Inheritance and variation are fundamental concepts in the study of genetics, a branch of biology that explores how traits are transmitted from one generation to the next. Inheritance is the process by which characters are passed on from parent to progeny; it is the basis of heredity and ensures the continuity of genetic information. Variation, on the other hand, is the degree by which progeny differ from their parents, contributing to the diversity observed within a species.
The foundation of these principles was laid by Gregor Mendel, whose pioneering work on pea plants gave rise to the laws of inheritance. Mendel’s Law of Segregation and Law of Independent Assortment describe how alleles (gene variants) segregate and assort independently during gamete formation, ensuring that genetic combinations vary from one generation to the next.
Variation plays a critical role in nature, contributing to the survival and evolution of species. For instance, while wild cows exhibit wide genetic diversity, we also have well-known Indian breeds like the Sahiwal cows in Punjab, which are prized for their adaptability and milk production. This natural variation allows selective breeding to improve specific traits beneficial for agriculture, such as milk yield, disease resistance, or physical strength.
Key Concepts
The term genetics was first introduced by British biologist William Bateson, marking the beginning of a scientific discipline that studies the mechanisms of heredity and variation. Genetics is the collective study of how traits are passed from one generation to another (heredity) and the differences observed within species (variation).
- Population: In genetics, a population is not simply a large group of individuals, but a group consisting of members of the same species. A population has specific characteristics such as a defined lifespan, birth and death rates, a balanced ratio of sexes, and a particular age structure. These factors are crucial in studying genetic variation and evolution over time.
- Heredity: The transmission of genetic traits from parent to offspring is referred to as heredity. This process ensures that offspring inherit genetic material from their parents, resulting in similarities in characteristics.
- Inheritance: Inheritance refers to the pattern or mechanism through which heredity operates. It is the basis of how traits, such as eye color or blood type, are passed down from generation to generation.
- Variation: Even within the same species, no two individuals are exactly alike. The differences or dissimilarities between individuals of the same species are known as variations. These variations can be influenced by genetic mutations, recombination during reproduction, or environmental factors and are essential for the survival and adaptation of species.
These concepts form the foundation of the study of genetics, which not only helps us understand the biological inheritance of traits but also plays a crucial role in fields like medicine, agriculture, and evolutionary biology.
Key Figures in Genetics and Their Contributions
The field of genetics has been shaped by the work of several pioneering scientists, each of whom contributed foundational knowledge that has advanced our understanding of heredity, variation, and genetic disorders. Below are some of the key figures and their contributions:
- Father of Genetics – Gregor Johann Mendel: Mendel is known for his groundbreaking experiments on pea plants, through which he formulated the basic laws of inheritance, such as the Law of Segregation and the Law of Independent Assortment. His work laid the foundation for modern genetics, even though it was not widely recognized during his lifetime.
- Father of Modern Genetics – William Bateson: William Bateson coined the term “genetics” and played a vital role in promoting Mendel’s work. He helped establish the study of heredity as a scientific discipline, thus earning the title of the Father of Modern Genetics.
- Father of Experimental Genetics – Thomas Hunt Morgan: Morgan conducted significant experiments on Drosophila melanogaster (fruit flies), which led to the discovery of key genetic concepts such as linkage, sex linkage, crossing over, crisscross inheritance, and the creation of the first linkage map or genetic map. His work demonstrated that genes are located on chromosomes, and his experiments with Drosophila became a cornerstone in experimental genetics.
- Father of Human Genetics and Biochemical Genetics – Archibald Garrod: Garrod was a pioneer in understanding human metabolic disorders. He discovered Alkaptonuria (commonly known as black urine disease), which was the first recognized human metabolic disorder. In this condition, the patient lacks the enzyme homogentisic acid oxidase, leading to the buildup of homogentisic acid in the body. Garrod also introduced the concept of “one mutant gene – one metabolic block,” linking genetic mutations to metabolic dysfunction, an idea that paved the way for the study of biochemical genetics.
