Do You Get More Genes From Your Mom or Dad?

In terms of raw DNA sequence, you inherit almost exactly half from each parent. Every person carries 23 pairs of chromosomes, one set from mom and one from dad, so the nuclear genome is a near-perfect 50/50 split. But “almost exactly” is doing real work in that sentence. Several biological quirks tilt the ledger, and most of them tilt it toward your mother. Between mitochondrial DNA, the size difference between X and Y chromosomes, the egg’s stockpile of molecular machinery, and even stray maternal cells that lodge in your tissues for decades, the idea of a clean even split turns out to be an oversimplification.

Mitochondrial DNA Comes Exclusively From Mom

The most clear-cut asymmetry is mitochondrial DNA. Mitochondria, the structures inside cells that generate energy, carry their own small genome of about 16,500 base pairs. You inherited all of yours from your mother’s egg cell, and none from your father’s sperm. For a long time the mechanism behind this was only partly understood, but a 2023 study in Nature Genetics clarified it at the molecular level. During sperm development, a key protein called TFAM, which normally protects and maintains mitochondrial DNA, gets redirected away from the sperm’s mitochondria and into the sperm cell’s nucleus instead. The result is that mature human sperm are essentially stripped of intact mitochondrial DNA before they ever reach the egg.¹

Even if trace amounts of paternal mitochondrial material do enter the egg at fertilization, backup systems exist to eliminate them. Research in animal models has identified at least two routes: an active degradation pathway, where the embryo’s own cellular recycling machinery (autophagy) breaks down paternal mitochondria, and a passive dilution pathway, where the tiny number of paternal mitochondria simply gets swamped by the much larger pool of maternal ones as the embryo’s cells divide.² Studies in roundworms have shown that paternal mitochondria and their DNA can be destroyed within two hours of fertilization.³

There have been rare, contested exceptions. A 2018 report described three unrelated families in which paternal mitochondrial DNA appeared to be transmitted across multiple generations, with the paternal type making up as much as 40% of detected sequences in some individuals.⁴ The findings were confirmed by independent labs using different methods, but the scientific community has debated their interpretation, and subsequent studies of other patients with mitochondrial DNA abnormalities have not reproduced similar results.⁵ For now, maternal-only inheritance of mitochondrial DNA remains the overwhelming rule in humans.

The X and Y Chromosome Gap

The second tilt comes from the sex chromosomes. If you are female (XX), you received one X chromosome from each parent, and the contribution is roughly symmetrical in size. But if you are male (XY), you got an X from your mother and a Y from your father. The human X chromosome carries around 800 protein-coding genes. The Y chromosome carries somewhere in the range of 50 to 70. That means a son inherits substantially more genetic material from his mother than from his father, simply because the X chromosome is so much larger than the Y. The difference amounts to a few percent of the total genome, but it is real and consistent.

In females, the two X chromosomes do not both stay fully active. Early in embryonic development, each cell randomly shuts down one of its two X chromosomes, a process called X-inactivation. Which X gets silenced varies from cell to cell, so adult women are essentially mosaics: some patches of tissue express the maternal X, others express the paternal X. This choice has real consequences. If one X carries a disease-causing variant, the proportion of cells that silence that copy versus the healthy copy can determine whether a woman shows symptoms or remains unaffected.⁶ X-inactivation does not change the amount of DNA inherited from each parent, but it does mean that the functional expression of that DNA can skew unpredictably.

Genomic Imprinting and Why Parent of Origin Matters

Even among the thousands of genes on the non-sex chromosomes, “inheriting a gene” and “using a gene” are not the same thing. For most genes, both copies are active. But for a small subset, only the copy from one specific parent is switched on, while the other is chemically silenced through a process called genomic imprinting. The silencing happens via chemical tags, primarily methyl groups attached to the DNA, that are set during egg or sperm formation. The result is that for these genes, you effectively rely on just one parent’s version.⁷

Roughly 100 to 200 human genes are known to be imprinted, a tiny fraction of the roughly 20,000 protein-coding genes in the genome. But their effects are disproportionately large because many of them are involved in growth, brain development, and metabolism. Some imprinted genes are active only when inherited from the father; others are active only when inherited from the mother. This creates situations where losing the “active” parent’s copy of a gene leaves no working copy at all, even though the other parent’s copy is physically present in the DNA.

When Imprinting Goes Wrong

The starkest illustration of how imprinting works comes from two conditions that involve the same stretch of chromosome 15. Prader-Willi syndrome results from the loss or silencing of genes in a region called 15q11-q13 that are normally active only on the copy inherited from the father. Angelman syndrome results from the loss or silencing of a gene in the same region that is normally active only on the maternal copy.⁸ Two people can have an identical-looking chromosomal deletion in the same spot, yet develop completely different disorders depending on whether the deletion sits on the chromosome they inherited from their mother or their father.⁹

Prader-Willi syndrome typically involves extreme hunger and obesity, intellectual disability, and hormonal problems. Angelman syndrome involves severe developmental delay, seizures, and a characteristic happy demeanor with frequent laughing.¹⁰ These two syndromes are a vivid reminder that the parent of origin for a gene can matter as much as which variant of the gene you carry.

