Carl Zimmer writes at Skeptical Inquirer Seven Big Misconceptions About Heredity. Excerpts in italics with my bolds

It’s been seven decades since scientists demonstrated that DNA is the molecule of heredity. Since then, a steady stream of books, news programs, and episodes of CSI have made us comfortable with the notion that each of our cells contains three billion base pairs of DNA, which we inherited from our parents. But we’ve gotten comfortable without actually knowing much at all about our own genomes.

If you want to get your entire genome sequenced—all three billion base pairs in your DNA—a company called Dante Labs will do it for $699. You don’t need whole genome sequencing to learn a lot about your genes, however. The 20,000 genes that encode our proteins make up less than 2 percent of the human genome. That fraction of the genome—the “exome”—can be yours for just a few hundred dollars. The cheapest insights come from “genotyping”—in which scientists survey around a million spots in the genome known to vary a lot among people. Genotyping—offered by companies such as 23andMe and Ancestry—is typically available for under a hundred dollars.

Thanks to these falling prices, the number of people who are getting a glimpse at their own genes is skyrocketing. By 2019, over twenty-five million worldwide had gotten genotyped or had their DNA sequenced. At its current pace, the total may reach 100 million by 2020.

There’s a lot we can learn about ourselves in these test results. But there’s also a huge opportunity to draw the wrong lessons.

Many people have misconceptions about heredity—how we are connected to our ancestors and how our inheritance from them shapes us. Rather than dispelling those misconceptions, our growing fascination with our DNA may only intensify them. A number of scientists have warned of a new threat they call “genetic astrology.” It’s vitally important to fight these misconceptions about heredity, just as we must fight misconceptions about other fields of science, such as global warming, vaccines, and evolution. Here are just a few examples.

Misconception #1: Finding a Special Ancestor Makes You Special

You can join the Order of the Crown of Charlemagne if you can prove that the Holy Roman Emperor is your ancestor. It’s a thrill to discover we have a genealogical link to someone famous—perhaps because that link seems to make us special, too.

But that’s an illusion. I could join the Mayflower Society, for example, because I’m descended from a servant aboard the ship named John Howland. Howland’s one claim to fame is that he fell out of the Mayflower. Fortunately for me, he got fished out of the water and reached Massachusetts. But I’m not the only fortunate one; by one estimate, there are two million people who descend from him alone.

Mathematicians have analyzed the structure of family trees, and they’ve found that the further back in time you go, the more descendants people had. (This is only true of people who have any living descendants at all, it should be noted.) This finding has an astonishing implication. Since we know Charlemagne has living descendants (thank you, Order of the Crown!), he is likely the ancestor of every living person of European descent.

Misconception #2: You Are Connected to All Your Ancestors by DNA

But genetics do not equal genealogy. It turns out that practically none of the Europeans who descend from Charlemagne inherited any of his DNA. All humans, in fact, have no genetic link to most of their direct ancestors.

The reason for this disconnect is the way that DNA gets passed down from one generation to the next. Every egg or sperm randomly ends up with one copy of each chromosome, coming either from a person’s mother or father. As a result, we inherit about a quarter of our DNA from each grandparent—but only on average.

If you go back a few generations more, that contribution can drop all the way to zero. . . While it is true that you inherit your DNA from your ancestors, that DNA is only a tiny sampling of the genes in your family tree.

Even without a genetic link, though, your ancestors remain your ancestors. They did indeed help shape who you are—not by giving you a gene for some particular trait, but by raising their own children, who then raised their own children in turn, passing down a cultural inheritance along with a genetic one.

Misconception #3: Ancestry Tests Are as Reliable as Medical Tests

Millions of people are getting ancestry reports based on their DNA. My own report informs me that I’m 43 percent Ashkenazi Jewish, 25 percent Northwestern European, 23 percent South/Central European, 6 percent Southwestern European, and 2.2 percent North Slavic. Those percentages sound impressive, even definitive. It’s easy to conclude that ancestry reports are as reliable as stepping on a scale at the doctor’s office to get your height and weight measured.

That is a mistake, and one that can cause a lot of heartbreak. To estimate ancestry, researchers compare each customer to a database of thousands of people from around the world. . . They can identify stretches of DNA that are likely to have originated in a particular part of the world. While some matches are clear-cut, others are less so. As a result, ancestry estimates always have margins of error—which often go missing in the reports customers get.

These estimates are going to get better with time, but there’s a fundamental limit to what they can tell us about our ancestry. . . Researchers are getting glimpses of those older peoples by retrieving DNA from ancient skeletons. And they’re finding that our genetic history is far more tumultuous than previously thought. Time and again, researchers find that the people who have lived in a given place in recent centuries have little genetic connection to the people who lived there thousands of years ago. All over the world, populations have expanded and migrated, coming into contact with other populations. . . If you want to find purity in your ancestry, you’re on a fool’s errand.

Misconception #4: There’s a Gene for Every Trait You Inherit

Mendel is a great place to start learning about heredity but a bad place to stop. There are some traits that are determined by a single gene. Whether Mendel’s peas were smooth or wrinkled was determined by a gene called SBEI. Whether people develop sickle cell anemia or not comes down to a single gene called HBB. But many traits do not follow this so-called Mendelian pattern—even ones that we may have been told in school are Mendelian.

