The passing down of similar characters generation after generation is termed heredity. Heredity is controlled by genes. Differences in gene combinations lead to variations or differences even among members of the same family. The science of heredity and variation is termed Genetics.

Mendel's Laws of Inheritance

Mendel’s laws state that:

1. Every feature or character (for example colour of flowers, height of plant, colour and texture of seed, colour and texture of pods and location of flower on the plant) is controlled by a pair of factors. During the formation of gametes, one factor goes to one gamete and its pair to another gamete. Thus the two factors of a pair segregate or separate during gamete formation. Upon fertilization, the combination of factors expresses the feature. (1st law).

2. Out of the two factors controlling a certain feature, the dominant one may express inspite of the presence of the other. The other factor expresses only in the absence of the dominant factor and is termed recessive (2nd law).

For example, factor for tallness in the pea plant always expresses in the offspring but dwarfness expresses only if factor for tallness is not present.


Genes are responsible for heredity. They are present on chromosomes at fix points.

The nucleus of every cell (except RBC of mammals) contains a fixed number of chromosomes. In all the cells of eukaryotes, chromosomes show the following characteristics:

  1. They are present in pairs, one from the father and the other from the mother.
  2. They can be seen only during cell division. In a non-dividing cell, they appear in the nucleus as a jumbled up network termed chromatin.
  3. The paired chromosomes are present in a fixed number. A fixed set of chromosomes is termed the "diploid" (paired) number and designated as 2n.
  4. Each chromosome is made of one molecule of the chemical called DNA or deoxyribonucleic acid and some proteins.
  5. Before cell division, the DNA molecule of a chromosome replicates (duplicates) to give two molecules of DNA which are called "chromatids". The two chromatids of a chromosome remain attached at a point called centromere and separate to form two chromosomes during cell division.

Human chromosomes

Every cell of a human being contains 46 chromosomes. In other words, the diploid number in humans is 46. This can be expressed as 2n = 46. Since gametes contain only half the number of chromosomes or the haploid number, a sperm and an ovum or an egg has only 23 chromosomes.

A pair of similar chromosomes (one received from the father and one from the mother) containing the same genes are termed homologous chromosomes.

Out of the 23 pairs of chromosomes, 22 pairs are termed autosomes. The 23rd pair (X and X in females and X and Y in males) are called sex chromosomes. X chromosome has several genes, some of which are necessary for survival. Y chromosome bears genes for maleness only.


Genes Genes are present on chromosomes. The Genes are the "Mendelian factors" present in pairs (one received from the father, other from the mother), on the chromosomes.

Genes are the bearers of hereditary characters or the units of heredity. A chromosome contains one molecule of the chemical called DNA. Genes present on chromosome are segments of DNA.

Since every individual begins life as a single cell, the DNA contained in all the cells of an individual is identical.

DNA Molecule

A DNA molecule is a polynucleotide (poly means many). It is made of units called nucleotides, each of which contains a nitrogenous base, a deoxy ribose sugar, and a phosphate. 

There are four nitrogenous bases Adenine, Guanine, Thymine and Cytosine and hence four kinds of nucleotides in a DNA molecule. The various combinations of these nucleotides in a segment of DNA form the different genes.

DNA Replication

Cell division takes place in a manner so that one cell divides into two identical cells with the same number of identical chromosomes. Therefore, prior to cell division, every chromosome should contain two chromatids made of two identical DNA molecules. This is achieved through the process of DNA duplication or DNA replication.

Upon DNA replication, each chromosome contains two identical molecules of DNA housed in its two chromatids. During cell division, the two chromatids, separate out as two chromosomes, one each, passing into the two daughter cells.

Blood Group Inheritance

Every one is born with genes inherited from our parents. Our blood group depends on the combination of a pair of genes, one of which is inherited from each parent.

There are four blood groups - A, B, AB and O. Every human being has one blood group out of the four. The genes which control the inheritance of these blood groups are designated as IA, IB and i. When a foetus (growing young one in mother’s womb) develops, its blood group is determined by the combination of any of the two above mentioned genes, one received from each of the parents.

Gene Combination Blood Group
IA IA or IA i A
IB IB  or  IB i B
ii O

Gene IA and IB are dominant and i is recessive. Apart from these blood groups, human beings may also belong to the groups designated as Rhesus positive (Rh+) or Rhesus negative (Rh-). Most humans are Rh+. Some are Rhesus negative. The Rh+ gene is dominant over Rh- gene.

Sex Determination in Humans

The combination of sex chromosomes with autosomes determines whether the foetus will be a boy or a girl. The foetus develops from the zygote which is formed by the fusion of the two gametes, the male gamete or sperm and the female gamete or egg. Gametes are haploid while the zygote is diploid.

Ova or eggs are of one kind only. These contain 22 autosomes and a single X chromosome. Sperms are of two kinds (i) having 22 autosomes and one X chromosome, or (ii) having 22 autosomes and a Y chromosome. When X bearing sperm fuses with the egg, a female child results with 44 autosomes and two X chromosomes. If Y bearing sperm fuses with the egg then a male child results with chromosomal constitution of 44 autosomes and one X and one Y chromosome.

Hereditary Disorders

Genes control all features of an organism. Some times a gene may change or mutate either in the gamete or zygote. Mutated gene may not remain normal. Also, sometimes a defective gene present in the parent may not be expressed in the parent as the dominant normal member of its pair may mask the effect of the defective gene. But if the child inherits the defective gene from both the parents, the presence of the defective pair of genes has a harmful effect. Such a disorder is termed hereditary or genetic disorder.

There are several kinds of hereditary disorders, some of which may be caused due to presence of only one defective gene which is dominant or sometimes by the presence of two defective recessive genes.

Common Genetic (Hereditary) Disorders

Three common hereditary disorders are Thallasemia, Haemophilia and Colour blindness.

The genes for both haemophilia and colour-blindness are located on X-chromosome, and hence, the disorder is passed down from mother to the son because a boy receives the X chromosome from the mother and Y chromosome from the father. In the mother, with two X-chromosomes, the defect does not show up.

Also in the daughter, the effect of defective gene on X-chromosome inherited from mother may be masked by a normal gene on the X-chromosome, inherited from her father. Since X chromosome bears the defective gene, the son suffers from the genetic disorder, as male has only one X chromosome and one Y chromosome and so the defective gene does not get masked.


Patients suffering from this disorder are unable to manufacture haemoglobin, the pigment present in red blood corpuscles which carries oxygen to tissues. This is because the pair of genes controlling haemoglobin production are defective. Thallasemics (persons suffering from thallasemia) require frequent blood transfusion in order to survive. The thallasemia gene is present on an autosome.


Those persons suffering from haemophilia have either a defective gene or lack genes, which control production of substance responsible for blood clotting. In the absence of such substance blood does not coagulate. Once bleeding starts, it does not clot easily.


Different kinds of colour-blindness have been detected but in the most common form of the disorder, a person is unable to distinguish the blue colour from green.