BREEDING DROSOPHILA MELANOGASTER

Abstract
The influence of Drosophila melanogaster on the understanding of basic genetic concepts is examined. Crosses were performed between different species of both wild type and mutant flies. The results of both the first filial generation (F1) and the second filial generation (F2) are presented with the use of Punnett squares. Basic genetic concepts such as autosomal dominance, autosomal recessive, sex-linked dominant and sex-linked recessive characteristics were illustrated.

Introduction
Gregor Mendel (1822  1884), an Austrian Monk, in the 1980s worked on the heritable traits of plants (pea plants) and revolutionalized the study of genetics. He carefully quantified and analyzed the ways these heritable traits were passed to offsprings. In doing this, he discovered several principles of genetics in use today. However, during the course of his experiments, he attributed the transmission of physical characteristics to heritable factors, which are now known as genes. A gene is said to be the basic unit of genetic information which comprises the DNA.

Drosophila melanogaster is a member of the melanogaster subgroup of the Drosophilidae family. It is commonly known as the fruit fly. This organism is one of the most commonly used species in the study of genetics, mainly because they are easy to handle, they can be bred easily, have relatively short life cycles, have physically evident characteristics which can be used for their gender differentiation, and more importantly, have a relatively simple genome (4 pairs of chromosomes  1 pair of sex chromosomes and 3 pairs of autosomes).

Although Charles W. Woodworth was credited with being the first to breed Drosophila in quantity and for suggesting to W. E. Castle that they might be used for genetic research during his time at Harvard University, it was not until 1910 that Thomas Hunt Morgan began using fruit flies in experimental studies of heredity at Columbia University. Thomas Morgan, some few years after Mendels death used the heritable physical characteristics of the fruit fly (Drosophila melanogaster) to expand the present knowledge and understanding of genetics at that time. He was able to reveal, through his experiments, that the genes that control the transmission of these traits were located on chromosomes. Also, he discovered that certain characteristics (e.g., vestigial versus curly wings) were found on the same chromosomes that also determine their sex.

Gestation period of the fruit fly
Drosophila melanogaster has a complete metamorphosis life cycle with an egg, 3 larval instars, pupa, and adult. They have an average lifespan of about one month (30 days) at an optimal temperature of 29oC (84oF). The time of development of these flies vary with the environmental temperature. As temperatures increase, the developmental time increases also. At temperatures of about 21oC, the gestation period takes an average of two weeks after mating and fertilization of eggs before offsprings mature as adults. The process of development goes through different stages. After fertilization, a worm-like larva emerges from the embryo after about 24 hours. The larva feeds on microorganisms and also on the decaying material which surrounds it, and continues to grow until it molts three times (first, second, and the third instar). The molting process occurs one, two and four days, respectively after hatching to produce a third instar larva which molts again one more time, forming a pupa. The pupa is usually immobile and its body is modified and developed to form a winged adult.

After emerging from the pupal shell, the adults are ripe enough for mating within 8 to 12 hours and in most cases can survive under laboratory conditions for over a month. Two days after mating, the females begin to lay eggs and within three to four days after molting, the first instar larva becomes active.

Characteristics of the fruit fly (Wild type)
The wild type fruit fly, which possesses the normal phenotype, has distinguishing characteristics. Both the male and female wild type flies have similarities and differences also. Usually, the older male flies have darker posterior abdominal parts when compared to the females which have lighter parts. Another distinguishing characteristic is the abdominal tip. The abdominal tip is more round in the males than in the females. Also, male wild type flies are considerably smaller than the females. However, this cannot be used as a reliable distinguishing character. The most reliable distinguishing factor is the presence or absence of sex combs (short dark bristles on the distal part of the first pair of legs). Sex combs are present in all male wild type flies, regardless of the age or level of maturity.

Characteristics of the fruit fly (Mutant)
Mutations in genes that control the phenotypic characteristics of the flies produce mutant flies. Mutant flies are phenotypically different from the wild type flies. A mutant fly is said to be a fly that deviates from the normal phenotypic characteristics of the wild type fly. The specific mutation of the fly is designated with a letter(s) appropriate for the specific phenotype of the mutation. Dominantly inherited mutations are recorded in upper case alphabets while recessively inherited mutations are represented with lower case alphabets. Sex-linked mutations are designated as superscripts to the corresponding X- or Y- chromosome.

