Suppose the gene that predetermines whether you have dimples on your cheeks is called 'gene D'. There are two variants of the gene: d and D. Persons with genotypes DD and Dd have dimples. Persons with genotype dd do not have dimples. Select the correct statement below:

Question

BlackTom AI · Accepted Answer

Because individuals with genotypes DD and Dd both have dimples while dd does not, the D allele must be dominant and the d allele recessive.

1) Relevant concepts
- Allele: a version of a gene (here D or d).
- Genotype: the two alleles an individual carries (DD, Dd, or dd).
- Phenotype: the observable trait (dimples or no dimples).
- Homozygous: both alleles the same (DD or dd).
- Heterozygous: two different alleles (Dd).
- Dominant allele: an allele whose trait appears in the phenotype when at least one copy is present (in a heterozygote).
- Recessive allele: an allele whose trait appears only when two copies are present (homozygous recessive).

2) Step-by-step reasoning
- Given phenotype mapping: DD → dimples, Dd → dimples, dd → no dimples.
- A heterozygote is one with two different alleles; Dd fits this definition, so Dd is heterozygous.
- Because the heterozygote (Dd) shows the dimple phenotype, the D allele is expressed in the presence of d. By definition, that makes D the dominant allele and d the recessive allele.

3) Intermediate “truth table” style summary
- DD: homozygous (two D alleles) → dimples
- Dd: heterozygous (one D, one d) → dimples
- dd: homozygous (two d alleles) → no dimples

4) Why the correct answer is correct
- The defining test for dominance is whether a single copy produces the phenotype. D does so (Dd has dimples), so D is dominant. Dd is heterozygous by allele composition.

5) Why the other options are incorrect
- (a) claims DD is heterozygous — incorrect because DD has two identical alleles (homozygous).
- (b) claims D is recessive — contradicted by Dd showing the dimple trait.
- (d) claims dd is heterozygous — incorrect because dd has two identical alleles (homozygous recessive).

Conclusion: Option c (genotype Dd is heterozygous, D is the dominant allele) is correct, matching the provided answer.

Suppose the gene that predetermines whether you have dimples on your cheeks is called 'gene D'. There are two variants of the gene: d and D. Persons with genotypes DD and Dd have dimples. Persons with genotype dd do not have dimples. Select the correct statement below:单项选择题

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