Explain the phenomenon of X-linked inheritance in Drosophila. (IAS 2018/15 Marks)

Introduction

X-linked inheritance in Drosophila refers to the inheritance pattern of genes located on the X chromosome. In Drosophila, the X chromosome is larger than the Y chromosome and carries a significant number of genes that are responsible for various traits. 

X-linked Inheritance in Drosophila

• Sex Chromosome Constitution in Drosophila:
  
  • Drosophila has a XX sex chromosome configuration in females and XY in males.
  • The X chromosome carries a number of genes responsible for various traits, some of which are involved in eye color, wing shape, etc.
  • The Y chromosome in males is much smaller and does not carry many genes, primarily involved in sex determination.
• X-linked Recessive Traits:
  
  • X-linked recessive traits are typically expressed in males, as they have only one X chromosome.
    
    • Example: Red-eye color in Drosophila is a typical example of a recessive X-linked gene.
  • Females require two copies of the recessive allele (one on each X chromosome) for the trait to be expressed, making them less likely to show the trait unless they are homozygous recessive.
  • Carrier Females (heterozygous for the trait) do not express the trait but can pass the allele to their offspring.
  • Example: A female carrier of red-eye color allele (X^r X^R) would have normal red eyes but could pass the recessive allele to her sons (who would express the trait if they inherit X^r).
• X-linked Dominant Traits:
  
  • X-linked dominant traits are expressed in both males (with one X chromosome) and females (with two X chromosomes).
  • A single copy of the dominant allele on the X chromosome is sufficient to express the trait.
  • Inheritance pattern:
    
    • A father will pass his X-linked dominant allele to all of his daughters (since daughters inherit the father’s X chromosome).
    • A mother will pass her X-linked dominant allele to both sons and daughters.
• X-linked Inheritance in Males and Females:
  
  • Males (XY) inherit their single X chromosome from their mother, meaning they will express any X-linked recessive or dominant traits carried on the X chromosome.
  • Females (XX) inherit one X chromosome from each parent. Thus, they have a backup X chromosome, which can compensate for the defective allele on the other X in case of a recessive trait.
• Examples of X-linked Traits in Drosophila:
  
  • Red eye color (X^R) vs white eye color (X^r): In this example, the white eye color trait is recessive and located on the X chromosome. Male flies with one X chromosome and the recessive allele will have white eyes, whereas females would need both X chromosomes carrying the recessive allele to express white eyes.
  • Bar eyes: Another example of a mutation on the X chromosome causing an abnormal eye shape in Drosophila.
• Meiotic Mechanisms of X-linked Inheritance:
  
  • Males inherit their single X chromosome from their mother. The Y chromosome is inherited from their father.
  • Females inherit one X chromosome from each parent, giving them a combination of X-linked traits from both the mother and father.
  • Crosses between male and female Drosophila can reveal the inheritance pattern of X-linked traits by examining the ratio of affected male and female offspring.

Consequences of X-linked Inheritance:

• Sex Bias in Expression:
  
  • Because males have only one X chromosome, X-linked recessive traits are more commonly expressed in males than in females.
  • In contrast, females need two copies of a recessive allele for the trait to be expressed, making the trait less common in females.
• Carrier Females:
  
  • Female carriers of X-linked recessive traits (heterozygous X^A X^a) do not express the trait but can pass the allele to offspring.
  • Sons of a carrier mother have a 50% chance of inheriting the X chromosome carrying the recessive allele and will express the trait if the allele is recessive.
• X-linked Dominant Traits:
  
  • In females, one copy of the dominant allele is sufficient to express the trait.
  • In males, the trait will be expressed if they inherit the dominant allele from their mother.

Conclusion

X-linked inheritance in Drosophila follows a unique pattern due to the presence of the X chromosome and its role in determining certain traits. This phenomenon is crucial for studying genetic inheritance patterns in Drosophila and can provide valuable insights into the mechanisms of inheritance in other organisms as well.