You want to determine whether short wings in fruit flies are the result of a dominant or recessive mutation. You cross a short-winged fly stock with a long-winged fly stock (which is considered to be wild type or / ). The progeny have a 1:1 ratio of short to long wings. You then allow the F1 short-winged flies to mate. The F2 progeny have a phenotypic ratio of 3:1 short to long wings. Short wings are likely the result of:

a dominant mutation

Explanation:

A monohybrid testcross is a cross-breeding experiment used to determine if an individual exhibiting a dominant phenotype is homo-zygous dominant or heterozygous for a particular phenotypic trait (in this case, wing length). In a monohybrid testcross, a 1:1 phenotypic ratio shows that the dominant parental phenotype was a heterozygote for a single gene that has complete dominance. Moreover, a 3:1 ratio in the F2 is expected of a cross between heterozygous F1 individuals, which means that 75% of individuals with short wings have the dominant allele that masks the expression of the long-wing trait (i.e. the recessive allele).

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