5.3 Mutation

Meaning and Types of Mutation

• Mutation: a spontaneous and random change in genes and chromosomes that may change the characteristics of offspring who inherit the modified genes.
• Two types: chromosomal mutation and gene mutation.

Chromosomal mutation	Gene mutation
Change in chromosome number or structure due to a defect during cell division	Chemical change in a gene that changes the characteristic controlled by that gene
Down syndrome, Turner syndrome, Klinefelter syndrome	Colour blindness, sickle-cell anaemia, thalassaemia, albinism, haemophilia

Examples of Chromosomal Mutation

• Down syndrome
  • One extra chromosome at chromosome pair 21.
  • Karyotype contains 47 chromosomes instead of 46.
  • Characteristics: physical and mental retardation, short neck, slanted eyes and a short, stocky body.
  • Textbook data: about 1 in 800 births; risk rises when maternal age exceeds 35 years.
• Turner syndrome (XO)
  • Female loses one X chromosome.
  • Karyotype: 45 chromosomes (44 + XO).
  • Female secondary sexual characteristics do not develop.
• Klinefelter syndrome (XXY)
  • Male gains one X chromosome.
  • Karyotype: 47 chromosomes (44 + XXY).
  • Characteristics: female characteristics such as breasts, small testes and sterility.

Examples of Gene Mutation

• Colour blindness
  • Caused by a mutant recessive gene on the X chromosome.
  • The individual cannot distinguish red from green.
  • More common in males.
• Sickle-cell anaemia
  • Spontaneous change in the gene responsible for haemoglobin production.
  • Abnormal, sickle-shaped red blood cells impair oxygen transport.
  • Caused by a recessive gene on an autosome.
• Thalassaemia
  • Mutation in a gene controlling haemoglobin production.
  • Red blood cells are small and have shorter lifespans, causing severe blood deficiency.
• Haemophilia
  • Mutation in a gene that produces a blood-clotting factor.
  • Blood clots slowly; an injured individual continues to lose blood.

Factors Causing Gene and Chromosomal Mutations

• A mutation may occur spontaneously (naturally) during cell division.
• External factors that cause mutations are called mutagens.
• Textbook factors:
  • pregnancy at a late age;
  • radioactive rays;
  • X-rays;
  • ultraviolet rays;
  • carcinogens;
  • natural occurrence.

Gene Disorder Diseases and Inheritance

• Alleles may carry disease traits that can be inherited in a family.
• Most genes controlling traits occur on autosomes; some occur on sex chromosomes.
• A trait on a sex chromosome is a sex-linked trait; its gene is a sex-linked gene.
• Haemophilia is an example of a sex-linked trait.
• Let $X^H$ = dominant normal allele and $X^h$ = recessive haemophilia allele.

Normal father × carrier mother

$X^{H}Y\times X^H{X}^h$

• Possible children: $X^H{X}^H$ normal daughter, $X^H{X}^h$ carrier daughter, $X^{H}Y$ normal son, $X^{h}Y$ son with haemophilia.

Normal father × mother with haemophilia

$X^{H}Y\times X^h{X}^h$

• Possible children: all daughters $X^H{X}^h$ are carriers; all sons $X^{h}Y$ have haemophilia.

Detecting Gene Disorder Diseases

• Amniocentesis and karyotyping detect chromosome abnormalities and allow earlier detection of gene disorder diseases.

Amniocentesis

• Used to detect abnormalities in foetal cells during the 15th–20th weeks of pregnancy.

1. Locate the foetus in the uterus by ultrasound to identify a safe position.
2. Carefully insert a needle through the mother’s abdomen and uterine wall.
3. Extract amniotic fluid containing suspended foetal cells.
4. Centrifuge the fluid to separate foetal cells from the amniotic fluid.
5. Use the isolated foetal cells to obtain a karyotype.

Karyotyping

1. Obtain a foetal-cell or body-tissue sample.
2. Add a chemical to stimulate mitosis; incubate for 2–3 days.
3. Add another chemical to stop mitosis at metaphase.
4. Transfer cells into a tube and centrifuge to concentrate them.
5. Transfer cells into a new tube containing a fixative.
6. Place drops of the solution on a microscope slide and add stain to make chromosomes clearer.
7. Observe and photograph the slide under a microscope.
8. Cut out and arrange the chromosome images to form a karyotype; inspect it for chromosome abnormalities.

Applications of Genetic Research

Forensic Science

• Field of science and technology that investigates crime by identifying and confirming an event’s chronology from scientific evidence.
• Provides scientific information to the legal system through analysis of physical evidence.
• Evidence is collected at a scene or from an involved person, analysed in a laboratory and presented in court.
• DNA from skin fragments, hair or blood may be matched with a suspect’s DNA sample.

Gene Therapy

• An experimental technique intended to repair mutant, abnormal or defective genes causing diseases such as cystic fibrosis, haemophilia and sickle-cell anaemia.
• A normal gene is inserted into the patient’s cells or tissues to replace the damaged gene.

1. Remove stem cells from the patient.
2. Insert a normal gene into a virus.
3. Mix the modified virus with the patient’s stem cells.
4. The patient’s stem cells become genetically modified.
5. Inject the cells into the patient.
6. Modified cells produce the required protein or hormone.

Genetic Genealogy

• Study involving the collection of genetic information to determine a family’s lineage, ancestry and history.
• Uses DNA testing.

Effects of Genetic Research on Human Life

• Genetic research is widely used in medicine and agriculture to improve quality of life.
• Genetic screening must be evaluated for advantages and disadvantages in these textbook aspects:
  • economy: insurance and employment opportunities;
  • well-being: family institution;
  • ethics;
  • psychology;
  • social effects.
• Ethical practice and good values are essential; without them, genetic research may create moral, religious, economic, psychological and social problems.