Is there any sort of natural mechanism that would explain why my giants are taller or shorter depending on how many horns they have?

Question

So I've got a species of humanoid giants in the setting. Each individual has their own set of horns, between 1 and 5, that extend from their forehead. The horns determine their adult height, with single-horns generally being 6 feet tall(1.828 meters) and penta-horns generally being 10 feet tall(3.048 meters). Family members tend to have horn counts that are the same among the individuals but sometimes one is born with one less or one more horn nub. The horns are mostly used by the parents to determine and prepare for the sizes of clothes they'll have to make during the growth of their child, but if push comes to shove they are great at impaling things if they can manage it.

Now there is some magic in the setting, but being that I want things to exist as independent from magic as possible, I'd like for it that there be an explainable reason as to why specifically their horn count causes such a large variation in height within this species of giants.

Is there any sort of natural mechanism that would explain why my giants are taller or shorter depending on how many horns they have?

Some extra information, if relevant:

• Most of them live in mountainous or snowy regions with the surrounding forest flora being mostly populated by pine trees. Those who travel or live elsewhere exist, but this is their native biome.
• Bone density and overall muscle size/strength increases proportionately with height, making each individual not look lanky in any sort of sense unless they were of the skinny sort.
• Except for the horns and a slight difference in how their ears and faces look like, they're basically like humans.

Answer 1

Growth hormones controlling horn development later control height:

It's fairly straightforward. Genetics controls the levels of a growth hormone that is present during development. The relative levels of hormone determine how many horns develop (increasing levels lead to more potential horn sites forming horns). Marry into families with similar numbers of horns, and you'll get offspring with similar numbers. Since it's all relative, some individuals happen to express a little more or less hormone based on developmental or genetic differences.

The same hormones are the ones which ultimately determine the upper limit of height in the species. So the number of horns is correlated with the amount of hormone AND the ultimate height the individual is likely to attain.

There are likely to be a few rare individuals or families where these two traits become un-linked. So someone could be born with few or no horns, yet still be tall, or conversely have the maximum (or more?) horns yet still be quite short. This would be because of a developmental abnormality that could be genetic or environmental. A certain region might have something that affects traits one way or another. Certain forms of albinism result in adults with dark hair and brown eyes, yet they have the abnormal foveal development and vision problems of people with albinism because of the timing of the expression of hormones.

• If you WANT it to be directly causative, then the hormones can be produced at the sites of the growth cones for the horns. That way, larger numbers of horns result in higher levels of hormone. Dehorning would thus end up stunting growth. The genes controlling horn number thus control hormone levels.
• Or you could have non-genetic (epigenetic) inheritance control the whole thing this way as well. Mom's hormones control the number of baby's horns, and baby's number of horns control baby's hormone levels. Thus, height is inheritable, but based on the hormone levels of the mother. That way, a female controls baby's ultimate height, and in a matriarchal culture, that way height is consistent throughout the family regardless of who a female reproduces with.

Answer 2

Genetics is a tricky subject. It isn't like computer programming, where each command has a single definite function. In genetics, stuff can be all mixed together to a greater or lesser degree.

If we're talking about Mendelian genetics, with dominant and recessive genes, if two genes are on seperate chromosomes, the usual rules of gene distribution apply, but if they are on the same chromosome, they are inherited together with a frequency determined by the inverse of the distance between the genes... i.e. the closer on the chromosome, the more often they are inherited together. This is genetic linkage.

Furthermore, a given gene may determine more than one trait. This is known as Pleiotropy.

So, for the number of horns to be related to height, this may be an example of genetic linkage if there is a strong correlation that is not always the case. Alternatively, if horn number and height always go hand in hand, this would be a case of pleiotropy.

Perhaps the genes that code for horns also directly affect height... it could be just that simple, two different sets of tissues affected by the same genes.

Answer 3

The link between horn count and adult height is just a very strong statistically significant correlation similar to how blue eyes and alcoholism are related.