Genetic disordersOxford AQA IGCSE Biology: Revision notes
Section 1
Why does teaching genetic disorders need particular care?
Genetic disorders are conditions caused by inherited alleles or by an abnormal number of chromosomes. These notes cover only what the specification asks for: interpreting data about three named conditions (polydactyly, cystic fibrosis, sickle cell anaemia) and understanding that some disorders are caused by having the wrong number of chromosomes. The focus throughout is on the biology — inheritance patterns, alleles and genetics — not on describing what living with a condition is like. When you see data in a question (a family tree, a set of numbers, a probability), your job is to read it correctly and apply genetics, not to speculate beyond what is given.
Section 2
What is polydactyly and how is it inherited?
Polydactyly is a condition in which a person is born with one or more extra fingers or toes. It is caused by a dominant allele.
Because the allele is dominant, a person only needs to inherit one copy of it (from either parent) to have polydactyly. This means:
- A person who is homozygous dominant (two copies of the allele) has polydactyly
- A person who is heterozygous (one copy of the allele, one copy of the normal allele) has polydactyly
- Only a person who is homozygous recessive (two normal alleles) does not have polydactyly
Example cross: if one parent is heterozygous for the polydactyly allele (Dd) and the other parent does not carry the allele (dd), the possible offspring genotypes are:
| D | d | |
|---|---|---|
| d | Dd | dd |
| d | Dd | dd |
This gives a ratio of 2 Dd : 2 dd, so on average half of the offspring (50%) are expected to inherit the allele and show polydactyly, and half are not.
Because the allele is dominant, polydactyly can appear in every generation of a family if an affected parent passes the allele on — it does not skip generations the way a recessive condition can.
In a genetic diagram, always show both the genotypes of the parents and the possible genotypes and ratios of the offspring. For a dominant condition, an affected parent is either homozygous dominant or heterozygous — check the data given in the question to work out which.
Section 3
What is cystic fibrosis and how is it inherited?
Cystic fibrosis is a disorder of cell membranes, caused by a recessive allele. It affects the movement of salts and water across cell membranes, particularly in the lungs and digestive system.
Because the allele is recessive, a person only has cystic fibrosis if they inherit two copies of the recessive allele (one from each parent) — they must be homozygous recessive.
- A person with one copy of the recessive allele and one copy of the normal (dominant) allele is heterozygous; they do not have cystic fibrosis themselves but are described as a carrier, because they can pass the recessive allele on to their children
- Two carrier parents can have a child with cystic fibrosis, even though neither parent has the condition
Example cross: if both parents are carriers (each Ff, where f is the recessive cystic fibrosis allele), the possible offspring genotypes are:
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
This gives a ratio of 1 FF : 2 Ff : 1 ff. So, on average, 1 in 4 (25%) of the offspring are expected to have cystic fibrosis (ff), 2 in 4 (50%) are expected to be carriers (Ff) without symptoms, and 1 in 4 (25%) are expected to have neither copy of the allele (FF).
When interpreting family-tree or probability data for cystic fibrosis, look for cases where two unaffected parents produce an affected child — this is a strong clue that the condition is recessive, because it means both parents must be carriers.
A common error is assuming that because a condition is rare, it must be dominant. Recessive conditions like cystic fibrosis can remain common in a population precisely because carriers show no symptoms and are not selected against — always use the pattern of inheritance in the data, not how common the condition is, to decide dominant or recessive.
Section 4
What is sickle cell anaemia and how is it inherited?
Sickle cell anaemia is a condition affecting red blood cells, caused by a recessive allele that changes the shape of haemoglobin, altering the shape of red blood cells.
As with cystic fibrosis, a person only has sickle cell anaemia if they inherit two copies of the recessive allele (homozygous recessive). A person with one recessive allele and one normal allele is a carrier and does not have the condition.
Example cross: if both parents are carriers (each Ss, where s is the recessive sickle cell allele), the possible offspring genotypes follow the same 1 : 2 : 1 pattern as cystic fibrosis:
| S | s | |
|---|---|---|
| S | SS | Ss |
| s | Ss | ss |
On average, 1 in 4 (25%) of offspring are expected to have sickle cell anaemia (ss), 2 in 4 (50%) are expected to be carriers (Ss), and 1 in 4 (25%) are expected to carry neither recessive allele (SS).
When a question gives you data on sickle cell anaemia (for example, numbers of affected and unaffected individuals in a population or family), apply the same recessive-inheritance reasoning used for cystic fibrosis: two unaffected carrier parents can still have an affected child.
Sickle cell anaemia and cystic fibrosis are both recessive, so they follow the same genetic-diagram method. When you are given data (rather than asked to construct the cross yourself), your task is to read off genotype ratios or probabilities from what is provided, applying the recessive inheritance pattern correctly.
Section 5
How can an abnormal number of chromosomes cause a genetic disorder?
Not all genetic disorders are caused by a single faulty allele. Some are caused by a person inheriting the wrong number of chromosomes.
Down's Syndrome is an example of this kind of disorder. It is caused by the presence of an extra chromosome — instead of the usual pair, an individual with Down's Syndrome has three copies of one particular chromosome.
This is a different mechanism from polydactyly, cystic fibrosis and sickle cell anaemia, which are all caused by inheriting particular alleles of a single gene. A chromosome-number disorder is instead caused by an error during the formation of gametes (sex cells), so the resulting individual has an unusual total number of chromosomes in every body cell.
When interpreting data about a chromosome-number disorder, the key point to identify is the number of chromosomes present, rather than which allele of a gene has been inherited.
Must Know
- Polydactyly is caused by a dominant allele — only one copy is needed for the condition to appear.
- Cystic fibrosis is caused by a recessive allele — a person needs two copies (homozygous recessive) to have the condition; a person with one copy is an unaffected carrier.
- Sickle cell anaemia is caused by a recessive allele, inherited in the same pattern as cystic fibrosis.
- Two unaffected carrier parents can produce a child with a recessive disorder — this is a key clue when reading family-tree or probability data.
- Down's Syndrome is caused by an abnormal number of chromosomes (an extra chromosome), not by a single faulty allele — a different mechanism from the other three conditions.
- When answering questions, describe conditions in terms of inheritance and genetics only.
That's the notes covered.
Carry on to the next subtopic.