inheritance:

• Law of Dominance
  
  
• Law of Segregation
  
  
• Law of Independent Assortment

·         Law of Dominance

Recessive alleles will always be hidden by dominant alleles. Therefore, a cross made between a homozygous dominant and a homozygous recessive consequently dominant phenotype expressed.

Law of Segregation

·         When  the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors.

·         In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions.  

 Law of Independent assortment-

·         The law of independent assortment states that when the inheritance of two or more genes occur at one time, their distribution in the gametes and in the progeny of subsequent generations are independent of each other.  To prove this, he did a dihybrid cross. In the dihybrid cross, we consider two characters. (e.g., seed color and seed shape)

·         Yellow color and round shape is dominant over green color and wrinkled shape.

Incomplete Dominance

·         In incomplete dominance, F1 generation has the phenotype that does not resemble either of the two parents but is a mixture of the two.

·         Example − Flower color in dog flower (snapdragon), where:

·         RR − Red flowers

·         rr − White flowers

·         Rr − Pink flowers  

·         Here, the genotypic ratio remains the same as in Mendelian crosses, but phenotypic ratio changes since complete dominance is not shown by R (hence, incomplete dominance).

·         Phenotypic Ratio − 1:2:1 that denotes Red: Pink: White

·         Genotypic Ratio − 1:2:1 that denotes RR: Rr: rr

Multiple Allelism / Codominance:
When a gene exists in more than two allelic forms, it shows the phenomenon of multiple allelism.  A well-known example is the inheritance of A, B, and O blood groups in human beings.  The gene for blood group occurs in three allelic forms  IA, IB and i.  A person carries any two of these alleles.  The gene IA produces glycoprotein (sugar) A and the blood group is A.  The gene IB produces glycoprotein B and the blood group is B.  The gene ‘i’ is unable to produce any glycoprotein and so the person homozygous for it, has O group blood. The genes IA and  IB are dominant over ‘i’.   When IA and  IB are present together, both are equally dominant and produce glycoproteins A and B and the blood group is AB.  They are called codominant alleles.  

Epistasis – Epistasis describes the situation in which a gene masks the genotypic effect of another gene. 

complementary gene

one of two or more genes that when present together produce effects qualitatively distinct from the separate effect of any one of them.  

Supplementary genes

one gene producing a characteristic and the second as only being able to ‘supplement’ this characteristic. 

Polygenic Trait

Polygenic traits are controlled by two or more than two genes (usually by many different genes) at different loci on different  chromosomes. For example, there are two major eye color genes, but at least 14 other genes that play roles in determining a person’s exact eye color.

Pleiotropy - a gene that affects more than one phenotype. Pleiotropic alleles are responsible for  the multiple symptoms of hereditary diseases  such as  cystic fibrosis, sickle cell anemia . 

Lethal gene - a gene that is capable of causing the death of an organism, usually during the development of the embryo. For Example sickle cell anemia disease.

           

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