Genetic Mapping of Drosophila Melanogaster

Genetic Mapping of Drosophila melanogaster

Introduction

Genetic mapping is the process and tool used to determine whether mutations affect different genes. This is based on their inheritance pattern, through charting the locations of genes on a chromosome or DNA molecule ( D. L. Hartl, E. W. Jones, 2012). Mapping can then establish the relative distance between the genetic markers on the chromosome in linkage units.

Drosophila melanogaster was used in the genetic crossing to demonstrate Mendelian inheritance and the inheritance patterns of genes, which are located on the X chromosome also known as “sex linkage”. (Figure 1)

Figure 1: Female vs Male Drosophila & their chromosomes

[pic 1]

This is due to the fact that Drosophila melanogaster has many variants including, body colour, eye colour, bristles, wing size, wing shape and wing vein.

The mutant used throughout this experiment was Purple which one of the three autosomal recessive eye colour mutations. The female Drosophila was chosen to undergo recombination, as male Drosophila displays an absence of crossing over during meiosis. During meiosis synaptonemal complexes do not form, these protein complexes are essential for the pairing of homologous chromosomes, without these complexes crossing over cannot occur. (J. Kammholz, A. W. Campbell, 2001)

Dihybrid and Trihybrid testcrosses are then conducted as they cross two or more genes one on a heterozygote and one homozygote that are recessive for all genes. This can define whether recombination has occurred in the heterozygous parent during the phase of gamete formation (J. Kammholz, A. W. Campbell, 2001).

Phenotypes of zygotes are equal to or the same as genotypes of heterozygous parent gametes. This allows parental gametes and the recombination number to be determined from the offspring’s of the phenotypes. Dihybrid test cross is first used with 7 other mutations from the other variant categories crossed with the purple autosomal recessive eye colour mutation. These testcrosses can resolve the two closest recombinant x-linked mutations to purple.

Trihybrid test cross is then used with the two x-linked genes along with the mutation purple. During this test cross a double recombinant can occur. This is when there is a double crossover between two loci in meiosis. The outside loci of the cross overs need to be brought back into parental combination. A double cross overs show that a 3-point cross is more accurate than a 2 point cross.

Aim

The aim of this experiment is to analyze the inheritance pattern of the two closest X-linked genes that are located on the X chromosome, to the recessive autosomal mutation of Purple eyed Drosophila melanogaster. Establishing the order, presence and degree of gene linkage on other recessive autosomal mutations will describe the relative distance, between the two genes that are linked on the chromosome.

 Results

 Dihybrid Test Crosses

 Parents
Female x Male
+ (Wild Type) x PR, BL (Purple - eyes colour & Black – body colour)

Table 1: F2 Progeny Ratios
Recombinant Value:
Parent 1= 455   Parent 2= 491   Recombinant 1= 36   Recombinant 2= 33

36+33/1015 = 0.067980296
0.067980296 x 100=6.8 mu

Parents
Female x Male
+ (Wild Type) x PR, E (Purple - eye colour & Ebony – body colour)

Table 2: F2 Progeny Ratios

Recombinant Value:
Parent 1= 256   Parent 2= 250   Recombinant 1= 249   Recombinant 2= 234

249+234/989= 0.488372093

0.488372093 x 100= 48.8 mu

Parents
Female x Male
+ (Wild Type) x PR, SS (Purple - eye colour & Spineless - bristles)

Table 3: F2 Progeny Ratios

Recombinant Value:
Parent 1= 257   Parent 2= 255   Recombinant 1= 249   Recombinant 2= 218