Genotype versus phenotype
Two of the most important words in genetics are easy to mix up. Your genotype is the actual set of alleles you carry — the instructions written in your DNA. Your phenotype is what those instructions produce that we can observe: flower color, seed shape, eye color. Genotype is the recipe; phenotype is the dish. The whole point of a cross is to predict the phenotype from the genotype, and to infer the genotype from what we see.
The reason these two words must stay separate is that different genotypes can give the same phenotype. Two purple-flowered peas might carry different allele combinations under the hood. You cannot always read the genotype straight off the organism — and learning to work around that gap is most of what this track teaches.
Homozygous, heterozygous, and notation
Since you carry two alleles per gene, there are two possibilities. If both copies are the same allele, you are homozygous for that gene. If the two copies differ, you are heterozygous. Geneticists write alleles as letters: a capital letter for one allele and the same letter in lowercase for the alternative — say P and p for purple versus white flowers.
- PP — homozygous for the capital allele (two purple-flower alleles).
- pp — homozygous for the lowercase allele (two white-flower alleles).
- Pp — heterozygous, carrying one of each.
Dominant and recessive
Now the payoff. When a heterozygote (Pp) shows the same phenotype as one of the homozygotes, that allele is acting as the dominant allele — it determines the visible trait even when only one copy is present. The allele it masks is the recessive allele; it shows up in the phenotype only when both copies are recessive (pp). In peas, purple (P) is dominant and white (p) is recessive: PP and Pp both look purple, only pp looks white.
Put together, this vocabulary lets you describe any simple genetic situation precisely. A white pea is homozygous recessive (pp) in genotype and white in phenotype. A purple pea is either PP or Pp — same phenotype, different genotype. Holding both the genotype and the phenotype in mind at once, and remembering which allele is dominant, is exactly the habit that makes the next guide's Punnett squares feel easy.