What Are Mendelian Disorders? / Recombinant DNA Technology (Genetic Engineering) / Types of Mendelian Disorders

Mendelian disorders are genetic disorders caused by a change in a single gene located at a specific position (locus) on a chromosome. These disorders follow Gregor Mendel’s laws of inheritance, which is why we call them Mendelian disorders.

In humans, Mendelian disorders usually:

• Appear at birth

• Are rare (affecting about 1 in 1,000 to 1 in 1,000,000 individuals)

• Can be traced through family history

To study how these disorders are passed from one generation to the next, scientists use a method called pedigree analysis, which is essentially a family tree showing inheritance patterns.

Inherited vs. Non-Inherited Disorders

• Inherited genetic disorders occur due to defects in germline cells (sperm or egg) and are passed to offspring.

• Non-inherited genetic disorders arise from new mutations, often influenced by environmental factors.

  • Example: Cancer may be inherited or may result from new DNA mutations during life.

Types of Mendelian Disorders

According to Mendel’s laws, Mendelian disorders are classified into the following types:

1. Autosomal dominant

2. Autosomal recessive

3. Sex-linked dominant

4. Sex-linked recessive

5. Mitochondrial disorders

These patterns can be easily identified using pedigree charts.

Common Examples of Mendelian Disorders

• Sickle-cell anemia

• Muscular dystrophy

• Cystic fibrosis

• Thalassemia

• Phenylketonuria (PKU)

• Color blindness

• Skeletal dysplasia

• Hemophilia

Let’s take a closer look at a few important ones.

Hemophilia

• Type: Sex-linked recessive disorder

• Chromosome involved: X chromosome

Since males have only one X chromosome, they are more frequently affected. Females are usually carriers and rarely show symptoms unless:

• The mother is a carrier or affected, and

• The father is affected

What happens in hemophilia?

The blood fails to clot properly because a clotting protein is missing or defective. This leads to:

• Excessive bleeding

• Prolonged bleeding from even small cuts

Sickle-Cell Anemia

• Type: Autosomal recessive disorder

This disorder occurs when both parents carry the faulty gene.

Molecular cause

• The amino acid glutamic acid is replaced by valine at the 6th position of the beta-globin chain of hemoglobin.

Effect

• Red blood cells change from a normal round shape to a sickle shape

• Oxygen-carrying capacity decreases

• Blood flow becomes restricted

Phenylketonuria (PKU)

• Type: Autosomal recessive disorder

Cause

The enzyme needed to convert phenylalanine into tyrosine is absent.

Effect

• Phenylalanine accumulates in the body

• Toxic by-products form

• Leads to intellectual disability if untreated

Thalassemia

• Type: Autosomal recessive disorder

What goes wrong?

The body produces abnormal hemoglobin, causing:

• Destruction of red blood cells

• Severe anemia

Common symptoms

• Facial bone deformities

• Enlarged abdomen

• Dark urine

Cystic Fibrosis

• Type: Autosomal recessive disorder

This disorder affects:

• Lungs

• Digestive system

Key feature

The body produces thick, sticky mucus, which:

• Blocks airways

• Interferes with digestion

Patients often have a shortened lifespan without proper treatment.

DNA Fingerprinting

What Is DNA Fingerprinting?

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

It focuses on satellite DNA, also known as VNTRs (Variable Number of Tandem Repeats). These are:

• Repetitive DNA sequences

• Non-coding (do not produce proteins)

• Highly polymorphic (vary greatly among individuals)

This variation makes them ideal for identification.

Applications of DNA Fingerprinting

• Forensic science (crime scene investigation)

• Paternity testing

• Identifying unclaimed children

• Population genetics and evolution studies

Steps in DNA Fingerprinting

Developed by Alec Jeffreys, the process includes:

1. Isolation of DNA

2. Cutting DNA using restriction enzymes

3. Separation of fragments by gel electrophoresis

4. Transfer to a nylon membrane (blotting)

5. Hybridization with labeled VNTR probes

6. Visualization using autoradiography

Recombinant DNA Technology (Genetic Engineering)

What Is Recombinant DNA Technology?

