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Basic Concepts in Drosophila melanogaster Genetics. E. David Peebles, Sharon K. Whitmarsh, and Matthew R. Burnham, 2001. Produced by MSU Department of Agricultural Commmunications. Video Script There are several features which may be used to distinguish the sexes of the fruit fly - Drosophila melanogaster. The most reliable method is through determination of the presence or absence of sex combs on the front legs of the fly. Sex combs are a fringe of about ten stout black bristles on the distal surface of the basal or uppermost tarsal joint of the first pair of legs. A sex comb looks like a small "ink spot". Sex combs are present on males but not on females. This feature is most reliable because of its ease in distinction and because it can be found at all levels of maturity, even while the individual is still within the pupal case or puparium. In older, mature flies, the posterior part of the abdomen is quite dark in males and considerably lighter in females. The tip of the abdomen is also more rounded in males than in females. In general, mature male fruit flies are smaller than females of the same strain, however, size is not a reliable character for sorting the sexes. Remember to use the sex combs as the best means to identify the sex of fruit flies of any age. We will not take time in this video to describe the classification, stages of development, duration of the stages at various temperatures, and the type of media that is used to grow and propagate Drosophila melanogaster. Please read this information in your instructional workbook and know it well for testing. You will also be responsible for being able to identify and understand the inheritance of two individual traits in Drosophila melanogaster. The first is wing type, which is controlled by an autosomal or non-sex-linked gene. Flies having the dominant gene for normal wings (V) would have either a VV or a Vv genotype. Flies with normal wings have fully developed wings and are able to fly. Those having no dominant (V) gene would have vestigial wings and would have a vv genotype. Flies with vestigial wings have nothing more than vestiges or degenerated elements of wings and are unable to fly. However, they can still jump and run quite well. The inheritance of these traits follows the simple rules for alleles having a dominant - recessive relationship with the sex of the fly having no influence. The second trait we will examine is eye color. Eye color is controlled by a sex-linked gene. More specifically, this gene is X-linked in Drosophila melanogaster. Male flies with red eyes having the dominant gene for red eyes will have an XWY genotype. Female flies with red eyes having the dominant gene will either have an XWXW or XWXw genotype. Male flies with white eyes (the recessive trait) will have an X wY genotype. Female flies with white eyes will have an XwXw genotype. Remember that in Drosophila melanogaster males are heterogametic and females are homogametic. Eye color in Drosophila melanogaster is actually controlled by multiple alleles, however, we are only examining the most dominant (red) and most recessive (white) alleles in the allele dominance heirarchy. Because the genes for eye color are X - linked, the sex of the fly has a definite bearing on the frequency of the red and white phenotypes in males and females. Remember to use the W (red) and w (white) symbols as superscripts on your X symbols in order to account for this fact. The reciprocal cross is the means by which you can differentiate between autosomal and sexlinked traits. When you perform a reciprocal cross, you switch the phenotype of the parent in relationship to its sex in the P-generation. This has no effect on the F1 and F2results for an autosomal trait like wing-type, but it does for a sex-linked trait like eye color. P-generation flies having normal wings will also have white eyes, however, normal wings may be found in red and white-eyed flies in both the F1 and F2 generations depending on the P generation cross performed. Reciprocal crosses 1 and 2 should be reviewed in your workbook (page 160) to understand these relationships.