Consider your ear lobes. For decades, teachers taught that they could either hang free or be attached to the side of our heads. The sort of ear lobes you had was a Mendelian trait, determined by a single gene. In fact, our ear lobes typically fall somewhere between the two extremes of strongly attached to fully free. In 2017, a team of researchers compared the ear lobes of over 74,000 people to their DNA. They looked for genetic variants that were common in people at either end of the ear-lobe spectrum. They pinpointed forty-nine genes that appear to play a role in determining how attached they are to our heads. There well may be more waiting to be discovered.

The genetics of ear lobes is actually very simple compared to other traits. Studying height, for example, scientists have identified thousands of genetic variants that appear to play a role. The same holds true for our risk of developing diabetes, heart disease, and other common disorders. We can’t expect to find a single gene in our DNA tests that determines whether we’ll die of a heart attack. Nor should we expect easy fixes for such complex diseases by repairing single genes.

Misconception #5: The Genes You Inherit Explain Exactly Who You Are

Take, for example, a recent study on how long people stay in school. Researchers examined DNA from 1.1 million people and found over 1,200 genetic variants that were unusually common either in people who left school early or in people who went on to college or graduate school. They then used the genetic differences in their subjects to come up with a predictive score, which they then tried out on another group of subjects. They found that in the highest-scoring 20 percent of these subjects, 57 percent finished college. In the lowest-scoring 20 percent, only 12 percent did.

But these results don’t mean that how long you stayed in school was determined before birth by your genes. Getting your children’s DNA tested won’t tell you if you should save up money for college tuition or not. Plenty of people in the educational attainment study who got high genetic scores dropped out of high school. Plenty of people who got low scores went on to get PhDs. And many more got an average amount of education in between those extremes. For any individual, these genetic scores make predictions that are barely better than guessing at random.

This confusing state of affairs is the result of how genes and the environment interact. Scientists call a trait such as how long people stay in school “moderately heritable.” In other words, a modest amount of the variation in education attainment is due to genetic variation. Lots of other factors also matter, too—the neighborhoods where people live, the quality of their schools, the stability of their family life, their income, and so on. What’s more, a gene that may have an influence on how long people stay in school in one environment may have no influence at all in another.

Misconception #6: You Have One Genome

According to this assumption, you will find an identical sequence of DNA in any cell you examine. But there are many ways in which we can end up with different genomes within our bodies.

Fairchild is known as a chimera. She developed inside her mother alongside a fraternal twin. That twin embryo died in the womb, but not before exchanging cells with Fairchild. Now her body was made up of two populations of cells, each of which multiplied and developed into different tissues. In Fairchild’s case, her blood arose from one population, while her eggs arose from another.

It’s unclear how many people are chimeras. Once they were considered bizarre rarities. Scientists became aware of them only in cases such as Lydia Fairchild’s, when their mixed identity made itself known. In recent years, researchers have been carrying out small-scale surveys that suggest that perhaps a few percent of twins are chimeras, but the true number could be higher. As for chimeric mothers, they may be the rule rather than the exception. In a 2017 study, researchers studied brain tumors taken from women who had sons. Eighty percent of them had Y-chromosome-bearing cells in their tumors.

Chimerism is not the only way we can end up with different genomes. Every time a cell in our body divides, there’s a tiny chance that one of the daughter cells may gain a mutation. At first, these new aberrations—called somatic mutations—seemed important only for cancer. But that view has changed as new genome-sequencing technologies have made it possible for scientists to study somatic mutations in many healthy tissues. It now turns out that every person’s body is a mosaic, made up of populations of cells with many different mutations.
Misconception #7: Genes Don’t Matter Because of Epigenetics

The notion that our genes are our destiny can trigger an equally false backlash: that genes don’t matter at all. And very often, those who push against the importance of genetics invoke a younger, more tantalizing field of research: epigenetics.

Our cells use many layers of control to make proper use of their genes. They can quickly turn some genes on and off in response to quick changes in their environment. But they can also silence genes for life. Women, for example, have two copies of the X chromosome, but in early development, each of their cells produces a swarm of RNA molecules and proteins that clamp down on one copy. The cell then only uses the other X chromosome. And if the cell divides, its daughter cells will silence the same copy again.

One of the most tantalizing possibilities scientists are now exploring is whether certain epigenetic “marks” can be inherited not just by daughter cells but by daughters—and sons. If people experience trauma in their lives and it leaves an epigenetic mark on their genes, for example, can they pass down those marks to future generations?

If you’re a plant, the answer is definitely yes. Plants that endure droughts or attacks by insects can reprogram their seeds, and these epigenetic changes can get carried down several generations. The evidence from animals is, for now, still a mixed bag. . . But skeptics have questioned how epigenetics can transmit these traits through the generations, suggesting that the results are just statistical flukes. That hasn’t stopped a cottage industry of epigenetic self-help from springing up. You can join epigenetic yoga classes to rewrite your epigenetic marks or go to epigenetic psychotherapy sessions to overcome the epigenetic legacy you inherited from your grandparents.