Discussion
The fruit fly, Drosophila melanogaster, has been used for several years to demonstrate Mendelian genetics. According to Mendel, some genetic traits are dominant, in the sense that, they are always expressed in heterozygous conditions, while others are recessive. Recessive traits are not expressed in heterozygous conditions. An organism is said to be homozygous for a particular trait if it expresses two identical alleles for the same trait, while it is said to be heterozygous if it has two different alleles for the same trait.
Crosses between different flies with either the homozygous allele or heterozygous allele confirm Mendels theories. The common phenotypes and notations of the wild type and mutant flies are as follows

Table 1. Abbreviations and descriptions of various common phenotypes. Source Mendelian Genetics Lessons from the Fruit Fly, 2009
Wild typeMutantEye color  Red ()White (w), Brown (se)Eye shapeNormal ()Lobed (L), Bar (B)AntennaeNormal ()Spineless (ssa)Wings Normal ()Vestigial (vg), Apterous (ap), DumpyJagged (dp)Body colorNormal ()Yellow (y), EbonyDark brown (e)BristlesNormal ()Singed (sn), Spineless (ss)

1. Cross between a light body male and a wild type female
The light bodied male will be designated (y), while the wild type female will be designated (). This cross is a monohybrid cross because it only involves a pair of alleles. This cross will pair a homozygous recessive (yy) male with a homozygous dominant () wild type female. The male will produce haploid sperms carrying the (y) allele while the eggs will carry the () allele. The first filial generation (F1) offsprings will all be heterozygous (y) for normal body colour. This is illustrated in the Punnett square below.

EggSpermyYyyyyFigure 1. Punnett square showing the genotypes of the P-generation and the expected genotypes of the F1 generation.

EggSpermyyyyyyFigure 2. Punnett square showing the genotypes of the F1-generation and the expected genotypes of the F2 generation.

A cross between the members of the first filial generation is shown in Fig. 2. The Punnett square reveals a genotypic ratio of 1 homozygous wild type, 2 heterozygous normal bodied, and 1 homozygous recessive light bodied. The phenotypic ratio is 1 light bodied and 3 normal bodied flies.

2. A cross between a light bodied female and a wild type male
Just like the cross illustrated in Fig. 1, the F1 generation offsprings would all be heterozygous (y) for normal body colour. The F2 generation offsprings will have a phenotypic ratio of 1 light bodied and 3 nornal bodied flies, and a genotypic ratio of 1 homozygous wild type, 2 heterozygous normal bodied, and 1 homozygous recessive light bodied.

3. A cross between a jagged winged female and a wild type male
The jagged winged female will be designated (dp), while the normal wild type male will have a notation of (). During meiotic division, haploid gametes are produced, with the male producing () sperms and the female producing (dp) eggs. The results of this cross is illustrated in the Punnett square below.

EggSpermdpdpdpdpdpdpFigure 3. Punnett square showing the genotypes of the P-generation and the expected genotypes of the F1 generation.

EggSpermdpdpdpdpdpdp Figure 4. Punnett square showing the genotypes of the F1-generation and the expected genotypes of the F2 generation.

Fig. 3 shows that all the offsprings of the first filial generation are heterozygous for normal wings. However, a second cross between the members of the first filial generation by mating a F1 male (normal) with a F1 female (normal) reveals a phenotypic ratio of 1 jagged winged and 3 normal winged flies. The genotypic ratio is 1 homozygous dominant normal, 2 heterozygous normal, and 1 homozygous recessive jagged winged fly. This is evident in Fig. 4.

4. A cross between a jagged winged male and a wild female
The notation for this cross is as the same for the cross done in Fig. 3 and 4. The first generation offsprings will all be heterozygous for normal wing. However, a second intra-F1 generation cross yields 3 normal wild type and 1 jagged winged phenotypes. The genotypic ratio is 1 homozygous wild type 2 heterozygous wild types 1 homozygous jagged.

5. A cross between a dark bodied male and a wild female
The dark bodied male will be designated (e) while the wild type female will be designated (). The Punnett square below reveals the predicted outcomes of the cross.

EggSpermeeeeeeFigure 5. Punnett square showing the genotypes of the P-generation and the expected genotypes of the F1 generation.

EggSpermeeeeeeFigure 6. Punnett square showing the genotypes of the F1-generation and the expected genotypes of the F2 generation.

The parents will produce a F1 generation in which all the offsprings are heterozygous wild type. A second cross between members of the F1 generation reveals a phenotypic ratio of 3 normal wild type 1 dark bodied. The genotypic ratio will then be 1 homozygous wild type 2 heterozygous wild types 1 homozygous dark bodied, as shown in Fig. 6.

