What Are Genetic Disorders?
Most of the time, inherited traits like eye colour or blood type don't cause health problems — they're just part of what makes you unique. But some inherited traits involve alleles that disrupt the normal functioning of the body, leading to genetic disorders.
A condition caused by an abnormal allele or set of alleles inherited from one or both parents that affects the normal development or functioning of the body.
Genetic disorders are not infections — you cannot catch them from another person. They are passed down through families in predictable patterns, which is why doctors ask about your family medical history.
To understand how genetic disorders are inherited, we need to revisit two key ideas:
- Dominant alleles — only one copy is needed for the trait to appear
- Recessive alleles — two copies are needed for the trait to appear
Think of dominant alleles like a loud friend who always speaks over everyone — even one dominant allele in the pair will be heard. A recessive allele is the quiet friend who only gets a turn to speak when the loud one isn't around (i.e., when both alleles are recessive).
Dominant vs Recessive Inheritance: A Quick Recap
Before diving into specific disorders, let's make sure the key vocabulary is solid.
One of two or more versions of a gene. For example, the gene for a particular protein may have a 'working' allele and a 'non-working' allele.
An allele whose effect is expressed in the phenotype even when only one copy is present (i.e., in a heterozygous individual). Represented by a capital letter (e.g., H).
An allele whose effect is only expressed in the phenotype when two copies are present (i.e., in a homozygous recessive individual). Represented by a lowercase letter (e.g., h).
An individual who is heterozygous for a recessive allele (e.g., Hh). They do not show the disorder themselves but can pass the recessive allele to their children.
The table below summarises the possible genotypes and their phenotypic outcomes for a single gene with one dominant (D) and one recessive (d) allele:
| Genotype | Description | Recessive disorder (e.g., CF) | Dominant disorder (e.g., Huntington's) |
|---|---|---|---|
| DD | Homozygous dominant | Unaffected | Affected |
| Dd | Heterozygous | Unaffected (carrier) | Affected |
| dd | Homozygous recessive | Affected | Unaffected |
Students often confuse dominant with common and recessive with rare. This is incorrect! A dominant allele simply masks the recessive one — it doesn't mean it's more common in the population. In fact, some dominant disorder alleles are quite rare.
10 more sections in this topic
Pick this up in your Library: it holds the whole topic, notes, cheatsheet and questions.