In One Parent or Two: Copies and Mixes you saw that genes come in pairs, one from each parent. Now meet the versions of a gene, and the reason a trait can skip a generation.
Key idea
A gene can come in different versions called alleles. You have two alleles for each gene, one from each parent. A dominant allele (written with a capital letter, like P) shows its trait even with one copy. A recessive allele (lowercase, p) shows only when there are two copies.
Gregor Mendel worked this out with pea plants in a monastery garden in what is now the Czech Republic, and published his results in 1866. Crossing purple-flowered and white-flowered plants gave all purple offspring; the white came back in the next generation.
Why it works
Often the dominant allele makes a working protein, such as an enzyme that makes purple pigment, and the recessive allele makes none. One working copy makes enough pigment, so Pp is purple. Only pp, with no working copy, is white. That is how two purple parents, each carrying a hidden p, can have a white-flowered offspring.
Worked example: Which shows?
In peas, round seeds (R) are dominant over wrinkled seeds (r). What do seeds with Rr look like?
- Rr has one dominant allele.
- One copy of a dominant allele is enough to show.
Answer: Round.
Worked example: Hidden allele
Two black guinea pigs have a white pup. White (b) is recessive. What alleles must each parent carry?
- A white pup is bb: one b from each parent.
- Both parents are black, so each also has a B.
Answer: Both parents are Bb.
Common mistake
Students think dominant alleles are the most common, or the "stronger, better" version. Dominant only means the trait shows with one copy. Some dominant alleles are rare: extra toes in cats come from a dominant allele, yet most cats have the usual number.