Puppies don’t consistently take after one parent more than the other when it comes to size. Adult size in dogs is controlled by many genes inherited from both parents, with no single parent dominating the outcome. That said, the mother’s body does play a unique role during pregnancy that can influence birth weight, and a few size-related genes sit on the X chromosome, which creates some interesting wrinkles in how size gets passed down.
Why Size Isn’t a Simple Mom-or-Dad Trait
Body size in dogs is what geneticists call a polygenic trait, meaning it’s shaped by the combined effects of many genes rather than a single one. Researchers have identified several genes associated with size variation, but there are likely dozens or even hundreds of additional genes that each contribute a small amount. The total effect is additive: each parent contributes roughly half the genetic blueprint, and the puppy’s final size reflects the sum of all those contributions.
This is why two large dogs can occasionally produce a puppy that grows even bigger than either parent. It’s also why puppies from the same litter can vary noticeably in adult size. Each puppy inherits a slightly different combination of size-related gene variants from mom and dad, leading to a range of outcomes rather than one predictable number.
The IGF1 Gene: The Biggest Single Factor
One gene stands out above the rest. A landmark study published in Science found that a single variant of the IGF1 gene is shared by virtually all small dog breeds and is nearly absent in giant breeds. This gene controls production of a growth-promoting protein that circulates in the blood. Dogs carrying two copies of the “small” variant have lower levels of this protein, smaller skeletal frames, and less overall mass. In one well-studied breed (Portuguese Water Dogs), this single gene accounts for about 15% of the variation in skeletal size.
IGF1 sits on a regular chromosome, not a sex chromosome, so puppies inherit one copy from mom and one from dad with equal probability. If one parent carries the small variant and the other carries the large variant, the puppy could end up with any combination. This is one reason mixed-size crosses are so unpredictable.
Genes on the X Chromosome Add a Twist
While most size genes are inherited equally from both parents, researchers have identified three genes on the X chromosome that influence body weight, muscling, and fat thickness. One of these interacts with growth factor receptors (including IGF1’s receptor), another affects thyroid hormone production, and the third plays a role in fat metabolism. Together, they help determine whether a dog has a lean or “bulky” build.
Here’s why this matters: male dogs have one X chromosome (from their mother) and one Y chromosome (from their father). Female dogs have two X chromosomes, one from each parent. So for these particular X-linked traits, a male puppy’s build is shaped entirely by the copy he got from his mother. A female puppy, on the other hand, gets X-linked input from both parents. This doesn’t mean male puppies always resemble their mother in size overall, but for this specific cluster of traits related to muscling and bulk, the mother’s genetic contribution carries more weight in sons than in daughters.
The Mother’s Body Limits Birth Weight
Beyond genetics, the mother’s physical size places a real constraint on how big puppies are at birth. Research has shown that for every additional kilogram of the dam’s adult weight, puppy birth weight increases by about 5 grams. The same relationship doesn’t hold for the sire’s weight. This makes intuitive sense: a smaller mother has a smaller uterus, less room, and a finite capacity to nourish developing puppies. Longer pregnancies also produce heavier pups, at roughly 6 grams per extra day of gestation.
Birth weight, however, is not the same as adult weight. A puppy born smaller because of a petite mother can still “catch up” during postnatal growth if its genetic potential (inherited from both parents) codes for a larger frame. Think of it as a temporary speed limit rather than a permanent ceiling. The mother’s diet during pregnancy also influences birth weight, adding another environmental layer on top of genetics.
Growth Timelines Vary Dramatically by Size
One factor that affects how big a puppy ultimately gets is simply how long it keeps growing, and this is determined largely by breed size rather than which parent it favors. Small breeds hit their peak growth rate between 3 and 5 months and typically reach adult size by 6 to 10 months, with growth plates closing as early as 6 to 8 months. Giant breeds, by contrast, keep growing until 18 to 24 months, with growth plates remaining open far longer.
For mixed-size crosses, this creates an interesting situation. A puppy that inherits more “large breed” genetics will generally have a longer growth window, continuing to add height and mass months after a smaller-framed littermate has plateaued. You often can’t tell which parent a puppy will favor in size until well past the one-year mark if a giant breed is involved.
Males Tend to Be Larger Than Females
Regardless of which parent a puppy resembles genetically, sex plays an independent role. Male dogs are typically larger than females of the same breed and parentage. In some breeds the difference is modest (a few pounds), while in others, particularly large and giant breeds, males can outweigh females by 20% or more. So a female puppy from two large parents may still end up noticeably smaller than her brothers, not because she “took after” a smaller ancestor but simply because of normal sex-based size differences.
Predicting Size in Mixed Breeds
If you’re trying to guess how big a mixed-breed puppy will get, averaging the parents’ sizes gives you a rough starting point, but the range of possible outcomes is wide. A puppy could land anywhere between the two parents’ sizes or occasionally outside that range entirely, depending on which gene variants it inherited.
A simple formula veterinarians sometimes use: take your puppy’s weight at four months and double it. This gives a ballpark adult weight, though it’s less reliable for giant breeds that continue growing well past that age. Knowing the specific breeds in your dog’s background and checking typical weight ranges for those breeds will usually give you a better estimate than trying to figure out whether your puppy “looks more like mom” or “looks more like dad.”
DNA tests that identify breed composition can improve predictions further, since breed averages are well documented. But even with genetic data, individual variation means your puppy’s adult size will always carry some element of surprise.