Download Basic Concepts in Drosophila Melanogaster Genetics Video Script

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.