Principles of Inheritance and Variation

Introduction

The principles of inheritance and variations help us study genetics. Genetics is basically the study of genes or traits that parents pass on to their children. Inheritance and variation deal with changes in the DNA sequence. These principles have helped us understand complicated genetic makeup and cure many diseases. Gregor Johann Mendel introduced the concept of genetics and extensively researched it. That is why he is the father of genetics. 

Some Important Terminologies

Here are some important terms in the principles of inheritance and variation:

• Genetics: The term genetics was given by William Bateson in 1906. It is the branch of biology that deals with the study of heredity and variations
• Heredity: It is the transmission of genetic character from parent to their offspring or progeny
• Inheritance: It is the process by which characteristics or traits pass from one generation to another
• Variation: It is the property by which the progeny vary from their parents

Branches of Genetics 

• Molecular genetics
• Transmission
• Population genetics

Mendel’s Experiment

Monohybrid test cross: When we consider the inheritance of one character at a time in a cross, this is called monohybrid cross.The ratio of  1:2:1  known as dihybrid ratio.

Dihybrid Cross-  A cross made to study the inheritance of two pairs of contrasting traits.  Mendel selected traits for dihybrid cross for his experiment as follows.The ratio of 9: 3: 3: 1 known as dihybrid ratio.

Mendel’s Theory of Inheritance

Gregor Johann Mendel (1822-1884) started his historical experiments of heredity on pea (Pisum sativum) plant. Mendel’s experiment involved 4 steps as selection, hybridization, selfing and calculations. Mendel’s three principles of inheritance are

– Law of dominance

– Law of segregation or law of purity of gametes.

­- Law of independent assortment.

Exception of Dominance

 There are two exceptions of law of dominance-(i) Incomplete dominance (ii)   Codominance

Incomplete Dominance

According to Mendel’s law of dominance, dominant character must be present in the F1 generation. But in some organisms, the F1 generation is different from both parents. Both factors such as the dominant factor are unable to express their character completely, resulting in a different type of generation which is different from the both parents.

Codominance

In codominance both the gene expressed for a particular character in F1 hybrid progeny. There is no blending of characters, whereas both the characters are expressed equally.

Chromosomal Theory of Inheritance

Chromosome theory of inheritance was first proposed by W.S. Sutton & Boveri in 1902. This theory states that chromosomes are vehicles of hereditary information & expression as genes are present over them. Gametes Serve as the bridge between two successive generations.  Chromatin in the nucleus is associated with the cell division in the form of chromosomes.

Any type of deletion or addition in the chromosomes can cause structural and functional changes in living beings. Determination of sex in most of the animals and plants is affected by specific chromosomes. These  chromosomes are called sex chromosomes.

Linkage

Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation. This is due to their location on the same chromosomes. Linkage was first time seen by Bateson and Punnett in Lathyrus odoratus and gave a coupling and repulsion phenomenon. But they did not explain the phenomenon of linkage.Sex linkage was first discovered by Morgan in drosophila and coined the term linkage. He proposed the theory of linkage.

Sex Determination

Establishment of sex through differential development in an individual at an early stage of life, is called sex determination.

Sex Determination in Human

Human beings have 22 pairs of autosomes and one pair of sex chromosomes.

There occur a special gene on differential region of Y-chromosome of human, called Sry-gene (Sex determine region on Y chromosome). This gene forms a proteinaceous factor called TDF (testis determining factor).

Human Genetics

The study and analysis of human genetics is performed by many methods like pedigree analysis, statistical analysis and human karyotyping.

Pedigree Analysis

Study of ancestral history of the transmission of genetic characters from one generation to next, is called pedigree analysis, Dwarfism albinism, colour blindness, haemophilia etc. are genetically transmitted characters.

Types of Genetic Crosses

Some of the crosses used in the principles of inheritance and variation are as follows:

Back cross: Here, an F1 individual is crossed with any of its parents or other genetically similar individuals. The generation obtained from a back cross is all dominant. As a result, an individual’s genotype cannot be determined.

Test cross: It is a cross in which the F1 progeny are crossed with a recessive parent. This type of cross helps in determining the genotype of an individual.

Reciprocal cross: In this type of genetic cross, two parents are used in two experiments and replaced as male and female parents.

Conclusion

Mendel’s laws were valid explanations for genetic variations in many living organisms. He concluded that characters could be divided into expressed and latent traits. Through the notes on principles of inheritance and variation, one can determine the offspring’s genetic makeup. These laws help us identify the dominant or recessive nature of each allele and subsequently cure many diseases.

Inheritance and variations were best studied after the inventions of Mendel’s laws. His experiments proved to be unique because he tended to use mathematics to explain his observations in natural events. The use of maths made the conclusion more convincing.