Genotype vs Phenotype
Genotype is the set of alleles an organism carries for a gene (e.g. Tt); phenotype is the observable characteristic that genotype produces (e.g. tall). The same phenotype can come from more than one genotype.
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Key definitions
- Gene: a section of a chromosome that controls one characteristic.
- Allele: an alternative form of a gene (e.g. T for tall, t for short).
- Genotype: the combination of alleles an organism has for a gene, written as a letter pair (e.g. TT, Tt, tt).
- Phenotype: the physical or observable characteristic produced by the genotype (e.g. tall or short).
Homozygous and heterozygous
A homozygous genotype has two identical alleles: homozygous dominant (TT) or homozygous recessive (tt). A heterozygous genotype has two different alleles (Tt).
Homozygous individuals are also called true-breeding, because all their gametes carry the same allele, so their offspring's genotype for that gene is predictable.
Dominant and recessive alleles
A dominant allele (shown with a capital letter, e.g. T) produces its phenotype even when only one copy is present. A recessive allele (lower case, e.g. t) only produces its phenotype when two copies are present, because the dominant allele masks it in a heterozygote.
| Genotype | Alleles present | Phenotype |
|---|---|---|
| TT | Homozygous dominant | Tall |
| Tt | Heterozygous | Tall |
| tt | Homozygous recessive | Short |
Why one phenotype can hide two genotypes
TT and Tt both look tall, so phenotype alone cannot tell you the genotype of a dominant-looking individual. A test cross (crossing with a homozygous recessive) is used to find out which genotype it actually has.
Worked problem 1 (easy): a homozygous cross
Question. In pea plants, the allele for tall (T) is dominant to the allele for short (t). A homozygous tall plant is crossed with a short plant.
State the genotype and phenotype of the F1 offspring.
Key. T = allele for tall (dominant), t = allele for short (recessive). Parents: tall (TT) x short (tt).
Gametes: T from the tall parent, t from the short parent.
| T | |
|---|---|
| t | Tt |
Worked problem 2 (medium): a test cross
Question. A tall pea plant could be TT or Tt. It is crossed with a short plant (tt) to find out which.
Show the cross for the case where the tall plant is heterozygous, and state how the result reveals the genotype.
Key. Parents: tall (Tt) x short (tt). Gametes: T and t from the tall plant, t from the short plant.
| T | t | |
|---|---|---|
| t | Tt | tt |
Worked problem 3 (SPM-level): working backwards from the offspring
Question. In rabbits, black coat (B) is dominant to brown (b). Two black rabbits are mated and one of the offspring is brown.
Determine the genotypes of both parents and the expected phenotype ratio of their offspring.
Answer. The brown offspring is bb, so it received one b allele from each parent. Both black parents must therefore carry a b allele, making each of them Bb (heterozygous).
Parents: black (Bb) x black (Bb); gametes B and b from each.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Practice problems
- In pea plants, round seed (R) is dominant to wrinkled seed (r). State all the possible genotypes of a round-seeded plant.
- A purple-flowered plant is crossed with a white-flowered plant (pp), where purple (P) is dominant. All the offspring are purple. State the genotype of the purple parent and explain your reasoning.
- Explain how a test cross can reveal whether a tall pea plant is homozygous dominant or heterozygous.
Answers to the practice problems
Common mistakes
Source:SRC-DSKP-EN
Frequently asked questions
What is the difference between genotype and phenotype?
Can two organisms with the same phenotype have different genotypes?
What does homozygous mean?
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