---
title: "Genotype: The Two Letters You Cannot See"
url: https://www.learningloftinstitute.com/glossary/genotype
source: Learning Loft Institute
language: en
updated: 2026-08-21
section: "Glossary › Genotype: The Two Letters You Cannot See"
---
# Genotype: The Two Letters You Cannot See
> **Answer.** A genotype is the combination of alleles an organism carries for a gene, written as two letters. With a dominant B and a recessive b there are exactly three genotypes: BB, homozygous dominant; Bb, heterozygous; and bb, homozygous recessive. BB and Bb both produce the dominant characteristic.
## Key facts
| | |
| --- | --- |
| Definition | A genotype is the pair of alleles an organism carries for a particular gene, usually written as two letters such as BB, Bb or bb. |
| Where students meet it | Grade 9 science, as soon as Punnett squares are drawn, and again in Grade 10 when inherited disorders and carriers are covered. |
## Two alleles, three genotypes
It looks as though two alleles filling two positions ought to give four combinations, and that is the source of a common error in Punnett squares. The square does produce four boxes, but two of them contain the same genotype written in a different order, so three distinct genotypes come out.
Each one has a name and a consequence:
- BB — homozygous dominant — shows the dominant characteristic, and can only pass on B
- Bb — heterozygous — shows the dominant characteristic, and can pass on either B or b
- bb — homozygous recessive — shows the recessive characteristic, and can only pass on b
## Homozygous and heterozygous
The word roots do the explaining: homo means same, hetero means different. Homozygous means both alleles for that gene are identical; heterozygous means they differ. An organism is described as homozygous or heterozygous for a particular gene, never in general, since it may be homozygous for one gene and heterozygous for the next.
The heterozygote is where the interesting cases live. Two brown-eyed parents can have a blue-eyed child only if both are Bb, because bb needs a b from each of them. Nothing about their appearance reveals this, which is why the genotype matters: it is the part of the story that gets passed on.
## Writing genotypes so they earn the mark
The conventions are narrow and they are marked. Both alleles of a gene use the same letter, differing only in case, so B and b are alleles of one gene while B and c are two unrelated genes. Choose a letter whose capital and lower case look clearly different — C and c or S and s cause trouble in handwriting, where B and b, T and t do not.
Two further points. Always write both letters: a genotype is a pair, and a single B answers nothing. And the word genotype is sometimes used more broadly to mean an organism's entire set of alleles, but at this level it almost always refers to one gene at a time.
## Questions
### What is the difference between genotype and phenotype?
The genotype is the pair of alleles carried; the phenotype is the characteristic that results and can actually be observed. BB and Bb are two genotypes with one phenotype between them, brown eyes. The genotype is what gets inherited and passed on; the phenotype is what a person shows to the world.
### What does homozygous mean?
That both alleles of a gene are the same, whether both dominant (BB) or both recessive (bb). A homozygous organism can pass on only one kind of allele for that gene, which makes its offspring easier to predict. Heterozygous means the two alleles differ, as in Bb, and either one may be passed on.
### Is Bb the same as bB?
Yes. They describe an identical pair of alleles, and the order the letters appear in has no biological meaning. The convention is to write the dominant allele first, so Bb is what you should put down. In a Punnett square you will often generate bB in one box; rewrite it as Bb before you count up the outcomes.
### Can you tell someone's genotype by looking at them?
Only when the recessive characteristic shows. A blue-eyed person must be bb, because a single B would have masked it. A brown-eyed person is BB or Bb and looking will not tell you which. Family history, the appearance of the children, or a genetic test are the ways to distinguish them.
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