The eyes of Drosophila develop from imaginal discs, groups of cells set aside in the fly embryo that differentiate into the adult structures during the pupal stage. Despite their importance in nature, eyes are dispensable for fruit-fly life in the laboratory.

What complications might arise from genetic screens targeting an organ that differentiates late in development?

The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd⁺/bcd⁻) and a male homozygous for the mutation (bcd⁻/bcd⁻).

Predict the outcome (normal vs. failed embryogenesis) in the F₁ and F₂ generations of the cross described.

The maternal-effect mutation bicoid (bcd) is recessive. In the absence of the bicoid protein product, embryogenesis is not completed. Consider a cross between a female heterozygous for the bicoid alleles (bcd⁺/bcd⁻) and a male homozygous for the mutation (bcd⁻/bcd⁻).

How is it possible for a male homozygous for the mutation to exist?

The eyes of Drosophila develop from imaginal discs, groups of cells set aside in the fly embryo that differentiate into the adult structures during the pupal stage. Despite their importance in nature, eyes are dispensable for fruit-fly life in the laboratory.

Devise a genetic screen to identify genes directing development of the fly eye.

Dipterans (two-winged insects) are thought to have evolved from a four-winged ancestor that had wings on both T2 and T3 thoracic segments, as in extant butterflies and dragonflies. Describe an evolutionary scenario for the evolution of dipterans from four-winged ancestors. What types of mutations could lead to a butterfly developing with only two wings?

In Drosophila, loss-of-function Ultrabithorax mutations result in the posterior thoracic segments differentiating into body parts with an identity normally found in the anterior thoracic segments. When the Ultrabithorax gene was cloned, it was shown to encode a transcription factor and to be expressed only in the posterior region of the thorax. Thus, Ultrabithorax acts to specify the identity of the posterior thoracic segments. Similar genes were soon discovered in other animals, including mice and humans. You have found that mice possess two closely related genes, Hoxa7 and Hoxb4, which are orthologs (see Genetic Analysis 14.2 for definition) of Ultrabithorax. You wish to know whether the two mouse genes act to specify the identity of body segments in mice.

How will you create loss-of-function alleles of the mouse genes?

Which of the following is not a germ layer formed during gastrulation?