Genetics of Drosophila melanogaster: Linkage Analysis

Abstract

The purpose of this experiment was to identify and analyze genetic linkages in Drosophila melanogaster by crossing individuals expressing mutant traits with wild type mates. Assuming Mendel's Laws hold true, we identified females with five visible mutant traits and crossed them with wild type males. We collected their progeny ($F_1$) and then analyzed their phenotypes to determine if certain genes reside on the same chromosome, as well as testing for sex linkage.

---

Introduction

Gregor Mendel, recognized as the father of genetics, demonstrated through extensive work with pea plants that discrete traits are transmitted from parents to offspring as alleles. From these observations, he formulated three foundational laws of inheritance:

1. Law of Segregation: Alleles for a trait segregate during gamete formation, such that each gamete receives only one allele.
2. Law of Independent Assortment: Alleles for different traits assort independently during gamete formation.
3. Law of Dominance: In a heterozygote, the dominant allele masks the phenotypic expression of the recessive allele.

Chromosomal theory of inheritance, established later by Thomas Hunt Morgan, revealed that genes reside linearly on physical chromosomes. Consequently, alleles situated in close physical proximity on the same chromosome exhibit genetic linkage and violate independent assortment unless separated by homologous recombination.

In modern Drosophila genetics, balancer chromosomes are indispensable tools for maintaining lethal mutations and suppressing crossing over. Balancers carry multiple complex chromosomal inversions that eliminate viable recombinant progeny during meiosis. They typically carry a dominant marker phenotype, such as Curly wings ($Cy$), and recessive embryonic lethal alleles. segregate as a discrete non-recombining unit across generations.

In this experiment, female parents possessed the genotype:

$$\frac{w,y}{+};\frac{Cy}{Scu};\frac{Stu}{+}$$

Mated with wild type male parents:

$$\frac{+}{Y};\frac{+}{+};\frac{+}{+}$$

The female stock carried the $CyO$ balancer chromosome on the second chromosome.

---

Materials and Methods

Materials

The female flies expressed five visible mutant markers:

• White eyes ($w$)
• Yellow body ($y$)
• Scutoid ($Scu$)
• Stubble bristles ($Stu$)
• Curly wings ($Cy$)

Wild type males expressed:

• Red eyes ($+$)
• Brown body ($+$)
• No scutoid ($+$)
• Normal bristles ($+$)
• Straight wings ($+$)

Reagents and equipment included standard cornmeal-yeast-agar media vials, live yeast paste, fly pads with continuous ${\text{CO}}_2$ anesthesia, fine sable brushes, and stereomicroscopes.

Methods

In fresh food vials, a small dab of active yeast paste was added to stimulate oogenesis. Six virgin mutant females and three wild type males were introduced per vial and maintained for 14 days at 25°C. On day 14, newly emerged $F_1$ adults were collected, anesthetized under ${\text{CO}}_2$, and scored for sex and phenotypic marker combinations.

---

Results

$F_1$ Progeny Phenotypes

Gender	Eye Colour	Wing Shape	Scutoid	Stubble	Body Colour
Female	Red	Curly	No	No	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Curly	No	Yes	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	No	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Female	Red	Straight	Yes	Yes	Brown
Male	White	Curly	No	No	Yellow
Male	White	Curly	No	No	Yellow
Male	White	Curly	No	No	Yellow
Male	White	Curly	No	Yes	Yellow
Male	White	Curly	No	Yes	Yellow
Male	White	Curly	No	Yes	Yellow
Male	White	Curly	No	Yes	Yellow
Male	White	Straight	No	No	Yellow
Male	White	Straight	Yes	No	Yellow
Male	White	Straight	Yes	No	Yellow
Male	White	Straight	Yes	No	Yellow
Male	White	Straight	Yes	No	Yellow
Male	White	Straight	Yes	Yes	Yellow
Male	White	Straight	Yes	Yes	Yellow
Male	White	Straight	Yes	Yes	Yellow
Male	White	Straight	Yes	Yes	Yellow
Male	White	Straight	Yes	Yes	Yellow
Male	White	Straight	Yes	Yes	Yellow

---

Discussion

Sex Linkage

Eye colour and body colour exhibit complete sex linkage: all 22 females showed red eyes and brown bodies, whereas all 18 males displayed white eyes and yellow bodies. This complete co-segregation demonstrates that both the white eye ($w$) and yellow body ($y$) loci reside on the X chromosome and are recessive to their wild type alleles.

Autosomal Chromosome Mapping

1. Wing Shape and Scutoid ($Cy$ and $Scu$):
   A strict negative association is observed between Curly wings and Scutoid bristles: 15 curly-winged flies have no scutoid, whereas 24 straight-winged flies possess scutoid bristles (with only 1 single individual displaying straight wings and no scutoid, likely an observational or scoring anomaly). This indicates that the genes for wing curvature ($Cy$) and scutellar bristle loss ($Scu$) are located in repulsion on homologous second chromosomes ($\frac{Cy}{Scu}$).

2. Stubble ($Stu$):
   The distribution of Stubble vs. normal bristles (25 stubble : 15 normal) assorts independently of sex, wing shape, and scutoid markers, confirming that Stubble resides on an independent autosomal linkage group (Chromosome 3).

Dominance and Genotypic Deductions

Eye and Body Colour (X-linked)

Female parents carried $w,y$; male parents were wild type $+/Y$.

• If females were heterozygous ($\frac{w,y}{+}$), 50% of male progeny would be wild type ($+/Y$).
• Because 100% of $F_1$ males (18/18) expressed white eyes and yellow body ($\frac{w,y}{Y}$) and 100% of $F_1$ females (22/22) were wild type ($\frac{w,y}{+}$), the maternal genotype was homozygous $\frac{w,y}{w,y}$, and wild type alleles ($w^{+},y^{+}$) are dominant.

$$\begin{array}{|ccc|}\hline \text{Gametes}& \mathbf{+}& \mathbf{Y}\\ \mathbf{w}\mathbf{,}\mathbf{y}& \frac{w,y}{+}\text{ (Red, Brown female)}& \frac{w,y}{Y}\text{ (White, Yellow male)}\\ \mathbf{w}\mathbf{,}\mathbf{y}& \frac{w,y}{+}\text{ (Red, Brown female)}& \frac{w,y}{Y}\text{ (White, Yellow male)}\\ \hline\end{array}$$

Wing Shape ($Cy$)

Curly wings ($Cy$) is a dominant marker: 15 out of 40 (37.5%) progeny displayed curly wings. Since the cross was $\frac{Cy}{+}\times \frac{+}{+}$, the expected phenotypic ratio is 1:1 curly : straight.

Scutoid ($Scu$)

24 out of 40 flies (60%) exhibited Scutoid, demonstrating that $Scu$ is dominant and present in a heterozygous configuration in the maternal stock ($\frac{Scu}{+}$).

Stubble ($Stu$)

25 out of 40 flies (62.5%) exhibited Stubble bristles, indicating that Stubble is also a dominant mutation maintained heterozygously ($\frac{Stu}{+}$).

---

References

1. Scitable by Nature Education. Gregor Mendel and the Principles of Inheritance. Nature Education
2. Bloomington Drosophila Stock Center. Introduction to Balancer Chromosomes. BDSC