The phenomenon where genetic characteristics are transmitted from ancestor to descendant through the genes is called Heredity. As a subject, it is tied closely to genetics, the area of biological study concerned with hereditary traits. The study of heritable traits helps scientists discern which are dominant and therefore is likely to be passed on from one parent to the next generation. On the other hand, a recessive trait will be passed on only if both parents possess it.
The 'factors' responsible for heredity and variations were first detected and analysed by Mendel. Subsequently many other scientists studied the pattern of their transmission from generation to generation. The term gene for the hereditary material was used for the first time in 1909 by Johannsen.
The experiments of Gregor Mendel on garden peas showed that the genes responsible for the characters of organisms were segregated or separated into different gametes at the time of gamete formation and these could independently recombine when the zygote was formed. Studies following the rediscovery of Mendel's work in the beginning of the 20th century revealed the nature of inheritance in living organisms. However, these studies provided no insight into the structure or molecular composition of genes.
Genes and Chromosomes
Shortly after the rediscovery of Mendel's laws, it was revealed that there are remarkable similarities between the behaviour of genes during their transmission from generation to generation and the behaviour of chromosomes during cell division. Mendel has postulated that the genes occur in pairs (allelomorphs) and the chromosomes also occur in pairs, one member of which is received from each parent. Mendel has also postulated that when the gametes are produced the paired genes separate and get redistributed to each gamete (Law Of Segregation). During meiosis the homologous pairs of chromosomes separate and only one member of each pair goes to a given gamete. According to Mendel, if the transmission of two pairs of contrasting characters is studied, the distribution of one pair of genes responsible for one pair of characters is independent of the distribution of the other pair (Law of Independent Assortment of Characters).
If we assume that hereditary factors (genes) for one trait, say seed shape, are located on one pair of chromosomes, and the genes for the other trait on another pair, then the independent assortment of chromosome pairs during meiosis will also result in independent assortment of characters. Such parallelism between the behaviour of genes and the behaviour of chromosomes was suggested by Sutton and Boveri (1901-1903), because there are many more traits in organisms than there are chromosomes, and chromosomes could not be identical to genes.
However, their studies led them to propose that chromosomes were bearers of the hereditary factors which determined the characters of individuals. Further studies on the association of chromosomes with inheritance of characters, by other scientists, confirmed the proposition made by Sutton and Boveri. Once it was firmly established that the nucleus and the chromosomes are responsible for heredity, further attempts were made to identify the hereditary material (genes) in chemical terms.
DNA, The Genetic Material
Chromosomes are composed of two types of macro-molecules namely proteins and nucleic acids. The nucleic acids are of two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). For many years, scientists disagreed as to which of these three macro-molecules carried the genetic information. However, studies between 1910 and 1952 showed that it is the DNA of the chromosomes that is responsible for the transmission of hereditary characters from generation to generation. DNA in the chromosomes also controls cellular metabolism and all other activities of living cells. The evidences in support of the theory that DNA is the genetic material come mainly from two phenomena occurring in bacteria: transformation and transduction.
In 1928, an English bacteriologist, Fred Griffith, conducted some experiments with the bacterium that causes pneumonia, Diplococcus pneumoniae (commonly known as Pneumococcus bacterium).
There are two main types of these bacteria, smooth (S) and rough (R). S type cells which have a capsule around each pair, are virulent and cause pneumonia. On the other hand, the R type bacteria which do not have capsules are avirulent and harmless. The S type bacteria when injected into mice cause pneumonia and subsequent death of the animal; whereas the R type cells when injected into mice are found to be harmless. Griffith found that the S type bacteria when killed by heat prior to injecting into mice did not cause the disease. However, when heat killed S type pneumococci were injected into mice along with avirulent R type bacteria, these mice suffered from pneumonia and ultimately died. Examination of the dead mice revealed that the death occurred due to the living S type pneumococci.
Griffith concluded from this experiment that the avirulent 'R' type bacteria had been transformed into the 'S' type, the capsulated variety. In other words the dead 'S' type bacteria had transmitted their virulence to the living 'R' type avirulent bacteria. It was shown, within a few years, that this kind of transformation could occur when the two types of bacteria are kept together in a culture medium. Later, a cell-free extract of the 'S' type cells was successfully used to transform 'R' type cells into 'S' type cells. These experiments showed that some component of the bacterial cell was responsible for the transformation. Griffith's experiments are summarised in this figure.
Diagrammatic summary of Griffith's experiments on transformation in Diplococcus
The cell extract from the bacteria had a variety of ingredients in it, including polysaccharides, proteins, DNA, RNA and lipids. By selectively destroying these ingredients one after the other, and testing the transforming activity of the remaining material, O.T. Avery, C. Macleod and McCarty (1944), were able to show that the active ingredient of the transforming principle was DNA. Further proof for the fact that DNA, and only DNA is the genetic material, came from another series of experiments with a bacteriophage (a virus that infects a bacterium).
Parent organisms pass traits to their offspring so there are often similar characteristics seen in both parent and offspring.
Inherited human traits include:
- Dimples or freckles,
- Naturally curly or straight hair,
- Attached or unattached earlobes, etc.
Rules for Inheritance of Traits
The rule of Inheritance of Traits are connected to both, the motherâ€™s and the fatherâ€™s contribution of the genetic material to the child. The DNA of the parents may influence the traits of the child. There will be two versions for every trait of the child, i.e. that of the mother and that of the father.