Heredity and Evolution

Important points

The parental generation provides information for basic body design for next generation, which is accompanied by small changes.

Different variations would provide different advantages to different individuals.

Selection of variants by environmental factors forms the basis for evolutionary process.

Both parents contribute equal amount of genetic material to the child i.e., for each trait there will be a maternal and a paternal version.

Gregor Johann Mendel, through consistent studies on Garden Pea arrived at laws of inheritance.

He used plants that were pure breeding for a trait and considered contrasting characters like – tall and short plant size, round wrinkled seeds, white and violet flowers etc.

In his experiments, he crossed plants with contrasting characters, studied the progeny of contrasting characters that were original parental type of different.

For example, he crossed a tall pea plant with a dwarf pea plant. He found that in first generation (F 1) all the plants produced were tall.

But when plants of F 1 were crossed to get second generation, he found that 3/4 of plants were tall while 1/4 were short.

Since an offspring receives half its genetic material from either parent and if we represent tallness as “T” and dwarfness as “t” then- The parent can be represented as TT and dwarf parent can be represented as tt.

Thus, F 1 generation can be shown as – fig will be loaded soon

When progeny of F 1 were self-pollinated the resultant progeny showed a ration of 3:1, i.e., three tall and one dwarf pea plant.

A single copy or “T” is enough to make the plant tall while both copies of “t” for the plant to be dwarf.

The trait which is expressed in F 1 is called dominant the one which remains hidden is termed recessive.

Similarly, when two pairs of contrasting characters were studied simultaneously, it was founded that two traits are inherited independent of each other.

For example , a plant with round seeds of yellow colour (RRYY) was crossed with a plant with wrinkled and green colour (rryy), the result was as follows : fig will be loaded soon

Here, in F 1 generation all plants produced resembled the dominant parent.

However, on selfing F 1 X F 1 to get F 2 progeny, the result showed that the two traits i.e., round/wrinkled seeds and yellow/green colour were inherited independently.


Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene. These genes control characteristics or traits.

Organisms have two sets of genes; one inherited from each parent. During formation of gamete, one set of these gene passes to the germ-cell. Thus germ-cells have only one set of gene.

Genes are located on the chromosomes. The chromosomes are present in pairs. During gamete formation one chromosome of the pair is passed on to each gamete.

When gametes fuse during sexual reproduction normal diploid number of chromosomes are restored.

In human beings, there are 23 pairs of chromosomes. Out of these, 22 pairs of chromosomes are similar in males and females. These are called autosomes.

One pair of chromosomes is XX in females and XY in males. These chromosomes are called dex chromosomes.

Thus, females produce all gametes having 22 autosomes + X chromosome while males produce two types of gametes i.e., 22 autosomes + X chromosome and 22 chromosomes + Y chromosome.

Since, female gametes always carry only X choromosome, thus sex of the child is determined by the sex chromosome of the sperm.

Fig will be loaded soon

 The gradual unfolding of organisms from pre-existing organism through change since the origin of life is termed as evolution.

Variation in a population provides protection from adverse conditions to certain member of a population.

Let us understand how variation leads to evolution. Consider a group of twelve red beetles which live in bushes with green leaves.Let us imagine that crows eat these beetles. The more beetles the crows eat, fewer beetles are available to reproduce. Now, let us think of three different situations that can develop in this beetle population.

A variation occurs such that beetles now produced are green, which are easily masked amongst leaves. Thus crows feed on only red beetles, resulting in increase in population of green beetles. Here nature selects fitter organism. This is natural selection.

There is yet another variation resulting in blue beetles. Now both red as well as blue beetles are not naturally selected. But an elephant passing by happens to stamp in such a manner that most beetles are killed, but few surviving ones are blue.

Thus, even though in first case there was survival advantage and hence natural selection, the second case it leads to genetic drift without any adaptation due to an accident.

Situation three : When the beetle population begins to expand, the bushes start suffering from a plant disease. The amount of leaf material available as food for the beetles is reduced. It resulted in reduction in weight of beetles. However, when the plant disease is eliminated the beetle size again increases. This is due to the that the fact weight of the beetle was reduced due to starvation which will not change the DNA of the germ-cell.

The changes that occur in germ cells are passed on to the progeny. Any trait that is acquired during the life time of an individual, cannot be inherited.

Charles Darwin came up with the idea of evolution of species by natural selection. His theory tells us how life evolved from simple to more complex forms.

Urey and Miller, experimentally proved that life must have arisen from simple inorganic molecules.

Speciation is formation of new species from a previously existing ones due to genetic drift and geographical isolation.

The branch of biology dealing with how such a wide variety of organisms come to exist is termed evolution. Various evidences which support evolution are : (i) Homologous organs (ii) Fossils                     (iii) Comparative embryology

Complex organs or organisms arise by step by step small variations. Hence, even very dissimilar looking structures would have evolved from a common ancestral design.

Evolution gives rise to more complex body designs even while the simpler body designs continue to flourish.

Study of evolution of human beings indicates that all of us belong to single species Homosapiens that can be traced back to Africa.