Mendelian Disorders, DNA Fingerprinting, and Recombinant DNA Technology

 

Mendelian Disorders

Mendelian disorders are genetic disorders caused by a mutation in a single gene located at a specific genetic locus. These disorders follow the basic principles of inheritance proposed by Gregor Mendel, which explain how traits are passed from parents to offspring.

In humans, Mendelian disorders usually arise due to:

• Alterations in a single gene

• Abnormalities in the genome

• Mutations inherited from parents or occurring newly (de novo mutations)

Such disorders are often present from birth and can be traced through generations using a family tree, a method known as pedigree analysis.

What Are Mendelian Disorders in Humans?

A Mendelian disorder in humans is a monogenic disorder, meaning it results from a defect in only one gene. These disorders are relatively rare, affecting approximately 1 in every 1,000 to 1,000,000 individuals.

Inherited vs Non-Inherited Genetic Disorders

• Inherited disorders originate in germline cells (sperm or egg) and are passed from parents to children.

• Non-inherited disorders occur due to new mutations, often influenced by environmental factors.

For example, cancer may result from an inherited mutation or develop due to environmental exposure such as radiation or chemicals.

Types of Mendelian Disorders

According to Mendel’s laws of inheritance, Mendelian disorders are classified into the following categories:

1. Autosomal Dominant

2. Autosomal Recessive

3. Sex-linked Dominant

4. Sex-linked Recessive

5. Mitochondrial Disorders

These inheritance patterns can be clearly identified through pedigree analysis, making it an essential diagnostic tool in genetics.

Examples of Mendelian Disorders in Humans

Some well-known Mendelian disorders include:

• Sickle cell anaemia

• Muscular dystrophy

• Cystic fibrosis

• Thalassemia

• Phenylketonuria (PKU)

• Colour blindness

• Skeletal dysplasia

• Haemophilia

Let us examine a few of these in detail.

Haemophilia

Haemophilia is a sex-linked recessive disorder caused by a defective gene located on the X chromosome.

Key Features:

• Affects males more frequently than females

• Females are usually carriers

• Blood does not clot properly due to missing or defective clotting proteins

A female can develop haemophilia only if:

• Her mother is a carrier or affected

• Her father is affected

Symptoms include excessive bleeding, even from minor injuries.

Sickle Cell Anaemia

Sickle cell anaemia is an autosomal recessive disorder inherited from two carrier parents.

Molecular Basis:

• A single amino acid substitution occurs in the beta-globin chain of haemoglobin

• Glutamic acid is replaced by valine at the sixth position

This mutation causes red blood cells to change from a biconcave disc shape to a sickle shape, reducing their oxygen-carrying capacity and leading to anaemia.

Phenylketonuria (PKU)

Phenylketonuria is an autosomal recessive metabolic disorder.

Cause:

• Absence or deficiency of the enzyme that converts phenylalanine to tyrosine

Effects:

• Accumulation of phenylalanine in the body

• Conversion into toxic derivatives

• Leads to intellectual disability if untreated

Early diagnosis and dietary management can prevent severe complications.

Thalassemia

Thalassemia is an autosomal recessive disorder characterized by abnormal haemoglobin production.

Symptoms:

• Anaemia

• Facial bone deformities

• Abdominal swelling

• Dark urine

The excessive destruction of red blood cells results in chronic fatigue and weakness.

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive disorder affecting the lungs and digestive system.

Characteristics:

• Production of thick, sticky mucus

• Blockage of lungs and pancreatic ducts

• Reduced life expectancy

DNA Fingerprinting

DNA fingerprinting is a technique used to identify individuals based on their unique DNA patterns.

Definition:

DNA fingerprinting analyzes differences in satellite DNA regions, which are non-coding repetitive sequences in the genome.

These regions show a high degree of polymorphism, making them ideal for identification purposes.

What Is Satellite DNA?

• Non-coding DNA

• Repetitive sequences

• Do not code for proteins

• Highly variable among individuals

Because these regions vary greatly, they are extensively used in forensic science and paternity testing.

Steps of DNA Fingerprinting

This technique was developed by Sir Alec Jeffreys, who used VNTRs (Variable Number of Tandem Repeats) as probes.

Steps Involved:

1. Isolation of DNA

2. Digestion using restriction endonucleases

3. Separation by gel electrophoresis

4. Blotting onto nylon membranes

5. Hybridization with labeled VNTR probes

6. Analysis through autoradiography

Applications of DNA Fingerprinting

• Crime scene investigations

• Paternity and maternity testing

• Population genetics

• Studying genetic diversity and evolution

Recombinant DNA Technology

Recombinant DNA Technology is a powerful method used to alter the genetic makeup of an organism by introducing foreign DNA.

Definition:

It is the technique of producing artificial DNA by combining genetic material from different sources. This process is also known as genetic engineering.

The technique emerged after the discovery of restriction enzymes in 1968 by Werner Arber.

Tools of Recombinant DNA Technology

1. Enzymes

• Restriction enzymes: Cut DNA at specific sites

• DNA ligase: Joins DNA fragments

• DNA polymerase: Synthesizes DNA

Restriction enzymes recognize palindromic sequences and produce sticky ends, allowing easy ligation.

2. Vectors

Vectors carry the desired gene into the host.

Common vectors include:

• Plasmids

• Bacteriophages

Key components of vectors:

• Origin of replication

• Selectable markers (e.g., antibiotic resistance genes)

• Cloning sites

3. Host Organism

The recombinant DNA is introduced into a host using methods such as:

• Microinjection

• Gene gun (biolistics)

• Heat shock

• Calcium ion treatment

Process of Recombinant DNA Technology

Step 1: Isolation of DNA

The desired gene is extracted in pure form.

Step 2: Cutting of DNA

Restriction enzymes cut DNA at specific sites.

Step 3: Amplification (PCR)

The gene of interest is amplified using Polymerase Chain Reaction.

Step 4: Ligation

DNA ligase joins the gene with the vector.

Step 5: Transformation

Recombinant DNA is introduced into the host cell, where it multiplies and expresses the desired protein.

Applications of Recombinant DNA Technology

• Medicine: Production of insulin, vaccines, hormones

• Gene therapy: Correction of defective genes

• Diagnostics: HIV detection, ELISA tests

• Agriculture: GM crops such as Bt cotton, Golden rice

DNA Cloning

A clone is a group of genetically identical cells derived from a single parent cell.

DNA Cloning:

• Involves inserting a DNA fragment into a vector

• The vector replicates inside a host cell

• Produces multiple identical copies of the gene

Common vectors:

• Plasmids

• Yeast cells

• Viruses

Applications of Gene Cloning

• Production of hormones, antibiotics, and vaccines

• Agricultural improvement and nitrogen fixation

• Identification and manipulation of specific genes

• Gene therapy for disorders like leukemia and sickle cell anaemia

Final Note

Stay connected with HN Series Biology to explore Mendelian disorders, DNA fingerprinting, recombinant DNA technology, and modern genetics with clear explanations, real-world applications, and exam-oriented clarity—designed to make biology both understandable and exciting for students.