From genes to phenotype
From genes to phenotype
- A gene codes for a protein, and that protein affects the phenotype.
- If the gene is faulty, the protein is faulty — and the phenotype changes.
- A few classic examples show this clearly.
Practice
How does a faulty allele change the phenotype?
A gene codes for a protein; a faulty allele makes a faulty protein, altering the phenotype.
Faulty alleles, faulty proteins
| Gene | Protein | Faulty allele causes |
|---|---|---|
| TYR | tyrosinase (enzyme) | albinism (no pigment) |
| HBB | haemoglobin | sickle cell anaemia |
| F8 | factor VIII (clotting) | haemophilia |
| HTT | huntingtin | Huntington's disease |
Practice
Albinism is caused by a faulty allele of the TYR gene because:
TYR codes for tyrosinase; a faulty version cannot make pigment, causing albinism.
Practice
Match each gene to the condition a faulty allele causes.
HBB → haemoglobin (sickle cell); F8 → factor VIII (haemophilia); HTT → huntingtin (Huntington's).
Gibberellin and stem height
- In pea plants, the dominant allele Le codes for a working enzyme that makes gibberellin, so the plant grows tall.
- The recessive allele le codes for a broken enzyme, so little gibberellin is made and the plant is short.
Practice
A pea plant with two le alleles is short because:
le codes for a broken enzyme; with little gibberellin, the stem does not elongate, so the plant is short.
You've got it
Key idea
- gene → protein → phenotype; a faulty allele → faulty protein → changed phenotype
- examples: TYR (albinism), HBB (sickle cell), F8 (haemophilia), HTT (Huntington's)
- pea height: Le (working enzyme → gibberellin → tall) vs le (broken → short)