6. A cross between a dark bodied female and a wild type male
Using the same notation as above, the dark bodied female is represented by (e) and the wild type male is represented by (). The male produces haploid sperms carrying the () allele, while the female also produces eggs carrying the (e) allele. A cross between these gametes will produce offsprings which are all heterozygous wild types. This represents the F1 generation (Fig 5). A cross between members of the F1 generation will produce two phenotypically different groups of offsprings. The offsprings produced are either wild type or dark bodied in a ratio of 31 respectively. The genotypic ratio is 1 homozygous wild type 2 heterozygous wild types 1 homozygous dark bodied.

7.  A cross between a brown eyed male and a wild type female
The genetic notation is a bit different from the notation of the other traits. This is because the trait for eye color is controlled by a sex-linked gene (Peebles, Whitmarsh,  Burnham, 2001). Therefore, male fly with brown eyes will be designated XseY while the female wild type will be designated XX. The male will produce haploid sperms with either Xse or Y, while the female will produce haploid eggs with X.

EggSpermXseYXX XseX  YXX  XseX  YFigure 7. Punnett square showing the genotypes of the P-generation and the expected genotypes of the F1 generation in a X-linked cross.

EggSpermXYXX  XXYXseX  XseXse YFigure 8. Punnett square showing the genotypes of the F1-generation and the expected genotypes of the F2 generation in a X-linked cross.

The above illustrated crosses involve sex-linked or X-linked characteristics. The first cross between the parents produced two kinds of genotypes, that is, X Xse and X  Y. This interpreted means that there are two wild type females and two wild type males, despite the fact that one of the parents was brown eyed. In the second cross illustrated in Fig 8, there are four genotypes and three phenotypes. The genotypic ratio is 1111 all through. Half of the males are wild types while the other half has brown eyes. Similarly, one half of the females are also wild types.  What if the phenotypes of the P1 generation are reversed

8. A cross between a brown eye female and a wild type male
The cross is demonstrated with the Punnett square in Fig. 9. The notation used above is still repeated, with the wild type male designated XY and the brown eyed female designated by XseXse.

EggSpermXYXseX XseXse  YXseX  XseXse  YFigure 9. Punnett square showing the genotypes of the P-generation and the expected genotypes of the F1 generation in a X-linked cross.

Again, the offsprings of this cross are an opposite of the P1 generation. The offsprings with the Y notation are the males while the others lacking the Y are females. Crossing these offsprings will yield the following

EggSpermXseYXX  XseXYXseXse  XseXse YFigure 10. Punnett square showing the genotypes of the F1-generation and the expected genotypes of the F2 generation in a X-linked cross.

Half of the F2 males are expected to be wild type, while the other half should have brown eyes. The F2 females are also expected to have a 1 wild type 1 brown eyed phenotypic ratio.

Although there were no brown eyed males in the parental generation, this does not discredit the result that there were both wild type and brown eyed males in the F2 generation. Also, examining the results of the F2 generation, heterozygous wild types can also be found.

Conclusion
Mendel, in the process of conducting several of his scientific experiments, observed the consistent appearance of some traits, which he called dominant, and the traits that reappeared recessive. The dominant traits are always expressed, no matter the number of alleles present (either one or two). This is demonstrated by the crosses in Figs. 1  7. The recessive alleles can only be expressed when both alleles are present. The inheritances of these traits abide by the Mendelian rules for characters having an autosomal dominant-recessive relationship.

Reciprocal crosses provide a means of distinguishing between autosomal traits and sex-linked characters. A reciprocal cross is performed by switching the phenotype of the parent in a direct relationship to its sex in the P1 generation. The fact that when reciprocal crosses were conducted in Figs. 1 -7 shows that the characters been inherited are not sex-linked. This makes them autosomal, and the dominant alleles are referred to as autosomal dominant while the recessive alleles are referred to as autosomal recessive. Reciprocal crosses have no effect on the F1 and F2 results for an autosomal trait like wing-type, but it does for a sex-linked trait like eye color.

X-linked characters can also be either dominant or recessive. There can never be any Y-linked dominant or recessive character. This is because the only major characteristic that may be located on the Y-chromosome are those that determine male differentiation. A male fly is said to be hemizygous because it can only possibly carry one copy of a gene on its X-chromosome, with the Y-chromosome being relatively redundant. Female flies with dominant wild type genes are asid to be X- linked dominant and those with the recessive genes are said to be X-linked recessive. This is because they carry two copies of the X-chromosome, unlike the males who have only one copy. A mutant fly must have the mutation on the single copy of its X-chromosome. In a mutant female fruit fly, the mutation must occur on both copies before it can be said to be mutant (X-linked recessive).

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