- A test cross was made with phenotypically wild type Drosophila files hiving genes for sepia eye and curled wing in heterozygous condition. The following results were obtained:
| Wild type | 400 individuals |
| Sepia eyed | 150 individuals |
| Curled winged | 100 individuals |
| Sepia eyed curled winged | 350 individuals |
The result indicates
(1) Independent assortment, as wild and the double mutant types are in almost equal proportion
(2) Unequal segregation, as it is showing departure from 1:1:1:1 ratio
(3) Complete linkage, as the two genes are very closely placed
(4) Linkage, the two genes are separated by 25 CM distance
Introduction:
Test crosses in Drosophila are fundamental experiments used to study the inheritance patterns and whether two genes assort independently or show linkage. This article explains how to interpret the phenotypic ratios from a test cross involving sepia eye and curled wing genes in heterozygous flies.
Analyzing Test Cross Result Phenotypes
Observed progeny counts from the test cross:
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Wild type: 400
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Sepia eyed: 150
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Curled winged: 100
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Sepia eyed curled winged: 350
Total progeny = 1000
Interpretation of Ratios and Genetic Linkage
In a classic Mendelian independent assortment for two heterozygous genes, the test cross progeny ratio is expected to be 1:1:1:1 for four phenotypes (wild type, sepia, curled, and double mutant). Here, the phenotype counts are far from equal: wild type and double mutant are much more frequent than the single mutants.
This pattern suggests the genes are not assorting independently but show linkage. The presence of higher numbers of parental types (wild and double mutants) compared to recombinant single mutants (sepia only and curled only) is characteristic of linked genes.
Explanation of Options
(1) Independent assortment, as wild and double mutant types are in almost equal proportion – Incorrect: Although wild and double mutants are similar in number, the disproportional combined with few single mutants indicates linkage, not independent assortment.
(2) Unequal segregation, as it is showing departure from 1:1:1:1 ratio – Incorrect: Unequal segregation implies problems in chromosome segregation, but here the deviation is explained by linkage.
(3) Complete linkage, as the two genes are very closely placed – Incorrect: Complete linkage would yield only parental types and no recombinants; some single mutants are present indicating linkage but not complete.
(4) Linkage, the two genes are separated by 25 CM distance – Correct: The presence of recombinant phenotypes indicates linkage with some recombination. The ratio supports linkage, and 25 cM is a reasonable estimate for moderate linkage distance.
This analysis shows that the sepia eye and curled wing genes are linked on the chromosome, separated by an estimated distance around 25 cM, explaining the deviation from the 1:1:1:1 ratio seen in independent assortment.
