Lesson 2 of 11 / Inheritance
Genotype, phenotype and dominance
Can two different genotypes produce the same phenotype?
I can use dominant, recessive, homozygous, heterozygous, genotype and phenotype correctly.
I can use dominance, codominance, genotype and phenotype terminology.Syllabus
Syllabus K325, Topic 14(b). Use dominance, codominance, genotype and phenotype terminology.
About 5 min
Make a guess. You are not marked.
Two pea plants both have round seeds. Must they have the same alleles for seed shape?
Show the answer
One could be RR and the other Rr.
With complete dominance, RR and Rr look the same. The same phenotype can hide different genotypes.
The key idea
Genotype describes the alleles present; phenotype describes the expressed characteristic. Under complete dominance, a heterozygote shows the dominant phenotype.
Stated model: R gives round pea seeds and is completely dominant to r.
Rr expresses round seeds, but r remains in its genotype and can be inherited. Dominance does not erase an allele.
Letter circles represent alleles; the seed represents phenotype. They are not atoms or a complete chromosome drawing. Dominant does not mean more common or better adapted.
Explanation
A genotype is the allele combination for the gene or genes being considered. A phenotype is the observable or measurable characteristic expressed, resulting from genotype and, for many characteristics, environmental influences.
Homozygous means having two identical alleles of a gene, such as RR or rr. Heterozygous means having two different alleles, such as Rr. Each genotype still contains two alleles in the diploid model; a capital letter is not an extra chromosome.
Under complete dominance, the dominant allele determines the phenotype when present with a recessive allele. In the stated pea seed-shape model, R gives round seeds and r gives wrinkled seeds. RR and Rr are round; rr is wrinkled. The recessive phenotype appears when no dominant allele is present.
Dominant does not mean stronger, more useful or more common. A recessive allele can remain in a heterozygote and be passed to offspring even when its phenotype is not expressed. Codominance is a different relationship: both alleles are expressed in a heterozygote, as explored with the ABO blood groups.
Step by step
- 1
State the allele relationship
Use the dominant/recessive relationship given in the question.
- 2
Read the genotype
Decide whether the two alleles are identical or different.
- 3
Predict the phenotype
For complete dominance, one dominant allele is sufficient for its phenotype.
Worked example
Two round seeds
In a complete-dominance model, one round-seeded plant is RR and another is Rr. Explain their similarity and difference.
One way to explain it
They share the round phenotype because both contain the dominant R allele. Their genotypes differ: RR is homozygous, while Rr is heterozygous and also carries the recessive allele.
Why this answer works
- Compare phenotype and genotype separately.
- Do not erase the recessive allele from the heterozygote.
Watch out for this
A student says: "A recessive allele disappears when paired with a dominant allele." What is wrong with this?
Show the answer
It remains in the genotype and can be passed on. Complete dominance affects the phenotype expressed, not the presence of the allele.
Check your understanding
R is dominant to r. Which statement about Rr is correct?
Show the answer
It is heterozygous and expresses the dominant phenotype.
Both alleles remain present, but the dominant phenotype is expressed in this model.
Quick recall
Can two different genotypes produce the same phenotype?