• Heredity : It means transmission of features or characters from one generation to another or from parents to offspring through their genes, for example, dogs reproduces to give birth to pups while cats will produce kittens. Heredity information is present in the fertilized egg or zygote.
  • Variation : Small differences among the members of a population. It is responsible for the diversity in a population. For example, shape of ear lobes, colour of eyes, hair colour, etc., in human beings.

  • Genetics : The branch of biology dealing with the study of heredity and variation, i.e., science and heredity.
  • Character/trait : Any structural, functional attribute, behavioural trait, or other characteristic of an organism.
  • Pure line : It is a strain of genetically pure true breeding individuals who have been derived from a single self-fertilized homozygous ancestor or identical homozygous ancestors.
  • Hybrid : In evolutionary biology, a cross between two species. In genetics, a cross between two genetic types. The organism produces after crossing two genetically different individuals. The term is synonym with heterozygote or heterozygous individual. Depending upon the number of characters in which parents differ from each other, there may be monohybrids (one character), dihybrids (two characters), trihybrids, polyhybrids, etc.
  • Mendelian factor : Traits in an individual are controlled by a pair of factors. Today factors are equated with genes. Factors can have two forms (alleles): dominant or recessive.
  • Allele : There is one pair of alleles, which can express itself whether present in homozygous or heterozygous state. Upper or capital letter is conventionally used to designate a dominant gene, e.g., T (tallness in pea), R (round seed in pea), A (axial flower in pea), Y (yellow seed in pea).
  • Gregor Johann Mendel (1822-1884) :  He is known as the Father of Genetics. He worked on sweet pea plant (Pisum sativum). This plant shows several pairs with contrasting features. Mendel worked with only seven pairs of varieties. All the selected varieties were pure lines, i.e., they were pure and bred true or gave offspring resembling the parents. Mendel was also the first to hypothesize independent assortment, the distinction between dominant and recessive traits. Gregor Mendel, who studies the segregation of heritable traits in pea plants observed that the flowers of each pea plant were either purple or white – and never an intermediate between the two colours. These different, discrete versions of the same gene are called alleles.  In the case of pea plants, each organism has two alleles of each gene, and the plants inherit one allele from each parent. When organisms are heterozygous, often one allele is called dominant as its qualities dominate the phenotype of the organisms, while the other allele is called recessive as its qualities recede and are not observed. Dominant alleles are often abbreviated with a capital letter, while recessive alleles are given a lower case version of the same letter.

  • Genotype : Genetic composition of an individual e.g., pure tall – TT, hybrid tall – Tt and pure dwarf – tt.
  • Phenotype : Visible traits of an individual e.g., tallness or dwarfness.
  • Mendel’s Principles of Inheritance
    1. Principle of paired factors : 
    A character is represented in diploid organisation by at least two factors. The two factors lie on the two homologous chromosomes at the same locus. They may represent the same (homologous, e.g., TT in case of pure tall pea plants ) or alternate expression (heterozygous, e.g., Tt in case of hybrid tall pea plants) of the same character.
    2. Principle of dominance : In heterozygous individuals or hybrids, a character is represented by two contrasting factors called alleles or allelomorphs. Out of the two alleles, only one is able to express its effect in the individual. It is called dominant factor or dominant allele. The other allele, which does not show its effect in the heterozygous individual, is called recessive factor or recessive allele.
    3. Principle of law of segregation : The two factors of a characters which remain together in an individual do not get mixed up but keep their identity distinct, separate at the time of gametogenesis or sporogenesis, get randomly distributed to different gametes and then get paired again in different offspring as per the principle of probability.
    4. Principle of law of independent assortment. According to this principle or law, the two factors of each character assort or separate independent of the factors of other characters at the time of gamete formation and get randomly rearranged in the offspring.
  • Mendelian factor or genes : Traits are inherited through heredity units called ‘genes’ which are segments of a DNA molecule and genes are located on the chromosome at fixed locations. For a trait, there is a pair of genes which segregate along with segregation of paired chromosomes during gamete formation. When male and female gametes fuse during fertilization, paired condition is restored.

  • Determination of sex : Gender is different organisms may be determined by environmental factors as in some snails, turtles and lizards. It is determined through chromosomes as in human beings, fruit flies, etc.
  • Determination of sex in human beings : It is determined by the sex chromosomes (XX in females and XY in males). If a sperm carrying ‘X’ fuses with ovum, female offspring is produced. If sperm with ‘Y’ fuses with ovum, male offspring is produced. Hence, sex chromosomes of male parent are involved in determining the sex of offspring.
  • Evolution : Gradual changes in traits of organisms form preexisting organisms is called evolution. It is the change in the inherited traits of a population from generation to generation. Change in the genetic make-up of a population occurs with time. All known species are descended from a single ancestral species through this process of gradual divergence. Errors in DNA copying (mutation) and sexual reproduction leads to variations which form the basis of evolution.
  • Evidences of Organic Evolution
    i) Morphological and Anatomical Evidences : These are the proofs of interrelationship between living organisms of different groups, which are based on the comparative studies of external and internal morphology (structure). Few cases of such evidence are:
    a) Homologous organs : Homologous organs provide an evident example of evolutionary relationships. The organs which have similar basic structure and mode of origin but perform different functions in different animals are called homologous organs, e.g., forelimbs of amphibians, reptiles, birds and mammals. Homologous organs give an evidence of common ancestory and evolutionary relationship between apparently different species.

b) Analogous organs : Analogous organs are those structures, which are different in their basic structure, development and origin but appear similar and perform similar function. Some examples of analogous organs are insect and bird wing, wing of bird and bat.

iii) Fossils : They are impressions of the body/ body parts or the remains of organisms living in the past, which got preserved in sediments of earth.

Fossils provide one of the most acceptable evidences in support of evolution, because we can study the evolutionary past of individuals in the form of their fossils. By studying fossils occurring in different strata or rocks, geologists are able to reconstruct the time course of evolutionary events. Age of fossils can also be found by time dating using isotopes of carbon (carbon dating).