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Chapter: Inheritance


Inheritance is the transmission of genetic information from one generation to the other generation.

Let us learns some important terms before get into the details of inheritance

1. Chromosome:

It is the thread of DNA made up of string of genes.

2. Gene:

Gene is the basic functional unit of heredity which is transferred from parents to offspring’s and is held to determine some of the characteristics of offspring.

3. Allele:

An allele is an alternative form of gene that is located at a specific position on a specific chromosome.

4. Haploid nucleus:

It is defined as a nucleus of the cell containing a single set of unpaired chromosomes. For example: nucleus of a sperm and egg cell.

5. Diploid nucleus:

It is defined as the nucleus of the cell containing two sets of chromosomes. For example: cells in our body.

Inheritance of sex in humans

Each cell in a human body contains 46 chromosomes. There are 22 pairs of chromosomes along with which are the two chromosomes which may not look alike. These are the sex chromosomes. Female cells have two sex chromosomes that are a like (XX) and males have two chromosomes which are not alike  (XY).  The man’s sperm decides the sex of his children since women produce only gametes with an X chromosome. All the children will inherit X chromosome from their mother. If the child inherits Y from their father the child will be a boy and if the child inherits X from the father she will be a girl.


It is the nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosomes.

Significance of mitosis

It is very important to understand the significance of mitosis in the life of an organism. The growth of multicellular organisms is due to mitosis

  1. Cell growth results in distributing the ratio between nucleus and the cytoplasm. It is therefore necessary for the cell to divide to restore the nucleo-cytoplasmic ratio.
  2. A very significant contribution of mitosis is cell repair
  3. The cells of the upper layer of the epidermis, cells lining the gut and blood cells are constantly being replaced.
  4. Mitotic divisions in the meristematic tissues-the apical and the lateral cambium, result in continuous growth of plants throughout their life.


Meiosis is defined as the reduction division in which the chromosome number is halved from diploid to haploid.

Significance of meiosis:

Meiosis is a mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms. It also increases genetic variability in the population of organisms from one generation to the next. Variations are necessary for the process of evolution.

Monohybrid inheritance

It is the pattern of inheritance involving only one pair of contrasting characters. In the beginning of his experiments on inheritance Mendel concentrated only on the pattern of inheritance of a single pair of contrasting characters. 

Here are some terms and definitions to help us understand the concept of monohybrid inheritance.

1. Genotype:

It is defined as the genetic makeup of a cell, an organism, or an individual usually with reference to a particular characteristic under consideration. The genetic constitution of an organism is referred to as its genotype. For example let us consider letters Tt (T is the dominant genotype and t- is the recessive genotype).

3. Phenotype:

The physical make up of an organism due its genotype and its environment. For example tall plant (it is tall in nature because it is expressing the dominant genotype T).

4. Homozygous:

If the nucleus carries the same alleles on both the members of the homologous pair, then the cell is homozygous.

5. Heterozygous:

If both alternative alleles are present in a particular cell nucleus, then the cell is called heterozygous.

6. Dominant allele: 

An allele that expresses its phenotypic effect even when heterozygous with recessive allele. For example if T is dominant over t then TT and Tt have the same phenotype.

7. Recessive allele:

An allele that is expressed only when there is no dominant allele of the gene present.Example t is recessive allele and T is dominant allele. t is expressed only if T is not present.

Monohybrid cross

  1. Mendel performed his first experiment by producing a progeny from a tall pea plant and a short pea plant and then he calculated the percentage of tall and short progeny.
  2. He observed that the first generation plants were all tall and there were no medium height plants. He called the first generation as the F1 generation.
  3. Mendel then self pollinated the F1 generation, the result was F2 generation or the second generation in which one quarter of them were short and the rest were tall this indicated that the first generation progeny, i.e. the F1 generation plants inherited both tall and short characters but only the tallness trait was expressed. Thus, two copies of the trait were inherited from each sexually reproducing organism.
  4. Let us understand this by making a cross:


Tall (TT) × short(Tt)    →   all tall off springs(Tt)

                                                                                [F1 generation]


                             F1 × F1              Tall (Tt), Tall (Tt), Tall (Tt), short (tt)

                                                                  (One quarter of the progeny is short)


From the above explanation it is understood that TT and Tt are representing tall plants and tt are representing short plants. Capital letters indicate dominant trait and small letters indicate a recessive trait. In the example given above tallness is the dominant trait and hence it is represented as T, whereas shortness is a recessive trait and is represented as t.