These scientists and their contributions have laid the foundation for modern genetics, helping to unravel the complexities of heredity, genetic disorders, and the biochemical processes underlying genetic traits. Their work continues to influence current research in medicine, agriculture, and evolutionary biology.
Genetics Related Terms
Factors: Unit of heredity which is responsible for inheritance and appearance of characters. These factors were referred to as genes by Johannsen (1909). Mendel used the term “element” or “factor“.
Morgan first used symbols to represent the factor. Dominant factors are represented by capital letters while recessive factors are by small letters.
Allele: Alternative forms of a gene that are located in the same position (loci) or the pair of genes on the homologous chromosome are called Alleles.
The term allele was coined by Bateson.
Characters: The morphological and physiological expression of an organism is called characters.
Examples – Plant height and flower color.
Trait: An alternative form of character is called a trait.
Example – Plant height (Dwarf, Tall)
Gene/Factor/Element: It is the basic unit of inheritance that is responsible for the transfer of characters from one generation to another generation.
Dominant: It is a character that does not allow the expression of contrasting characters in a hybrid.
Example – In a heterozygous tall plant (Tt) here, the “T” gene is dominant.
Recessive: It is a gene whose expression is suppressed by the dominant allele.
Example – In heterozygous tall plant (Tt) here, the “t” gene is recessive
Homozygous: A zygote is formed by the fusion of two gametes having identical factors called homozygotes and the organism developed from this zygote is called homozygous.
Example – TT, RR, tt
Heterozygous: A zygote is formed by the fission of two different types of gamete carrying different factors is called heterozygote (Tt, Rr), and individually developed from such zygote is called heterozygous.
The term homozygous and heterozygous is coined by Bateson.
Hemizygous: If an individual contains only one gene of a pair then the individual is said to be Hemizygous. The male individual is always Hemizygous for the sex-linked genes.
Example – Heterogametic organism (Like in man having X/Y chromosome)
Pure Breed: It is a homozygous individual having the same genes (both recessive or both dominant) for a character on its pairs of homologous chromosomes.
Phenotype: It is the external and morphological appearance of an organism for a particular character.
Genotype: The genetic constitution or genetic make-up of an organism for a particular character.
Genotype and phenotype terms were coined by Johannsen.
Phenocopy: If different genotypes are placed in different environmental conditions then they produce the same phenotype. Then these genotypes are said to be Phenocopy of each other.
Hybrid vigor or Heterosis: The superiority of offspring over its parent is called Hybrid vigor and it develops due to Heterozygosity.
- Hybrid vigor can be maintained for a long time in vegetatively propagated crops.
- Hybrid vigor can be lost by inbreeding (selfing) because inbreeding induces Homozygosity in offspring.
- Loss of Hybrid vigor due to inbreeding is called inbreeding depression.
Eugenics: Improvement of the human beings by the use of the principle of inheritance is called Eugenics, Like restrictions in mating two disease-carrier individuals.
Euthenics: The improvement of human beings by improving their environmental conditions is called Euthenics.
Euphenics: The improvement of human beings by the use of genetic engineering and medical science is called Euphenics.
Cross-Pollination: In this type of pollination heterozygosity increases and hybrid vigor increases.
Self-Pollination or Inbreeding: In this type of pollination heterozygosity decreases and hybrid vigor decreases, it’s also called inbreeding depression.
Do you know how the hybrid vigor is maintained?
Hybrid vigor is maintained by vegetative propagation.
Forward Genetics: Forward genetics is an approach based on the molecular genetics of determining the genetic basis that is responsible for a phenotype. In this genetic bases are responsible for a particular phenotype. It is studied from phenotype to genotype.
Example – Mendelian genetics
Reverse Genetics: It is experimental molecular genetics that enables researchers to elucidate gene function by analyzing the changes made to phenotypes (of cells or in an organism) caused by genetic engineering in a specific nucleic acid sequence (within a DNA or RNA).
Example – Genomics (the study of the genome)