Question
In Drosophila, vestigial (partially formed) wings (vg) arerecessive to normal long wings (vg+)
and the gene for this trait is autosomal. The gene for thewhite-eye trait (w) is on the X-chromosome
and is recessive to wild type red eye color (w+). Suppose ahomozygous red-eyed, long-winged
female fly is crossed with a homozygous white-eyed,vestigial-winged male fly.
(a) What will be the appearance of the F1?
(b) What will be the appearance of the offspring of a cross of theF1 back to each parent?
(c) What will be the appearance of the F2?
Explanation / Answer
Eye color is X-linked, so has to be passed on by the Xchromosome- therefore the male can only pass on 1 allelle for this,seeing as he only has 1 X to pass on to his daughters. Themonohybrid cross for eye color is as follows: malegametes Xw Y female Xw+ Xw+Xw Xw+Y gametes Xw+ Xw+Xw Xw+Y F1 result: all offspring have red eyes (females areheterozygous, males are hemizygous, or only having one allele foreye color) Next the wings (this is an autosomal triat, so doesn'thave to be passed on by the X chromosome like eye color): malegametes: vg vg female + vg+vg vg+vg gametes + vg+vg vg+vg F1 result is all offspring have normal wings and are carriersfor the vestigial wing. (heterozyous) This will get you started, just remember eye color is Xlinked. In this case it is easier to make twomonohybrid crosses (as above) and determine the possiblecombinations of offspring with the two traits (eye color and wingtype), just replace the alleles used above with those called for inyour problem. For example, in part b, replace one parent's geneswith those of one of the offspring (make sure it's the one of theopposite gender in your X-linked cross). For the F2 generation, youshould end up with a 9:3:3:1 ratio, 9 being dominant in bothtraits, 3 dominant in eye color but recessive in wing type, 3dominant in wing type but recessive in eye color, and 1 beingrecessive in both traits.