---
title: "Oncogenes and Tumour Suppressors: Why One Allele Is Sometimes Enough"
description: "Gain-of-function and loss-of-function lesions behave differently in a pedigree and in a tumour: a single altered allele can drive, while a suppressor usually needs both copies lost. This stop reads th"
canonical: https://lightmysky.com/learn/science/oncogenes-and-tumour-suppressors-why-one-allele-is-sometimes-enough-mt_7WTgv1dLNx
source: https://lightmysky.com/learn/science/oncogenes-and-tumour-suppressors-why-one-allele-is-sometimes-enough-mt_7WTgv1dLNx.md
retrieved: 2026-09-12
---

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# Oncogenes and Tumour Suppressors: Why One Allele Is Sometimes Enough

Gain-of-function and loss-of-function lesions behave differently in a pedigree and in a tumour: a single altered allele can drive, while a suppressor usually needs both copies lost. This stop reads that difference out of inheritance patterns and tumour genotypes.

Subject: Science · Area: Genetics & Evolution · Ages 22 to 23
Page: https://lightmysky.com/learn/science/oncogenes-and-tumour-suppressors-why-one-allele-is-sometimes-enough-mt_7WTgv1dLNx

## Ready when they can

- Predict dominance or recessiveness from the mechanism of a stated mutation
- Explain loss of heterozygosity and how it is detected in tumour sequence data
- Use a familial cancer pedigree to argue which class a gene belongs to

## Lesson: Why one bad copy can be enough, or not

You sort cancer genes by mechanism, not by name. Proto-oncogenes relay growth orders from the surface to the nucleus. A mutation locking one relay in the on position makes the cell divide without orders, and a single mutant copy suffices because the stuck signal dominates.

You treat suppressors the opposite way. They build the brakes, so losing one copy still leaves a working brake and the defect is recessive at the cellular level. Trouble starts when the second copy is lost in some unlucky cell.

**Example.** You compare normal and tumour sequence from one patient. The blood shows two variants at nearby markers, but the tumour shows only one: the region with the healthy copy was deleted. That loss of heterozygosity reveals the second hit directly.

**Tip.** You read pedigrees for the gene class. Early bilateral tumours across generations point to an inherited suppressor defect, since each cell already lacks one copy and needs one further hit. Sporadic single tumours fit an acquired dominant oncogene event instead. Repair gene loss raises the mutation rate without driving growth itself, supplying raw material that accelerates every later malignant step.

**Recap.** You call dominant stuck signals oncogenes, recessive lost brakes suppressors, and you confirm the second hit with tumour versus normal sequence.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [What a Cell Has to Break to Become a Tumour](https://lightmysky.com/learn/science/what-a-cell-has-to-break-to-become-a-tumour-mt_cjg3DtLHQL)
- [Receptor Tyrosine Kinases and Phosphorylation Cascades](https://lightmysky.com/learn/science/receptor-tyrosine-kinases-and-phosphorylation-cascades-mt_EvIwqsLVGA)

## Opens up

- [The Tumour Microenvironment: Stroma, Hypoxia and Immune Exclusion](https://lightmysky.com/learn/science/the-tumour-microenvironment-stroma-hypoxia-and-immune-exclusion-mt_OIrMAjomX_)