You will decide what allele pairs mean, infer hidden alleles from inheritance and separate dominance from unrelated ideas such as frequency or harm. Begin with the two allele slots in one individual, then ask what each allele does when paired.
Reason from the pair, not from the label
An allele is one version of a gene, not a separate gene or a whole chromosome. An individual has two allele copies for a gene because the chromosome pair contributes two positions, one inherited through each parent. A population may contain several versions of the gene, but one individual still has only two positions.
| Signal | Decision method |
|---|---|
| At least one dominant allele is present | The dominant characteristic appears, whether the second allele is dominant or recessive. |
| The recessive characteristic appears | Rule out every pair containing a dominant allele; both alleles must be recessive. |
| A recessive allele is hidden | Keep it in the genotype. Masking changes what is seen, not the DNA, so the allele can still pass to offspring. |
| Allele symbols are requested | Use one letter for the gene: capital for the dominant allele and the matching lower-case letter for the recessive allele. |
| A recessive offspring appears from dominant-looking parents | Trace one recessive allele back to each parent. Each parent must carry the hidden allele as well as a dominant allele. |
Dominant and recessive describe expression in a mixed pair. They do not say how frequent, strong, healthy or harmful an allele is. A carrier has one recessive allele that is masked by a dominant allele, so the recessive characteristic is not shown even though the allele remains inheritable.
Where the reasoning goes wrong
| Tempting route | How to reject it |
|---|---|
| Calling two genes or two chromosomes alleles | Check whether both forms occupy the same gene position and affect the same characteristic. Alleles are alternatives of one gene. |
| Making a recessive characteristic appear with one recessive allele | A dominant partner masks it. The recessive appearance requires the absence of any dominant allele. |
| Reading dominant as common or recessive as rare, weak or harmful | Population frequency and biological effect are separate questions. Dominance only describes expression when different alleles meet. |
| Letting a masked allele disappear or change | The allele stays in the DNA and can enter a gamete. Hidden does not mean removed. |
| Calling an individual with two recessive alleles a carrier without the characteristic | Two recessive alleles produce the recessive appearance. A carrier instead has a mixed pair. |
Try it
Which symbol pairs follow the usual convention for the dominant and recessive alleles of one gene? Select all that apply.
- B for the dominant allele and b for the recessive allele
- T for the dominant allele and t for the recessive allele
- B for the dominant allele and d for the recessive allele
- r for the dominant allele and R for the recessive allele
A gene in a population of snails exists as five different alleles. How many of them can one snail carry?
- One of them, chosen at random
- All five of them, one after another
- Five, because the population has five
- Two of them, one on each chromosome of the pair
Two guinea pigs both show the dominant form of a coat characteristic, yet one of their offspring shows the recessive form. Which explanation fits?
- One parent must be showing the recessive form after all
- The recessive form appeared without being inherited
- Both parents carry one dominant and one recessive allele
- A recessive form can appear only if a parent already shows it