CAIE IGCSE Biology 17.4.9 Defining a Recessive Allele
Practise defining a recessive allele as expressed only when no dominant allele of that gene is present.
- Syllabus
- 2026–2028
- Course
- Biology 0610
Exam points
- define a recessive allele as expressed only when no dominant allele is present
- identify a recessive phenotype with a homozygous recessive genotype
- use unaffected parents with an affected child as evidence that both parents are carriers
17.4.9—Recessive allele as an allele that is question 1
[Maximum number: 1]
Some fruit flies have orange eyes and others have red eyes.
If two orange-eyed fruit flies are crossed, their offspring always have orange eyes.
If two red-eyed fruit flies are crossed, their offspring can have orange eyes or red eyes.
What can be concluded from these observations?
A
The allele for orange eyes is dominant.
B
The allele for orange eyes is recessive.
C
The alleles for orange and red eyes are codominant.
D
This is an example of sex linkage.
B
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