Recombinant DNA technology is a method used to alter the genetic makeup of an organism by introducing a foreign gene into its genome.

This technology allows scientists to:

• Produce useful proteins

• Modify traits

• Treat genetic disorders

Key Tools of Recombinant DNA Technology

1. Enzymes

• Restriction enzymes – cut DNA at specific sites

• DNA ligase – joins DNA fragments

• DNA polymerase – synthesizes DNA

2. Vectors

Vehicles that carry the desired gene:

• Plasmids

• Bacteriophages

Each vector contains:

• Origin of replication

• Selectable marker (e.g., antibiotic resistance)

• Cloning sites

3. Host Organism

Usually bacteria or yeast cells that:

• Take up recombinant DNA

• Express the inserted gene

Steps of Recombinant DNA Technology

1. Isolation of genetic material

2. Cutting DNA using restriction enzymes

3. Amplification using PCR

4. Ligation of gene with vector

5. Insertion into host cell (transformation)

Once inside the host, the gene is expressed and passed on to future generations.

Applications of Recombinant DNA Technology

• Production of human insulin

• Detection of HIV

• Gene therapy

• Agricultural advances (Bt cotton, Golden rice)

• Medical diagnostics (ELISA)

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

• Allowing it to replicate inside a host cell

Common vectors

• Plasmids

• Viruses

• Yeast cells

Applications of Gene Cloning

• Production of hormones, antibiotics, and vaccines

• Crop improvement and nitrogen fixation

• Gene therapy

• Treatment of diseases like leukemia and sickle-cell anemia

Multiple Choice Questions (MCQs)

Mendelian Disorders

1. Mendelian disorders are caused due to mutation in:
   A. Multiple genes
   B. A single gene at a specific locus
   C. Entire chromosome
   D. Mitochondrial DNA only

Answer: B

2. Which method is used to trace the inheritance pattern of genetic disorders in a family?
   A. Karyotyping
   B. DNA sequencing
   C. Pedigree analysis
   D. Gel electrophoresis

Answer: C

3. Which of the following is an autosomal recessive disorder?
   A. Haemophilia
   B. Colour blindness
   C. Sickle cell anaemia
   D. Huntington’s disease

Answer: C

4. Haemophilia is more common in males because:
   A. It is autosomal dominant
   B. The gene is present on the Y chromosome
   C. The gene is present on the X chromosome
   D. Females do not inherit the gene

Answer: C

5. In sickle cell anaemia, which amino acid replaces glutamic acid in haemoglobin?
   A. Alanine
   B. Valine
   C. Glycine
   D. Leucine

Answer: B

6. Phenylketonuria occurs due to the absence of the enzyme that converts:
   A. Tyrosine to phenylalanine
   B. Phenylalanine to tyrosine
   C. Glucose to pyruvate
   D. Amino acids to proteins

Answer: B

7. Thalassemia is caused due to:
   A. Absence of red blood cells
   B. Abnormal haemoglobin synthesis
   C. Defective platelets
   D. Vitamin deficiency

Answer: B

8. Which Mendelian disorder affects the lungs and digestive system?
   A. Thalassemia
   B. Haemophilia
   C. Cystic fibrosis
   D. PKU

Answer: C

DNA Fingerprinting

9. DNA fingerprinting is based on differences in:
   A. Coding DNA
   B. Ribosomal RNA
   C. Satellite DNA
   D. Mitochondrial DNA

Answer: C

10. VNTR stands for:
    A. Variable Nucleotide Tandem Repeats
    B. Variable Number of Tandem Repeats
    C. Very Narrow Tandem Regions
    D. Variable Nuclear Transcription Regions

Answer: B

11. DNA fragments are separated based on size using:
    A. Autoradiography
    B. Blotting
    C. Electrophoresis
    D. Hybridization

