The correct answer is: A. Some alleles are dominant, while others are recessive.
An allele is one of two or more forms of a gene that arise by mutation and are located at the same position on a chromosome. The principle of dominance states that when two alleles for a trait are present in an individual, only one allele will be expressed. The allele that is expressed is said to be dominant, while the allele that is not expressed is said to be recessive.
For example, the gene for eye color has two alleles, one for brown eyes and one for blue eyes. If a person has two brown eye alleles, they will have brown eyes. If a person has one brown eye allele and one blue eye allele, they will also have brown eyes, because the brown eye allele is dominant. Only if a person has two blue eye alleles will they have blue eyes, because the blue eye allele is recessive.
The principle of dominance is important in genetics because it helps to explain how traits are passed down from parents to offspring. It also helps to explain why some traits are more common than others in a population.
Here is a brief explanation of each option:
- Option A: Some alleles are dominant, while others are recessive. This is the correct answer.
- Option B: All alleles are recessive. This is not correct. Some alleles are dominant, while others are recessive.
- Option C: All alleles are dominant. This is not correct. Some alleles are dominant, while others are recessive.
- Option D: Alleles are neither dominant nor recessive. This is not correct. Alleles are either dominant or recessive.