Model Answer
0 min readIntroduction
Sex determination, the biological process that determines the sex of an individual, is a fundamental aspect of reproduction. While various mechanisms exist across species – including chromosomal, hormonal, and environmental factors – the genic balance theory provides a unique explanation for sex determination in *Drosophila melanogaster*. Proposed by Calvin Bridges in the 1920s, this theory posits that sex is not determined by the presence or absence of a specific chromosome (like the Y chromosome in humans), but rather by the ratio of X chromosomes to autosomes (non-sex chromosomes). This ratio dictates the developmental pathway towards either male or female characteristics.
Sex Determination: An Overview
Sex determination is the process by which the sex of an organism is established. Several mechanisms are known, including:
- Chromosomal Determination (e.g., Humans): Presence of Y chromosome determines maleness.
- Hormonal Determination (e.g., some reptiles): Temperature during incubation influences sex.
- Genic Balance Theory (e.g., Drosophila): Ratio of X chromosomes to autosomes determines sex.
The Genic Balance Theory in Drosophila
Bridges proposed that sex in Drosophila is determined by the balance between ‘female-determining’ genes located on the X chromosome and ‘male-determining’ genes distributed on the autosomes. He termed the effective genetic contribution of the X chromosome as ‘X’ and the genetic contribution of all autosomes as ‘A’. The sex of an individual is determined by the ratio X/A.
Key Principles:
- A ratio of X/A = 1.0 results in a female.
- A ratio of X/A = 0.5 results in a male.
- Any deviation from these ratios leads to intersex phenotypes.
The ‘female’ genes on the X chromosome produce a female-determining factor, while the ‘male’ genes on the autosomes produce a male-determining factor. The balance between these factors determines the sexual phenotype.
Normal Karyotypes and Sex Determination
A normal Drosophila female has two X chromosomes (XX) and a normal set of autosomes. Therefore, X/A = 2/2 = 1.0, resulting in a female. A normal male has one X chromosome (X) and a normal set of autosomes. Therefore, X/A = 1/2 = 0.5, resulting in a male.
Abnormal Karyotypes and Phenotypes
The power of the genic balance theory lies in its ability to explain the phenotypes observed in individuals with abnormal chromosome numbers. Let's examine some examples:
| Karyotype | X/A Ratio | Phenotype | Explanation |
|---|---|---|---|
| XXY | 3/3 = 1.0 | Female | The increased X/A ratio favors female development, overriding the presence of the Y chromosome. |
| X0 | 1/2 = 0.5 | Male | Despite the absence of a second X chromosome, the X/A ratio remains at 0.5, resulting in a male. |
| XXX | 3/2 = 1.5 | Metafemale (sterile female) | The significantly increased X/A ratio leads to a sterile female with broader abdomen and other female characteristics. |
| XYY | 1/3 = 0.33 | Metamale (sterile male) | The decreased X/A ratio leads to a sterile male with smaller size and other male characteristics. |
| XX | 2/2 = 1.0 | Female | Normal female. |
These examples demonstrate that the absolute number of X or Y chromosomes is less important than the relative ratio of X chromosomes to autosomes. The theory accurately predicts the sex phenotype even in cases where the typical chromosomal sex determinants are altered.
Conclusion
The genic balance theory of sex determination in *Drosophila* provides a compelling explanation for how sex is determined not by the presence of specific chromosomes, but by the quantitative balance of genes on the X chromosomes relative to the autosomes. The theory’s predictive power, as demonstrated by its ability to explain phenotypes resulting from abnormal karyotypes, solidified its importance in the field of genetics. While more complex mechanisms are now known to influence sex determination in other organisms, the genic balance theory remains a cornerstone of our understanding of sex determination and a classic example of how genetic ratios can dictate developmental pathways.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.