Biology - 5090




It is the distinction between individuals of the same species. Genetic variation is the difference between the genotypes of the individuals, while the phenotypic variation is the difference between the features of different individuals, caused by both genetic and environmental factors.

The continuous variation results in a range of phenotypes between two extremes, for example, height and eye color in humans. Many alleles are involved in continuous variation. The discontinuous variation results in a limited number of phenotypes with no intermediates, for example, tongue rolling. It is mostly caused by genes alone, like A, B, AB, and O blood groups in humans.

Chromosomes and DNA

Chromosomes are made up of a long strand of DNA. A gene is a piece of the DNA molecule that codes for a protein. It can be copied or passed from one generation to the next. An allele is a specific form of a gene, which distinguishes from other ones by a few bases, and it occupies the same gene locus as the others.

Structure of DNA

DNA is a double helix biological molecule that is formed when two strands of DNA are coiled together. Both strands consist of chemicals known as bases, which have shapes that complement each other. Cross-links between the strands of DNA are formed by pairing of the bases. The bases always pair up in the same way, i.e. A with T and C with G.

Protein Synthesis

1. DNA is present inside the nucleus. Protein synthesis occurs on ribosomes present freely in the cytoplasm or those attached to the rough endoplasmic reticulum. 

2. To carry the information from the DNA to the ribosome, a messenger molecule, known as mRNA, is used. mRNA is made from DNA, and this process is named as transcription. mRNA is a single helix molecule. 

3. The gene that is coding for the protein molecule remains intact in the nucleus of the cell. mRNA molecules carry a copy of the gene from the nucleus to the cytoplasm, attaching to the ribosomes present there. The ribosomes assemble the amino acids into a specific sequence to form the protein molecule. The order in which amino acids assemble depends on the sequence of the bases present on the mRNA. The endoplasmic reticulum is extended membranes of the nucleus present in the cytoplasm. The mRNA attaches to the ribosomes which are attached to these. 

4. Against every code, an amino acid will connect, making a polypeptide chain that forms the protein molecule. This is known as translation.

Monohybrid Inheritance

The genotype is the genetic make-up of an organism in terms of the alleles present.

Phenotype is the observable features of an organism. 

Homozygous means having the same two alleles of a particular gene. Two identical homozygous individuals that breed together will be termed as pure breeding.

Heterozygous refers to the presence of two different alleles of a particular gene. A heterozygous individual does not classify as pure breeding. 

The term ‘dominant’ means that an allele that is expressed if it is present in heterozygous condition. 

The term ‘recessive’ refers to an allele that is only expressed when then there is no dominant allele of the gene present, i.e. in recessive condition.


It is the change in the base sequence of the DNA molecule. It leads to the formation of new alleles. Ionizing radiation and some chemicals increase the rate at which mutation occurs. Many different chemicals are known to increase the risk of mutations. Lead and mercury can interfere with the process in which DNA is replicated, while radiation can damage the base sequence of the DNA molecule.


When both the alleles in heterozygous condition are expressed and the resulting gametes either show a mixture of both or are completely different from each other, this condition is referred to as codominance or incomplete dominance. For example, A blood group is not dominant over the B blood group, but A and B blood groups are dominant over the O blood group. 

Test Cross

It is used to identify an unknown genotype or a dominant character, for example, if we know that tallness is dominant to dwarfness in a certain species of peas, then the genotype of any tall plant can be identified by crossing it with a dwarf plant. If any of the two offsprings are dwarf, then this means that the tall parent had an allele for dwarfness i.e. it must have been heterozygous. If none of the offspring are dwarf, then the tall parent was homozygous for the tallness allele.

Sex Linkage

A sex-linked characteristic is a character in which the gene responsible is located on a sex chromosome, and it makes it more common in one sex compared to the other. A sex-linked gene is any gene present on one of the sex chromosomes (X or Y). For most chromosomes (22 pairs), there are two copies of each one. They contain the same gene in the same position so we have two copies of each gene. Whereas, for the sex chromosomes, the Y chromosome is tiny with a few genes, while the X is larger. Most of the genes on the X chromosomes only have one copy. There are also few genes present on the Y chromosome, which are not present on the X chromosome. Genes present on the non-homologous parts of the chromosomes are the sex-linked genes. Color blindness is an example of sex linkage.


The species that are well adapted to the environment are the best variety. Nature chooses the variety that is best suited to that environment. These characters pass on to the offspring. The other unfit varieties perish from that area. There are two types of selection, natural and artificial. 

Natural Selection

There are several reasons for this preference:

Variation within populations: Slight variations between individuals of the population may adopt some organisms better than the rest. 

Production of many offspring: Most organisms produce more offspring than will survive to adulthood.

Competition For resources: Organisms experience environmental resistance. They compete for the limited resources present within the environment. 

Struggle for survival: Organisms that are well-adapted are the best fit.

Reproduction by the best fit individuals: Best fit individuals reproduce successfully and pass on advantageous characters to their offspring.

Passing on of good alleles to the next generation: Favourable characteristics pass on the next generation, which leads to evolution. 

Artificial selection

It is the selection, done by humans, of individuals with desirable features. These individuals are crossbred to produce the next generation. It is a quicker process, as compared to natural selection. 

Genetic Engineering

It is changing the genetic material of an organism by removing, altering or inserting individual genes or DNA.

Production of Human Insulin Through Genetic Engineering

1. The DNA that makes a human gene is isolated using restriction enzymes and it forms sticky ends. Restriction enzymes identify the base sequences. 

2. The DNA of a bacterial plasmid is cut open with the help of the same restriction enzymes that form complementary sticky ends. 

3. The DNA of the human gene is inserted into the bacterial plasmid DNA, using DNA Ligase to form a recombinant plasmid. 

4. The plasmid continuing the human gene is put back into the bacterium.

5. The bacteria, along with the human gene, containing recombinant plasmids, divides and makes the human insulin.

6. Insulin is extracted from the bacterial culture in the fermenter and purified

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