BioMasterMendelian genetics: dominant, recessive and the Punnett square
The basics of Mendelian inheritance. Alleles, genotype, phenotype and how to predict crosses with the Punnett square.
Direct answer: A dominant allele (written uppercase, e.g. A) is expressed whenever it is present; a recessive allele (lowercase, a) is only expressed when it is homozygous (aa). The genotype is the combination of alleles and the phenotype is the visible trait.
Key concepts
- Homozygous: two identical alleles (AA or aa).
- Heterozygous: two different alleles (Aa) — shows the dominant phenotype.
- Genotype: the alleles it has (AA, Aa, aa).
- Phenotype: the observable trait (e.g. purple or white flower).
Punnett square (Aa × Aa)
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Result: 3:1 in phenotype (3 dominant : 1 recessive) and 1:2:1 in genotype (1 AA : 2 Aa : 1 aa).
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Study Biology free on BioMasterFrequently asked questions
What is the difference between genotype and phenotype?
The genotype is the combination of alleles (AA, Aa, aa); the phenotype is the observable trait.
What ratio does an Aa × Aa cross give?
3:1 in phenotype and 1:2:1 in genotype.
When is a recessive allele expressed?
Only when homozygous (aa), when there is no dominant allele to mask it.