How traits get passed on

Two copies, one from each parent

For most genes, you carry two copies — one inherited from each parent. That simple fact is the engine of inheritance. When an organism reproduces, each parent passes on just one of its two copies of each gene, chosen at random, and the offspring ends up with a fresh pair: one copy from the mother, one from the father. This is why a child is a genuine blend of two lineages rather than a clone of either.

Because you have two copies, the two alleles at a gene can be the same or different. If they are different, which one shows up in your phenotype? Often one wins out. A dominant allele shows its effect even when only one copy is present. A recessive allele only shows when both copies are the recessive version — a single dominant copy is enough to mask it.

Watching a cross play out

Because each parent passes one allele at random, we can predict offspring as proportions, not certainties. The classic tool for this is the Punnett square: a small grid that lines up the alleles each parent can give and shows every combination the offspring could receive. Let us walk through the simplest case.

1. Pick a gene and use letters: B = dominant brown allele, b = recessive blue allele.
2. Cross two parents who each carry one of each: both are Bb (one brown copy, one blue copy).
3. Each parent can pass B or b. Fill every combination into the grid.
4. Read off the results: collect the four boxes and count genotypes, then phenotypes.

Cross:  Bb  ×  Bb   (B = brown, dominant; b = blue, recessive)

             from parent 1
              B        b
          +--------+--------+
   from  B |   BB   |   Bb   |
  parent  +--------+--------+
      2  b |   Bb   |   bb   |
          +--------+--------+

Genotypes:  1 BB : 2 Bb : 1 bb     (ratio 1 : 2 : 1)

Phenotypes: BB, Bb, Bb all show BROWN (have a B)
            bb shows BLUE
  →  3 brown : 1 blue     (ratio 3 : 1)

Note: two brown-eyed parents can have a blue-eyed child (the bb box).
The blue allele was carried hidden in both parents all along.

Notice the payoff: two brown-eyed parents can have a blue-eyed child, because each carried a hidden blue allele and both happened to pass it on. This is not magic — it falls straight out of “two copies, one passed at random.” That single idea, made precise, is the heart of classical inheritance.