Figure 1 | Outline of the crossing schemes used in the Heidelberg screen and the first screen for enhancers of sevenless.   a | A typical crossing scheme for a screen for mutants on an autosome that produce a zygotic phenotype, based on those used in the Heidelberg screens for mutants that affect the pattern of the larval cuticle. Male flies are fed ethyl methane sulphonate (EMS) to induce mutations and are crossed en masse to virgin females (indicated by the symbol ) that carry a BALANCER for the chromosome to be screened (grey). As the mutations are induced in mature spermatids, each F₁ male inherits a mutagenized chromosome (red) carrying a different spectrum of mutations (asterisk). Single F₁ males that carry a mutagenized chromosome in trans to the balancer are then backcrossed to balancer stock to generate F₂ males and females that carry the same mutagenized chromosome. When these are crossed to each other, 25% of the F₃ progeny will be homozygous for the mutagenized chromosome. As only one chromosome is screened at a time, the other chromosomes are not shown. In the Heidelberg screens, the lines were first screened for the absence of flies that were homozygous for the mutagenized chromosome, which indicated that it carried a zygotic lethal mutation, and cuticle preparations were then done on the embryos from these crosses to see if 25% showed a phenotype. In the screens for MATERNAL-EFFECT MUTATIONS (see main text), the eggs laid by homozygous females from non-lethal lines were screened for phenotypes. b | The crossing scheme for the original screen for enhancers of sev (Ref. 32). Males that are hemizygous for a null allele of sev (sev^d2) were mutagenized with X-rays and crossed to sev^d2 homozygous females that carry a temperature-sensitive allele of sev (sev^B4) as a transgene inserted on the third chromosome balancer, TM3. The F₁ TM3 flies were then screened for a reduction in the number of R7 photoreceptor cells in the eye.

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© 2002 Nature Publishing Group