Sex-linked Inheritance in Drosophila

The Discovery: Morgan (1910) found a mutant white-eyed fly within a normal red-eyed population. He discovered that the results of crosses depended on the sex of the parent introducing the trait, which differs from standard Mendelian characters.

Cross 1: White-Eyed Male × Red-Eyed Female

• F1 Generation: All offspring were red-eyed, proving that red eye colour is the dominant trait.
• F2 Generation: All females were red-eyed, whereas the males were half red-eyed and half white-eyed (making one-fourth of the total F2 offspring white-eyed).

Cross 2: White-Eyed Female × Red-Eyed Male

• F1 Generation: Females were red-eyed, but the males were white-eyed.
• F2 Generation: Both sexes produced an equal number of red-eyed and white-eyed flies.

Rules of Transmission (The "Why")

• Male Vulnerability: The original mutant observed by Morgan was male because males possess only one X-chromosome, and there is no gene on the Y-chromosome to mask the expression of the recessive white trait.
• Inheritance Path: Sex-linked genes are consistently transmitted from mother to son and never from father to son.

Other Sex-Linked Traits

• Wing Length: The genes determining wing length in Drosophila are also sex-linked and follow a similar inheritance pattern to eye colour. Normal wings are a dominant trait over miniature wings.

Sex-linked inheritance in Drosophila

• Sex-linked inheritance involves genes on sex chromosomes.
• In Drosophila, the white-eye trait is X-linked and recessive.
• Morgan’s reciprocal crosses showed different results, proving sex linkage.
• Males show recessive X-linked traits more often because they are hemizygous.
• Criss-cross inheritance is a hallmark of X-linked inheritance.