Hereditary disorders, genetic counselling, the human genome and genetic engineering
Defective genes cause hereditary disorders such as thalassemia, haemophilia and colour blindness. Genetic counselling, the human genome and genetic engineering help prevent or treat them.
How a hereditary disorder arises
Genes control all our features. Sometimes a gene changes (mutates) in a gamete or a zygote and stops working normally. A parent may carry one defective recessive gene and still be healthy, because the normal dominant gene of the pair masks it. Such a person is a carrier. If a child gets the defective gene from both parents, the disorder shows. Some disorders are caused by just one defective dominant gene. Medicines cannot cure these disorders. Scientists are working on gene replacement therapy, which aims to replace a defective gene with a normal one.
Three common hereditary disorders
| Disorder | What goes wrong | Where the gene is | Effect |
|---|---|---|---|
| Thalassemia (spelt thallasemia in your NIOS lesson) | The pair of genes for making haemoglobin, the oxygen-carrying pigment of red blood cells, is defective | An autosome | Patients need frequent blood transfusions to survive |
| Haemophilia | A gene for a blood-clotting substance is defective or missing | The X chromosome | Once bleeding starts, the blood does not clot easily |
| Colour blindness | A gene for colour vision is defective or missing | The X chromosome | The commonest kind cannot tell red from green; your NIOS lesson says blue from green |
Worked example: why X-linked disorders show up in sons
A woman carries the haemophilia gene on one of her X chromosomes; her husband is normal. Write Xh for an X with the defective gene. What can happen to their sons and daughters?
- Mother: Xh X (a carrier, healthy, because her normal X masks the defect). Father: X Y.
- Sons get Y from the father and one X from the mother: Xh Y or X Y.
- So each son has a 1 in 2 chance of haemophilia, since his only X cannot be masked.
- Daughters get the father's normal X and one X from the mother: Xh X or X X.
- So daughters are healthy, but each has a 1 in 2 chance of being a carrier.
Sons: 1 in 2 chance of haemophilia. Daughters: healthy, with a 1 in 2 chance of being carriers
Genetic counselling and marriage between relatives
Marriage between close relatives is called consanguineous marriage. It is discouraged because relatives may carry the same defective recessive gene, so the chance that a child gets two copies goes up. Genetic counsellors are trained professionals. They study the family history and work out the chance of a disorder in a child. This helps people make informed decisions before they have children.
The human genome
All the genes of a species together form its genome. Humans are complex, so many genes are needed. By 2003, scientists had found where most human genes lie on the chromosomes and the order of nucleotides in them. Knowing the human genome helps doctors find and plan treatments for genetic defects, which gives hope to people with hereditary disorders.
Genetic engineering in simple steps
Genetic engineering is also called the recombinant DNA technique.
- 1The useful gene is cut out of the DNA of a cell by enzymes called restriction enzymes, which act like scissors. Your NIOS lesson does not name them, but the exam has asked about them.
- 2★The gene is joined into a plasmid, a small circular DNA found in bacteria that is not part of the bacterial chromosome; an enzyme called ligase seals the join. A bacteriophage (a virus that attacks bacteria) can also carry genes.
- 3The plasmid enters a bacterium, which now carries a foreign gene.
- 4★The bacterium multiplies into a clone (a group of identical copies), all carrying the gene.
- 5The gene or its product is collected and used, for example to replace a defective gene in gene therapy. An organism that carries a gene from another species is a genetically modified organism (GMO).
★ key step
Thalassemia testing before marriage or pregnancy is encouraged in India, and thalassemic children depend on regular blood donations. Insulin for people with diabetes is now made by genetically engineered bacteria.
1 marks: “Name one hereditary disorder whose gene lies on an autosome.” Thalassemia.
2 marks: “A mother with normal blood clotting has a son with haemophilia. Explain how this can happen.” The mother is a carrier: one of her X chromosomes has the defective gene, masked by her normal X. The son got that X; with only one X, nothing masks it.
3 marks: “What is genetic engineering? Explain the role of plasmids in it and give two of its uses.” Transfer of a gene from one species to another (recombinant DNA). Plasmid: circular bacterial DNA, outside the chromosome, that carries the gene into a bacterium, which then multiplies. Uses: making insulin; gene therapy; pest-resistant crops.
T on an autosome, H and C on the X: thalassemia, haemophilia, colour blindness. One X in boys means no back-up.