Lesson 3 of 8 / Inheritance
Genotype, phenotype and dominance
Can two different genotypes produce the same phenotype?
In this lesson: Use dominant, recessive, homozygous, heterozygous, genotype and phenotype correctly.
About 5 min
The key ideaGenotype 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 remains in a heterozygote and can be passed to offspring even when its phenotype is not expressed. Use the dominant/recessive relationship stated in a question rather than assuming an allele is dominant because it is common.
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.
Is this true? "A recessive allele disappears when paired with a dominant allele."
It remains in the genotype and can be passed on. Complete dominance affects the phenotype expressed, not the presence of the allele.