Genotype is the alleles an organism carries, written as letters. Phenotype is the observable feature that results, such as flower colour or height. Exam questions ask you to name one or the other, and the marks are lost if the wrong type of answer is given.
This lesson builds on the grids in constructing a simple inheritance grid and prepares you for the probability ideas in explaining repeated offspring.
How are the two connected?
The alleles set up what the organism can become. A dominant allele shows in the phenotype when it is present, even with only one copy. A recessive allele shows only when there is no dominant allele alongside it.
Organisms with two identical alleles (RR or rr) are homozygous. Organisms with two different alleles (Rr) are heterozygous. The environment can also shape the phenotype, so the genotype is not always the whole story.
How to separate them, step by step
- Look at the letters. If the answer is written as alleles, such as Rr, it is a genotype.
- Look at the description. If the answer names a feature you could see or measure, such as white flowers, it is a phenotype.
- To go from genotype to phenotype, check which allele is dominant. Any genotype with a capital letter shows the dominant feature.
- To go from phenotype to genotype, a recessive feature must be homozygous (rr). A dominant feature could be RR or Rr.
Worked example
A fictional plant has tall stems (allele T, dominant) or short stems (allele t, recessive). Complete the table.
| Genotype | Phenotype | Homozygous or heterozygous |
|---|---|---|
| TT | tall | homozygous |
| Tt | tall | heterozygous |
| tt | short | homozygous |
Now a question: a tall plant is crossed with a short plant, and some offspring are short. State the genotype of the tall parent.
Step 1: a short offspring is tt, so it got one t from each parent.
Step 2: the short parent is tt and gives t.
Step 3: the tall parent must also have given a t, so it carries t. It is tall, so it also carries T.
Answer: the tall parent is Tt.
Check: Tt × tt gives Tt, tt, Tt, tt, which is 1 tall to 1 short, so short offspring are expected.
The environment matters too. Two seedlings with the same genotype, one grown in good light and one in deep shade, can differ in height and leaf colour. The genotype is the same, but the phenotype is not.
The mistake to watch for
A common slip is to say “the genotype is purple” or “the phenotype is Rr”.
Mistaken answer: “The phenotype of the plant is Tt.”
Tt is written in letters, so it is the genotype. The phenotype is “tall”.
A second slip is to say a recessive allele is weak or disappears. It is still present in a heterozygous organism and can be passed on to offspring, where it may show if two copies meet.
Check yourself
1. Is “RR” a genotype or a phenotype? Is it homozygous or heterozygous?
Show answer
It is a genotype and it is homozygous, because both alleles are the same.
2. A fictional beetle has the allele G for green (dominant) and g for brown (recessive). State the phenotype of Gg and of gg.
Show answer
Gg is green, because G is dominant. gg is brown.
3. Two green beetles have a brown offspring. State the genotype of both parents and explain.
Show answer
A brown offspring is gg, so it received g from each parent. Both parents are green, so each also carries G. Both parents are Gg.
Where this leads next
The final lesson explains why these results are chances, in probability across repeated offspring. The inheritance model board gives fictional organisms where you can test genotype to phenotype links.
Students sometimes understand the two terms in isolation but mix them when a long question mentions both. That mixing is something our teachers in online one-to-one Biology tuition can find and fix.