Inheritance from the previous generations provides both a common basic body design along with some changes .The second generation will inherit differences from the first generation as well will have newly created differences. In asexual reproduction, let us look at bacteria for example. The bacteria first divides into two which will further divide into four and eight respectively. All these bacteria generated would be similar with minor differences between them because of small inaccuracies in DNA copying. In case of sexual reproduction we can observe much more variations in the second generation because of the involvement of two individuals in the process of reproduction. There are two types of variations. 1. Continuous variation 2. Discontinuous variation.

1. Continuous variation:

The variation which is influenced by genes and environment, resulting in a range of phenotypes between two extremes is called continuous variation. Height is an example of continuous variation. Individuals can have complete range of heights, for e.g. 1.5, 1.51, 1.52 etc meters high. Shoe size, hand span, milk yield in cow is some of the other examples of continuous variation.

2. Discontinuous variation:

The variation which is influenced by genes alone and results in limited number of distinct phenotypes with no intermediates is called discontinuous variation. Blood groups are the best example of discontinuous variation. We can be of any one blood group between A, B, AB or O but we cannot be in between.


It is defined as the permanent change in the nucleotide sequence of the genome of an organism, virus, extrachromosomal DNA or other genetic elements. Mutation is a source of variation. Let us understand this by taking an example of Down’s syndrome.

Down’s syndrome

Down’s syndrome also called as trisomy 21 is a genetic disorder caused due to the presence of third copy of chromosome no: 21. In a normal meiosis, chromosomes are equally distributed between the gametes. In unusual meiosis the chromosomes are unequally distributed between the gametes. Unusual distribution results in gametes with extra chromosomes. It contains chromosome no: 21 from both the parents. This gamete with extra chromosome will have 24 chromosomes. When this gamete fuses with a gamete having normal number of chromosomes it will result in zygote with 47 chromosomes, there are three copies of chromosome 21. This results in Down’s syndrome.

Effect of radiations and chemicals on mutations


Gamma radiations, ultraviolet radiations and X-radiation can cause damage DNA and so can cause mutations.


Tar in tobacco smoke, high concentrations of some preservatives and some plant control hormones can cause mutations.

Agents causing mutations are called as mutagens. Mutagens causing uncontrolled cell division are called carcinogens.

Artificial selection

Variations occur naturally and randomly in all living organisms, but natural environment is not the agent of selection. Ever since human began to domesticate animals and plants they have been trying improve them. This improvement is brought by selecting those individuals for breeding with more useful characteristics. This process is called artificial selection. Some of the examples of selective breeding are as follows.

  1. Jersey cattle have been bred for producing milk with high cream content
  2. All the domestic dogs are of same species but some are bred for the appearance , some are bred for hunting and some as aggressive guards.
  3. Wheat has been bred so that all the stems are of the same height and ears separate easily from stalk

Some species can be bred in different ways for different purposes. Let us look at an example.

  1. In a group of calves if two are larger than the others, the larger ones are selected for breeding as they contain more meat and so are valuable for butcher. By selecting large cattle for the purpose of breeding, the size character can be inherited and the next generation individuals may be even larger with more meat.
  2. We can produce many vegetables from one ancestor species. For example if we take a wild mustard. The larger leaves of it are selected to produce kale. Cauliflower is produced by selecting large flower head. Cabbage is produced by selecting large bud. A swollen stem is selected for producing kohlrabi. Side buds can be selected for producing Brussels sprouts.

Natural selection:

It is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. The theory of natural selection was given by Charles Darwin.

Genetic engineering

It is the process of manually adding new DNA to an organism. The main goal of genetic engineering is the development of organisms with new traits.