The Egg’s Extra Cargo

Beyond DNA itself, your mother’s egg contributed something your father’s sperm did not: a massive cellular environment packed with proteins, RNA molecules, and organelles. In the earliest hours and days after fertilization, before the embryo’s own genome switches on, development runs almost entirely on supplies stockpiled in the egg during the mother’s lifetime. The genes responsible for producing this stockpile are called maternal effect genes, and they control critical early events including the first cell divisions, the activation of the embryo’s own genome, and the initial establishment of cell types that will become different tissues.¹¹

This is not “more genes” in the sense of more DNA, but it is more biological influence at a stage of development that shapes everything that follows. If the egg’s supply of these maternal factors is deficient, the embryo may fail to develop at all, regardless of the quality of the DNA contributed by either parent. Sperm, by contrast, contribute DNA and very little else: the cell is stripped down to basically a nucleus and a tail.

How a Mother’s Environment Reprograms the Genome

During pregnancy, the environment inside the uterus can alter how the baby’s genes behave without changing the DNA sequence. Maternal nutrition, stress, and exposure to certain chemicals can modify the chemical tags on the fetal genome, adjusting which genes get turned up or down. These epigenetic modifications can have lasting effects. Research has shown that a mother’s diet during pregnancy and the neonatal period helps establish epigenetic patterns in the fetus that influence susceptibility to conditions like obesity and metabolic disease later in life.¹²

Fathers are not entirely absent from the epigenetic picture. Small RNA molecules carried in sperm appear to play a role in early embryonic development, and disruptions to these molecules, such as those caused by oxidative stress, may affect offspring outcomes.¹³ But the sheer duration and intimacy of the maternal-fetal connection during nine months of gestation gives the mother’s body far more opportunity to shape how the shared genome gets used.

Maternal Cells That Stick Around

Here is something that surprises most people: your body likely contains a small number of cells that are not genetically yours at all. During pregnancy and breastfeeding, maternal cells cross the placenta and enter the developing baby’s body, where they can integrate into various tissues and organs. This phenomenon is called maternal microchimerism, and these cells can persist throughout the child’s entire life.¹⁴

Maternal microchimeric cells have been detected in the blood, skin, thyroid, liver, and other organs of children and adults. Their presence appears to influence the developing immune system, and researchers have found associations between microchimerism and both protective immune effects and certain autoimmune conditions.¹⁵ Fathers do not contribute cells to their children’s bodies in this way. So while microchimerism is not inheritance in the textbook sense, it is another channel through which a mother’s biological material directly shapes her child.

New Mutations and Paternal Age

One area where fathers have an outsized genetic influence is in de novo mutations, the brand-new genetic changes that appear in a child but were not present in either parent’s original genome. Most de novo mutations in a child’s DNA originate from the father’s sperm rather than the mother’s egg. The reason is biological: sperm-producing cells divide continuously throughout a man’s reproductive life, and each division is an opportunity for copying errors. Egg cells, by contrast, are largely formed before a woman is born, so they undergo far fewer divisions. As a result, the number of new mutations in a child’s DNA rises with paternal age, a pattern that has been linked to increased risk for certain conditions in offspring.¹⁶

This is an interesting counterweight to the maternal tilts described above. In terms of inherited DNA, the split is roughly even. In terms of new DNA variants that neither parent had, fathers are the dominant source. Whether those mutations are harmful, neutral, or occasionally beneficial varies case by case, but the pattern itself is one of the clearest examples of an asymmetric parental contribution to a child’s genome.

When Both Copies Come From One Parent

In rare cases, the normal 50/50 split breaks down dramatically. Uniparental disomy occurs when a child inherits both copies of a particular chromosome from the same parent and none from the other. This can happen through errors during egg or sperm formation, or through a rescue mechanism early in embryonic development where a cell with three copies of a chromosome loses one, and by chance eliminates the copy from the opposite parent.¹⁷

Most of the time uniparental disomy has no obvious effect, because for the majority of genes it does not matter which parent’s copy is active. But when it involves an imprinted region, where one parent’s copy is supposed to be silent, inheriting both copies from the wrong parent can cause disease. This is actually one of the mechanisms behind Prader-Willi and Angelman syndromes: a child with two maternal copies of the relevant region on chromosome 15 can develop Prader-Willi syndrome, because the genes that are supposed to be active only on the paternal copy have no paternal copy to be active on.

Three-Parent Babies and Mitochondrial Replacement

The fact that mitochondrial DNA is inherited separately from nuclear DNA has opened the door to a medical technique that challenges the traditional two-parent model entirely. Mitochondrial replacement therapy allows a woman who carries harmful mutations in her mitochondrial DNA to have a biological child without passing those mutations on. The technique works by transferring the nuclear DNA from the mother’s egg (or fertilized embryo) into a donor egg that has had its own nucleus removed but retains healthy mitochondria.¹⁸

The resulting child has nuclear DNA from both biological parents and mitochondrial DNA from a third person, the egg donor. Two main approaches exist: one moves the genetic material before fertilization at the egg stage, and the other moves it after fertilization at the embryo stage.¹⁹ The mitochondrial donor’s contribution is small in terms of gene count, roughly 37 mitochondrial genes compared to over 20,000 nuclear genes, but it is enough to prevent devastating mitochondrial diseases. The technique has been approved for clinical use in a small number of countries, and the first children born through it are still young, so long-term outcomes are still being tracked.

In a sense, mitochondrial replacement therapy is a deliberate engineering of the maternal tilt. It takes the one piece of genetic material that already comes exclusively from the mother and splits it between two women, creating a child whose genetic heritage is drawn from three biological sources rather than two.