You will translate between allele symbols and visible characteristics, classify allele pairs and judge how much appearance reveals about genotype. Keep the genetic information and its observable result in separate columns as you reason.
Move between symbols and characteristics
A genotype records both alleles for one gene, so it uses a pair of symbols. If the symbols match, the genotype is homozygous; if they differ, it is heterozygous. A phenotype describes what can be observed or measured. Under complete dominance, a homozygous dominant pair and a heterozygous pair produce the dominant phenotype, while the recessive phenotype comes only from a homozygous recessive pair.
| Evidence | What you may conclude |
|---|---|
| A dominant phenotype | Two genotypes remain possible: homozygous dominant or heterozygous. Appearance alone cannot separate them. |
| A recessive phenotype | The genotype is homozygous recessive because any dominant allele would mask that phenotype. |
| The same phenotype in two individuals | Do not assume identical genotypes; the dominant phenotype can arise from two allele pairs. |
| Different measurements despite the same genotype | Consider environmental influences. Light, water or other conditions can affect some phenotypes without changing alleles. |
| A true-breeding or purebred description | Treat both terms as homozygous. The pair may be dominant or recessive. |
To reveal an unknown genotype behind a dominant phenotype, cross the organism with a homozygous recessive partner. That partner always supplies a recessive allele, so the offspring can reveal whether the tested organism sometimes contributes one too. This is evidence from inheritance, not a closer inspection of appearance.
Where the reasoning goes wrong
| Tempting route | How to reject it |
|---|---|
| Swapping genotype and phenotype | Ask whether the statement names allele symbols or an observable characteristic. Symbols describe genotype; observations describe phenotype. |
| Calling only a dominant pair homozygous, or calling a mixed pair homozygous | Homozygous means same, regardless of case. Heterozygous means the two allele symbols differ. |
| Expecting three phenotypes from three possible genotypes | Complete dominance makes two of the genotypes look alike, so genotype count and phenotype count need not match. |
| Treating a dominant appearance as proof of two dominant alleles | Keep the mixed pair open until inheritance evidence distinguishes it from the homozygous dominant pair. |
| Assuming true-breeding means dominant | The term describes matching alleles, not which allele is expressed. A recessive line can also breed true. |
Try it
In an exercise on genotype and phenotype, a student writes that the genotype of a tall plant is T. What is wrong with this?
- A genotype needs two allele symbols, not one
- A genotype must always use a lower-case letter
- A genotype must name the characteristic in words
- Nothing is wrong with writing it this way
Two tomato plants have the same genotype for height but grow to different heights. Which statements are true? Select all that apply.
- One plant must have a different allele after all
- A measured difference here does not prove an allele difference
- Growing conditions such as light and water can affect height
- A phenotype is fixed by the genotype alone in every case
A breeder wants to find out whether a rabbit showing the dominant form also carries a recessive allele. Which cross answers the question?
- Cross it with a rabbit that has two dominant alleles
- Cross it with one of its own offspring, chosen at random
- Do not cross it at all, and examine its fur closely instead
- Cross it with a rabbit that has two recessive alleles