Answer: C

12. The scientist who developed DNA fingerprinting is:
    A. Watson
    B. Crick
    C. Mendel
    D. Alec Jeffreys

Answer: D

Recombinant DNA Technology

13. Recombinant DNA technology is also known as:
    A. Cloning
    B. Mutation
    C. Genetic engineering
    D. Transcription

Answer: C

14. Which enzyme is responsible for cutting DNA at specific sites?
    A. DNA ligase
    B. DNA polymerase
    C. Restriction endonuclease
    D. RNA polymerase

Answer: C

15. Sticky ends are produced by:
    A. DNA ligase
    B. Restriction enzymes
    C. RNA polymerase
    D. Exonucleases

Answer: B

16. Plasmids are commonly used in recombinant DNA technology because they:
    A. Are part of the nucleus
    B. Have high copy numbers
    C. Cannot replicate
    D. Are linear DNA molecules

Answer: B

17. The process of introducing recombinant DNA into a host cell is called:
    A. Translation
    B. Transcription
    C. Transformation
    D. Translocation

Answer: C

18. Which technique is used to amplify DNA?
    A. ELISA
    B. Southern blotting
    C. PCR
    D. Autoradiography

Answer: C

DNA Cloning

19. A clone is defined as:
    A. Genetically different cells
    B. Cells with random mutations
    C. Genetically identical cells
    D. Sexually reproduced cells

Answer: C

20. Which of the following is NOT a vector used in gene cloning?
    A. Plasmid
    B. Yeast
    C. Virus
    D. Ribosome

Answer: D

FAQ

1. Define Mendelian disorder.

A Mendelian disorder is a genetic disorder caused by a mutation in a single gene and inherited according to Mendel’s laws of inheritance.

---

2. What is pedigree analysis?

Pedigree analysis is the study of a family tree to trace the inheritance pattern of a genetic trait or disorder across generations.

---

3. Why are females rarely affected by haemophilia?

Haemophilia is an X-linked recessive disorder. Females have two X chromosomes, so a normal gene on one X usually masks the defective gene. A female must inherit the defective gene from both parents to be affected, which is rare.

---

4. State the molecular cause of sickle cell anaemia.

Sickle cell anaemia is caused by the replacement of glutamic acid with valine at the sixth position of the beta-globin chain of haemoglobin.

---

5. What is phenylketonuria and why is it dangerous if untreated?

Phenylketonuria is an autosomal recessive metabolic disorder caused by the absence of the enzyme that converts phenylalanine to tyrosine. If untreated, phenylalanine accumulates and causes intellectual disability.

---

6. Define satellite DNA.

Satellite DNA consists of repetitive, non-coding DNA sequences that show a high degree of variation among individuals.

---

7. What is DNA fingerprinting?

DNA fingerprinting is a technique used to identify individuals by analyzing variations in their satellite DNA regions.

---

8. Mention any two applications of DNA fingerprinting.

• Forensic investigations

• Paternity or maternity testing

---

9. What is recombinant DNA technology?

Recombinant DNA technology is the process of combining DNA from different sources to produce artificial DNA and express desired genes in a host organism.

---

10. Name any two restriction enzymes used in genetic engineering.

• EcoRI

• HindIII

---

11. What is the role of plasmids in recombinant DNA technology?

Plasmids act as vectors that carry the desired gene into the host cell and help in its replication and expression.

---

12. Define transformation in genetic engineering.

Transformation is the process by which recombinant DNA is introduced into a host cell.

---

13. What is DNA cloning?

DNA cloning is the process of producing multiple identical copies of a specific DNA fragment or gene.

---

14. Give two applications of gene cloning.

• Production of hormones such as insulin

• Gene therapy for genetic disorders

---

15. Why are sticky ends important in recombinant DNA technology?

Sticky ends allow complementary base pairing between the vector and the foreign DNA, making ligation efficient and accurate.

Final Thought

Understanding Mendelian disorders and modern genetic technologies gives us powerful tools to diagnose diseases, improve agriculture, and develop life-saving therapies. Genetics is no longer just theory—it’s transforming real lives every day.