Home > Articles

  • Print
  • + Share This
This chapter is from the book

How Genes Work and Why They Come in Different Flavors

Even if you haven’t asked yourself why it is that genes makes us sick, perhaps you have wondered why it is that your sister has legs up to her ears and piercing blue eyes that haven’t been seen in the family since Great-Aunt Bessie, while you seem to have inherited a horrible mix of dad’s stockiness and mom’s frumpiness? And what’s up with your brother’s moroseness: Where did that come from?

It is not much of an explanation, but the straight answer is that genetics is a lot more complex than the idea that there’s a gene for every trait. Most traits, or attributes, are regulated by many genes, not just one. Furthermore, while it is a nice abstraction to suppose that genes come in normal, or good, versions and mutant, or bad, ones, the reality is that there are always multiple different flavors of normal. The gradation from the most common allele to various types of normal alleles to abnormality is continuous. Just having certain alleles is insufficient to predict whether a person will get a disease.

Crucially, too, the environment has a pervasive effect on the way our genes function. “Environment” means much more than the temperature outside or the nutritional content of the food we eat. It also includes influences as diverse as a mother’s health during pregnancy and the pressure that peers and society put on us to behave in certain ways. As we shall see, in many cases environmental interventions are likely to have a much greater impact on public health than pharmaceutical ones. Unfortunately, most of us find it easier to pop a pill than to buck a social trend, so drugs are likely to have an ever-increasing role in disease control.

Without going into any mechanistic details, it is helpful to recognize that genes function on two levels, the biochemical and the biological. The biochemical is hidden to most observers, and therefore typically excluded from general conversation. The biological is what we actually see.

Each of the 23,000 or so genes scattered along our chromosomes encodes the information to perform a specific biochemical function. Less than a third of these genes function in every cell in your body to provide the basic building blocks and to generate energy—they are the bricks and mortar, if you like. Another third of our genes makes every one of the hundreds of different cell types in your body different. Neurons need proteins that process electrical signals, muscle cells are full of actin and myosin that make them stretch and contract, and white blood cells carry around the components of your immune system. These are the doors and windows and furniture and appliances. The final third of our genes is responsible for regulating which genes are used when and where and in what amount. Turning on hair keratin in your pancreas wouldn’t be good, and light receptors have no place in your heart, so development and physiology are highly regulated processes. These genes are the architects, foremen, and designers.

We hear and read about genes for cancer and for autism, or are given to believe that there is an aggression gene or a blonde hair gene. The reality is that these attributes are many steps removed from the molecular functions that the genes perform. If a gene contributes to cancer, it is because it normally performs a role in making sure that the right number of cells are produced at the right time and place. The reason there may be a genetic contribution to spirituality is not because some genes function to ensure that we have a belief in God, but rather because there are genes that affect how the neurons are wired together and the strength of signaling across synapses.

Fly geneticists like to name genes after the way flies look when the gene is mutated. Antennapedia flies have legs on their heads, technical knock out ones fall over when you bump their heads, and shaven baby embryos don’t have any hairs. It is an amusing, but unfortunate habit, because it reinforces the notion that there are genes for traits. Time after time it turns out that the same gene does completely different things in different contexts. A favorite example of mine is staufen, which is required both for sperm development and for memory. It is not that male flies think with their penises, but rather that both of these attributes turn out to depend on a biochemical process called intracellular RNA localization, which staufen is involved in. Almost without exception the biological functions of genes are not written in the DNA, but rather emerge from the network of biochemical interactions within cells, and in turn the manner in which cells work together to build tissues and organs.

It follows that the reason we are all a little different from one another is because these interactions occur between ever so slightly different copies of the genes. Each gene comes in multiple different flavors—I mean, alleles—that have cropped up during the evolution of the species. These different alleles have their origin in the process of mutation, which is basically what happens to genes when you leave them out in the sun or exposed to poisons.

Mutations are ultimately the source of all things good, but for the most part are harmful, tending to break genes. Every one of us has a few mutations that neither of our parents had, simply because mistakes are made every time the genome is copied. (But don’t get too upset about this: The error rate is only about one in a billion letters in the DNA. Most of us would be thrilled to make a mistake only once in every hundred times we do something.) Mutations are also so plentiful that we all carry several of them that would kill us if we got the same one from both parents.

Mutations are so plentiful in fact that there is no way that natural selection can possibly purge them all. Obviously alleles that would tend to kill a person will not generally last long in the gene pool, and similarly ones that would tend to make us sick should not fare well either. But all new mutations are extremely rare when they appear, and nature has bigger fish to fry. It is more concerned with common alleles that affect the fitness of a large percentage of the population, so the fate of new mutations is largely governed by chance. Consequently, some mutations manage to drift around for a while and can even become reasonably common before they start having a noticeable effect on public health. The process is called mutation-selection-drift balance, which is a fancy way of saying that a lot of bad things happen to genomes, and evolution deals with them, but it is so busy that some of the bad things hang around for a while.

Some mutations are also good for you. Maybe they offer protection from diabetes; maybe they make a person more fertile. These tend to be favored by natural selection, but before they become the standard allele, they necessarily share real estate in the genome with the original allele. Typically it takes thousands of generations for one allele to replace another, so in the meantime you have variation. Sometimes the new allele will be better under some conditions, while the ancestral one is better under others. Maybe they have different effects in men and women, or in rural and urban settings. In such cases, geneticists speak of balanced polymorphisms, the classic case being sickle cell anemia, which is bad under some circumstances but protects a person from malaria in others.

You will also see it argued that many of the bad effects are actually offset by some absolute good that they do. Perhaps at a different stage of life they are sufficiently beneficial that natural selection overlooks their contribution to disease. Or perhaps at some earlier phase of human evolution they were the right gene in the right place at the right time. It is easy to get carried away with devising clever stories along these lines. Some, particularly in the domain of psychology, are even tempted to postulate that promoting disease is in itself advantageous to the selfish genes, but it really stretches credulity to suppose that there is some benefit to having genes that make us suicidal. We won’t go down that road. Rarely is it necessary anyway.

It turns out that as species go, humans are actually among the least variable, at least at the level of their DNA. Nevertheless, the average person has a few million differences between the copy of his genome received from his mother and the copy received from his father. Somewhere among all those differences are the genetic variants that are responsible for all genetic diseases, but no more than a couple dozen have a big enough impact on any particular disease for us to have any hope of finding them. Finding a few dozen out of a few million is a genuine needle-in-a-haystack problem.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020