Question

Define tumor-suppressor genes. Why is a mutation in a single copy of a tumor- suppressor gene expected to behave as a recessive gene?

Short answer

Answer: Tumor-suppressor genes are a type of genes that regulate the cell cycle and prevent tumor formation by controlling cell growth and division. A mutation in a single copy of these genes behaves as a recessive gene because the other non-mutated copy can still produce enough functional protein to regulate the cell cycle and prevent tumor formation, thus masking the effect of the mutated gene.

Step-by-step solution

Define tumor-suppressor genes

Tumor-suppressor genes are a type of genes that play a critical role in regulating the cell cycle and preventing the formation of tumors. They act as a safety mechanism that controls cell growth and division, ensuring that cells with damaged DNA do not replicate uncontrollably and develop into cancerous tumors. When functioning properly, tumor-suppressor genes produce proteins that inhibit cell division and promote the repair or elimination of damaged cells.

Understand the way genes are inherited

In humans, genes are inherited in pairs, with one copy coming from each parent. They can be dominant or recessive, depending on how they affect an individual's traits. A dominant gene is one that masks the effects of another gene, while a recessive gene shows its effect only when both copies are mutated or non-functional. In other words, an individual must inherit the mutated gene from both parents for the recessive trait to manifest.

Explain why a mutation in a single copy of a tumor-suppressor gene behaves as a recessive gene

A mutation in a single copy of a tumor-suppressor gene is expected to behave as a recessive gene because it usually takes more than one mutation in the gene for it to become non-functional and lead to abnormal cell growth. When one copy of the tumor-suppressor gene is mutated, the other (non-mutated) copy can still produce enough functional protein to regulate the cell cycle and prevent the formation of tumors. Thus, an individual with just one mutated copy of a tumor-suppressor gene would typically not develop cancer, as the normal function of the gene is maintained by the non-mutated copy. In this case, the mutated gene is recessive because its effect is masked by the presence of a functional (dominant) copy of the gene. So, to summarize: 1. Tumor-suppressor genes regulate the cell cycle and prevent tumor formation. 2. Genes are inherited in pairs and can exhibit dominant or recessive inheritance patterns. 3. A mutation in a single copy of a tumor-suppressor gene behaves as a recessive gene because the other non-mutated copy still maintains its normal function and can prevent tumor formation.

Key concepts

Cell Cycle Regulation

The cell cycle is an orderly sequence of events in which a eukaryotic cell duplicates its contents and divides into two. Tumor-suppressor genes are intrinsic components of the regulatory system that orchestrates the cell cycle.

They function as 'cellular brakes,' halting cell division if DNA damage is detected, allowing time for repair or, if repair is unfeasible, directing the cell towards programmed cell death, or apoptosis. Without the control provided by these genes, cells can multiply wildly, setting the stage for cancer.

BRCA Genes: Protectors Turned Perpetrators

BRCA1 and BRCA2 are well-known tumor-suppressor genes related to breast cancer. When they mutate, their protective role falters, potentially leading to unchecked cell proliferation. These mutations serve as poignant examples of how crucial the integrity of tumor-suppressor genes is to cell cycle regulation and cancer prevention.

Genetic Inheritance

Understanding how traits are passed down from parents to offspring is foundational to genetics. In humans, genes come in pairs, with each parent contributing one gene to the pair. This is where dominant and recessive genetic patterns play a role.

With dominant genes, just one copy of a gene variant is sufficient to express a particular trait. In contrast, recessive genes require both gene copies to be the same variant for the trait to manifest.

Cystic Fibrosis: A Classic Case of Recessive Inheritance

For cystic fibrosis, a life-threatening disorder, two copies of the mutated CFTR gene—one from each parent—are necessary for the disease to develop. This underscores the fundamental nature of inheritance patterns in understanding genetic diseases and responses to mutations such as those occurring in tumor-suppressor genes.

Recessive Genes

Recessive genes come into the spotlight when discussing genetic conditions that require two copies of a mutated gene for the trait or disease to appear. These genes can be metaphorically considered the 'underdogs' of genetics — they don't get to show their effects unless they're in the majority, which means having both alleles (gene variants) in the recessive form.

Such is the case with many genetic disorders, but most relevantly for our discussion, tumor-suppressor genes. When one copy of a tumor-supressor gene is mutated and the other is not, the healthy gene can often compensate. This means that the individual remains healthy, or 'phenotypically normal', despite having a mutation.

Understanding Carriers

An individual with one mutated suppressor gene is a carrier. While they may not display symptoms of a condition, they can pass the gene to their offspring, who, if they inherit another mutated copy from the other parent, may exhibit the disease or condition. This puts recessive genes at the core of many genetic testing and counseling discussions.