Law of Dominance — Definition

Definition

Imagine you have two types of pea plants: one that always grows tall, and another that always grows short (dwarf). These are 'pure' varieties, meaning they only carry the genetic information for their specific height.

Now, if you cross-pollinate a pure tall pea plant with a pure dwarf pea plant, what do you expect the offspring to look like? According to Mendel's Law of Dominance, all the offspring in the first generation (we call this the F1 generation) will be tall.

Even though they received genetic information for 'dwarfness' from one parent and 'tallness' from the other, only the 'tall' characteristic shows up. The 'dwarf' characteristic seems to disappear, or rather, it's hidden.

This 'tall' characteristic is what we call the 'dominant' trait, because it dominates or masks the other trait. The 'dwarf' characteristic, which is hidden, is called the 'recessive' trait.

So, in simple terms, the Law of Dominance tells us that when an organism inherits two different versions (alleles) of a gene for a particular trait – one from each parent – often only one of those versions will be visible or expressed.

The version that shows up is the dominant one, and the one that gets hidden is the recessive one. It's like having two instructions for the same job, but only one instruction gets followed. This doesn't mean the hidden instruction is gone; it's still there, just not active.

If these F1 generation tall plants are then allowed to self-pollinate or cross with each other, the hidden 'dwarf' trait can reappear in the next generation (the F2 generation), typically in a predictable ratio, demonstrating that it was merely masked, not destroyed.

This law is foundational to understanding how traits are passed down and expressed from one generation to the next in many organisms.