Lesson 8 of 10 / Inheritance
Gene and chromosome mutations
How is a change in one gene different from an extra chromosome?
In this lesson: Describe gene and chromosome-number mutations, with sickle-cell anaemia, Down syndrome and factors increasing mutation rate.
About 5 min
The key ideaMutation changes genetic information. It may alter a gene sequence or chromosome number; ionising radiation and chemical mutagens can increase its rate.
A gene mutation changes its DNA sequence and may affect a polypeptide. Sickle-cell anaemia involves a haemoglobin-gene change; the short sequence here is an invented illustration, not the real mutation.
Gold: changed base or extra chromosome. Ionising radiation and chemical mutagens can increase mutation rates. Neither exposure nor need guarantees a specific change.
Explanation
A gene mutation is a change in the DNA sequence of a gene. This can change the polypeptide made and affect a characteristic. Some sequence changes have little or no observable effect; mutations are not all harmful or all beneficial.
Sickle-cell anaemia provides an example involving a gene for a haemoglobin polypeptide. A base-sequence change alters the polypeptide, producing a form of haemoglobin that can cause red blood cells to become sickle-shaped under low-oxygen conditions. This is not a change in the total chromosome number.
A chromosome-number mutation changes the number of chromosomes. A common form of Down syndrome involves three copies of chromosome 21 and a total of 47 chromosomes, rather than the usual 46. It can arise when chromosomes fail to separate normally during cell division.
Mutations can arise spontaneously. Ionising radiation, including X-rays, and certain chemicals called chemical mutagens can increase the mutation rate. Exposure raises probability; it does not guarantee a specific useful mutation. In sexual reproduction, a mutation present in a gamete can be inherited through fertilisation; a mutation confined to a body cell is not passed on that way.
Step by step
- 1
Identify the level of change
Decide whether the evidence describes a sequence change or an altered chromosome number.
- 2
Link to an example
Sickle-cell anaemia involves a haemoglobin gene; the common trisomy-21 form of Down syndrome involves chromosome number.
- 3
Explain risk probabilistically
Mutagens increase the rate of changes, not a guaranteed chosen outcome.
Worked example
Two different observations
A shows an altered DNA base sequence in one haemoglobin gene. B has 47 chromosomes with an extra chromosome 21. Classify the changes.
One way to explain it
A is a gene mutation. B is a chromosome-number mutation, illustrating the common trisomy-21 form of Down syndrome.
Why this answer works
- Use the kind of evidence provided.
- A changed gene sequence does not necessarily change chromosome number.
- An extra chromosome contains many genes.
Is this true? "Organisms produce the mutations they need when conditions become difficult."
Mutations do not arise because an organism chooses a helpful change. Existing heritable variation can later be affected by natural selection.