Allelic genes are genes located at identical loci (positions) on homologous chromosomes that control the same trait. The classic definition is the first option.
The correct answer is: Genes located in identical loci of homologous chromosomes and responsible for one trait.
Here's why the other options are wrong:
- "Different variants of the same gene in a population" - that describes polymorphism/allele variants at the population level, not the definition of allelic genes as a pair.
- "A pair of genes responsible for the development of the same trait in a phenotype" - too vague; genes on non-homologous chromosomes can also affect the same trait (that would be epistasis or polygenic inheritance, not allelic genes).
- "Different genes that control the manifestation of variants of the same trait in a phenotype" - describes modifier genes or polygenic traits, not alleles.
- "Different variants of the phenotypic manifestation of a trait within a normal response" - this describes a norm of reaction, not allelic genes.
Definition summary: Allelic genes (alleles) occupy the same locus on each of a pair of homologous chromosomes. They may be identical (homozygous) or different (heterozygous), and they govern expression of the same trait. This is the foundational concept from Mendelian genetics.
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I now have all the information needed from multiple authoritative textbooks. Here is the complete answer:
Leishmania tropica major (Zoonotic Cutaneous Leishmaniasis) - Correct Features
The question is about L. tropica major (now classified as Leishmania major), which causes zoonotic cutaneous leishmaniasis (the rural/moist/wet type). Here is what the textbooks say about each option:
Correct answers to select:
| Feature | L. major (Zoonotic/Rural type) |
|---|---|
| Incubation | 1-4 weeks (short) |
| Reservoir | Rodents (burrowing rodents, gerbils) |
| Ulcer type | Moist/wet ("ulcer wetting") |
Analysis of each option:
-
Ulcer - Mostly dry - WRONG. The dry ulcer is characteristic of L. tropica (the anthroponotic/urban type), NOT L. major. L. major causes the moist/wet ulcer.
-
Incubation - 2-6 months - WRONG. This is the incubation for visceral leishmaniasis (kala-azar), not L. major cutaneous disease.
-
Incubation - 3-8 weeks - Partially overlapping, but the classic quoted range for the rural/wet type is 1-4 weeks (Andrews' Diseases of the Skin). Some sources say "weeks to months" broadly.
-
Ulcer - Ulcer wetting - CORRECT. L. major causes the moist/wet type ulcer.
-
Reservoir - Cats - WRONG. Cats are not a significant reservoir for L. major.
-
Incubation - 1-4 weeks - CORRECT. Andrews' Dermatology states: "The incubation period is relatively short (1-4 weeks)" for the rural/wet type caused by L. major. - Andrews' Diseases of the Skin, p. 3691
-
Reservoir - Humans - WRONG. Humans as reservoir = anthroponotic type = L. tropica (urban/dry). L. major is zoonotic.
-
Reservoir - Rodents - CORRECT. Jawetz Microbiology states: "The 'wet' type, caused by L. major, is rural, and burrowing rodents are the main reservoir." - Jawetz Medical Microbiology, 28e, p. 1030
The three correct answers are:
- Incubation - 1-4 weeks
- Ulcer - Ulcer wetting (moist/wet type)
- Reservoir - Rodents
The already-checked answer "Incubation - 2-6 months" in the image is incorrect for L. major - that range applies to visceral leishmaniasis.
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