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1 INVESTIGATION 1 - DIHYBRID CROSS IN DROSOPHILA Examination of the phenotypic ratio produced from a dihybrid cross involving two mutant phenotypes is the classic means by which the linkage relationships between gene loci is determined. In a cross involving two heterozygous loci, each with complete dominance, the expected phenotypic ratio for unlinked traits that show independent assortment is the classic 9:3:3:1 ratio. In the case of genes that are physically linked on the same chromosome, two loci that are closely linked on the same chromosome will tend to segregate together and distort the expected ratio. Further, in many species, the male and female members have characteristic sex chromosomes , and gene loci that are located on these chromosomes will show differ patterns of segregation in the two genders. You will be provided with either data or culture bottles containing the F 2 generation of two dihybrid Drosophila crosses involving flies with two distinctive mutant phenotypes (e.g., white eyes and/or vestigial wings ). Your task is to determine the linkage relationships of the genes responsible for the phenotypes: they may be independently assorting, or they may be linked on autosomal chromosomes, or they may be sex-linked. The F 2 generations you will observe are the result of the parental (P 1 ) cross (Cross A) between: Phenotype 1( White eyes) ± X Phenotype 2 (Vestigial wings) ² and the reciprocal P 1 cross (Cross B) between: Phenotype 2 (Vestigial Wings) ± X Phenotype 1(White eyes) ² The two mutant phenotypes you are looking at therefore are: Mutant phenotype 1- White eyes and Mutant Phenotype 2 - small deformed wings (Vestigial) OBSERVATIONS: Write a description of phenotypes 1 and 2; draw these phenotypes showing or explaining what a normal wing/eye would look like. Count the number of males and females separately to see if the trait is expressed in equalling in both sexes. (Note: The number of males and females may not be equal as females tend to hatch earlier)
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This note was uploaded on 03/06/2011 for the course BIOL 340 taught by Professor Margaretjefferson during the Fall '09 term at California State University Los Angeles .

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