Inheritance, often known as Heredity, is the process of passing on traits from one generation to the next, either through gametes, sperm, and ova in sexual reproduction or by asexual reproductive bodies in asexual reproduction.
Heredity is the cause of similarities between individuals, while variation is the cause of differences between individuals.
Every cell arises from preexisting cells, so genetic material must be replicated and transferred from parent to offspring cell during each cell division.
Theories for Heredity
The theories to explain the phenomenon of inheritance can be categorized under the following headings:
- Vapor and fluid theories
- Preformation theories
- Particulate theories
Vapor and Fluid Theories
Pythagoras (500 B.C.) suggested that every organ in an animal’s body is said to release a certain form of vapor. A new person is created when these vapors combine.
Hippocrates (400 B.C.) believed all areas of an individual’s body contribute to reproduction, which is how traits are passed down to offspring.
Similarly, Aristotle (350 B.C.) hypothesized that the semen of men had a “vitalizing” effect and compared it to highly purified blood. He claimed that the father gives a new life its motion while the mother provides the inert material.
W. Harvey (1578–1657) hypothesized that semen solely has a vitalizing function and that all animals develop from eggs.
R. de Graaf (1641–1673) hypothesized that both parents should contribute to the heredity of the progeny after observing that the offspring would exhibit traits of both the father and the mother.
J. Swammerdam (1679) examined the growth of frogs and insects and proposed that the development of an organism is only the simple enlargement of a small, already-formed individual.
These theories, which advanced the concept of the presence of preformed embryos in the sex cells, are known as preformation theories.
The preformation theory was finally disproved by K.F. Wolff (1738-1794), who suggested that neither the egg nor the sperm had a structure like a homunculus but that the gametes contained an undifferentiated life substance capable of producing the organized organism after fertilization. Wolff, however, thought that unidentified vital forces were responsible for these tissues and organs’ development.
Maupertuis (1689–1759) postulated that each parent’s body produces tiny particles and the particles of both parents combine to create a new individual during sexual reproduction.
He theorized that in some cases, the male parent’s particles might predominate over the female parent’s, leading to the birth of a male individual. In contrast, the female parent’s particles predominate over the male parent’s particles, leading to the birth of a female individual.
He, therefore, put forth the idea of biparental inheritance via elementary particles.
Charles Darwin (1868) presented his pangenesis theory which stated that every component of an animal’s body produces several tiny particles known as gemmules. These gemmules are at first collected in the blood and later on are concentrated in the reproductive organs. When the animal reproduces into a new individual, these gemmules pass on to it, and it has the blending of both parents. By this mechanism, acquired characters would also be inherited because as the body parts changed, so did the pangenes or gemmules they produced.
However, the theory of pangenesis was disapproved by Galton (1823–1911) and Weismann (1835–1934). To explain heredity, Weismann proposed the germplasm theory. His theory stated that there are two types of cells in an organism’s body; somatic and reproductive. The reproductive cells create sperm and ova, whereas the somatic cells create the body and its many organ systems.
It was Gregor Johann Mendel who laid the foundation of our modern concept of the particulate theory. Through his well-known studies of the pea plant, he determined general rules for the inheritance of several distinct traits, including seed color. He made the accurate assumption that each trait is caused by a pair of inherited variables, now known as genes, and thus correctly understood the observed patterns of heredity.
He was able to observe the mathematical patterns of inheritance from one generation to the next generation. Mendel’s Laws of Heredity are typically expressed as follows:
- Law of Dominance: An organism with alternate forms of a gene will express the dominant form.
- Law of Segregation: A gene pair controls every inherited trait. Parental genes are arbitrarily distributed among the sex cells so that each sex cell contains just one of the pairs of genes. When sex cells combine to form embryos, offspring receive one genetic allele from each parent.
- Law of Independent Assortment: To prevent the inheritance of one feature from influencing the inheritance of another, the genes for various traits are arranged in separate groups.
However, he failed to explain the exact process by which these factors pass on to the sex cells.
By 1860, the sperm and egg were known to be the means of transmitting genetic information. Ernst Haeckel proposed that the nucleus is responsible for inheritance after observing that sperm was mostly composed of nuclear material.
Walter S. Sutton (1902) proposed the chromosome theory of heredity in which he postulated that the newly rediscovered Mendel’s hereditary factors were physically located on chromosomes. This idea combined the fields of genetics and cytology and offered a transmission mechanism to explain the behavior of Mendel’s components.
Despite reporting the first occurrence of linkage in a sweet pea in 1906, Bateson and Punnett failed to explain the linkage phenomena.
Johannsen (1909) developed the genotype-phenotype concept to differentiate between genetic and environmental changes. He proposed that a person’s phenotype represents the observable structural and functional characteristics formed by the interaction between genotype and environment. In contrast, the genotype of an individual represents the totality of heredity.
T.H. Morgan (1866–1945) proposed the linkage theory in 1911. He turned the chromosome theory of inheritance into the concept of genes being located in a linear array on each chromosome.
The age of molecular genetics began after the discovery of DNA structure in 1953, which revealed that DNA nucleotides are the basic unit of heredity and that a gene is made up of a collection of nucleotides.
Importance of Heredity
Understanding the mechanisms of heredity is key to studying biodiversity for at least three reasons.
- Evolutionary processes such as population adaptability, lineage diversification, and the emergence of new species all depend on genetic variation.
- Existing genetic variety among communities may reflect their recent past and signify potential future change.
- The diversity of molecular genetic markers can be used to determine the taxonomic status of certain populations and the relationships between living groups of animals.
- For biological fields of study like development, cytology, physiology, and morphology, having a basic understanding of how genes function is a prerequisite.
- Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Heredity, Genes, and DNA. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9944/
- Children resemble their parents. DNA Learning Center, Cold Spring Harbor Laboratory. Accessed from: http://www.dnaftb.org/1/bio.html. Accessed on: 01.09.2022
- Michael F. Antolin and William C. Black. (2007). Genes, Description of. In Encyclopedia of Biodiversity, Elsevier Inc., pg 1-11
Verma P.S. and Agarwal V.K. (2005). Genetics, Human Genetics and Eugenics, In Cell Biology, Genetics, Molecular Biology, Evolution and Ecology. Multi-color Edition. S. Chand & Company Ltd. Ram nagar, New Delhi, pg 3-9