How long does the Drosophila melanogaster lifecycle take?

The complete Drosophila lifecycle takes approximately 10 days at 25°C, progressing through four main stages: embryo, larva, pupa, and adult. Temperature significantly affects development speed. At 18°C, the lifecycle extends to about 19 days, while at 29°C it shortens to only seven days. Maintaining consistent temperature control is essential for predictable experimental timing.

What is the purpose of creating nursery vials in fly genetics experiments?

Nursery vials allow adult flies to mate and lay eggs over 3-5 days in a controlled environment. After this period, adults are removed and returned to stock vials, leaving eggs and larvae to develop. Checking nursery vials every 6-8 hours ensures newly eclosed females are virgin, since they require 8-10 hours to reach sexual maturity. This timing is critical for controlled genetic crosses.

How do you distinguish male and female Drosophila flies?

Fly sexing requires a magnifying glass to examine the posterior abdomen. Males are slightly smaller with darker, solid pigmentation on the abdominal tip. Females are larger and display striped patterns rather than solid coloring. Accurate sexing is essential for maintaining separate experimental vials and ensuring virgin females for controlled breeding experiments.

Why must you obtain Drosophila cultures at least two weeks before the experiment?

Two weeks allows sufficient time to generate virgin females through controlled breeding. Adult females store sperm after mating, so this preparation period is necessary to produce flies of known mating history. Additionally, it takes approximately two weeks for 25 healthy adults to produce about 50 new adults, ensuring adequate fly populations for all student groups and class spares.

What equipment and materials does each lab group need for the fly genetics experiment?

Each group requires five virgin red-eyed females, five virgin white-eyed females, five red-eyed males, and five white-eyed males in separate vials. Additional materials include a morgue vial with 70% ethanol, two anesthesia wands, fly anesthetic, a paintbrush, magnifying glass, plain paper, and two empty vials for crosses. Proper organization of these materials ensures smooth experimental execution.

What happens if you miss the 6-8 hour window when collecting newly eclosed flies?

If females are not collected within 6-8 hours of eclosion, they may not be virgins, as they reach sexual maturity in 8-10 hours. Non-virgin females should be discarded into a morgue tube containing 70% ethanol or returned to the stock vial. Males do not require virgin status but must be kept in male-only containers to prevent unwanted matings.

How should Drosophila cultures be stored and maintained during the experiment?

Keep flies in an incubator or controlled room maintained at 23-25°C when not being moved or counted. This temperature range ensures a predictable 12-day lifecycle for experimental planning. Proper environmental control is critical for synchronizing fly development across multiple experimental vials and maintaining consistent phenotypic expression of eye color traits.

Genetics of Organisms

Concepts Instructor Prep Student Protocol

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An inheritance is something passed from one generation to another. In some contexts, this means stuff like houses and money. In the context of biology, however, the study of genes and how they are inherited, is called genetics. Gregor Mendel is credited with being the father of modern genetics. His work is responsible for our understanding of how noticeable physical traits, or phenotypes, are passed from one generation to the next. Famously, he studied such traits in pea plants.

The pieces of information controlling those phenotypes are called genes. As there are two copies of each gene, known as alleles, we can represent these as letters. Here, we'll use the letter P. This is the first step of Mendel's famous experiment on pea flower coloration, represented here, using a tool called the Punnett square. Mendel found that when he crossed purple flowers with white ones, all of the progeny, or the first-generation plants, had purple flowers. This is because the purple color is dominant, shown using uppercase P. And carrying even one dominant allele means the phenotype will be expressed. But interestingly, when these purple flowers were crossed again, 1/4 of them were white. Where were the white flowers in the first generation? All of the first generation of purple-flowered plants were heterozygous, meaning that they had one capital purple P and one lowercase, or white, P. When they passed on their alleles in the F2 generation cross, this meant that 1/4 of the offspring received two small P alleles, and so, expressed the white phenotype. Mendel didn't have the Punnett square tool to use, he had to figure all of this out by keeping track of thousands of plants, and then noticing patterns in their numbers.

From this evidence, and a lot more, we now know that genes are also present in two copies in other organisms, too, like humans, and flies. In following up on Mendel's work, several scientists found that not all inheritance patterns followed the simple, basic model that Mendel proposed. For example, in hemophilia, a genetic clotting disorder, unaffected mothers were capable of transmitting the disease to their male children. The reason behind this lies in chromosomes, which were studied by Thomas Hunt Morgan, using his famous fruit flies, Drosophila. Because of Morgan and others, we now know that chromosomes are long strands of DNA that typically exist in pairs. Here, we can see that Drosophila has four of them. These chromosomes have genes on them for different traits, much like how a cookbook contains lots of different recipes. Nowadays, using modern microscopy, we can actually see these chromosomes and even organize them. The product of this process is called a karyotype. Here, you can see a human one. In both humans and flies, there are autosomes, and sex chromosomes. Humans, like flies, have X and Y chromosomes controlling their sex. However, most of the genes on these chromosomes control things that have nothing to do with sex. In the rare form of hemophilia that we mentioned earlier, the reason that it occurs more frequently in males, is because the phenotype is controlled by a gene found on the X chromosome, in a section which has no partner on the Y chromosome. If a female has a bad copy of the gene, and her male child inherits this copy, he will have the disorder, he has no backup copy of the X chromosome. Since one copy of the gene is sufficient for a person to clot normally, a female must inherit two bad alleles of the gene, one from each parent, in order to exhibit the disease. Which, in this case, is not possible, because the father is unaffected. As a result, this type of hemophilia affects more males than females.
In this lab, we'll look at inheritance in Drosophila. Eye color in flies is controlled by a series of genes, some controlling what kinds of pigments are made, and one particularly important gene, called the ABC transporter, which controls transports of pigments into granules in the eye. If that gene is broken, even if the fly is making pigment, that pigment will be invisible, and the fly will have white eyes. The lab exercise is to recreate one of Thomas Hunt Morgan's most famous experiments, and explore the genetic inheritance pattern of the gene encoding the pigment transporter. Is it inherited like the purple color of Mendel's pea flowers, or is it sex-linked, like hemophilia?

At the end of this lab, students should know...

$$\rightleftharpoonup{xx}$$ $$\longleftharp{xx}$$, $$\longrightharp{xx}$$,

What are alleles?

Alleles are different versions of a gene encoding similar, but distinct, versions of a gene product.

What characterizes an allele as dominant versus recessive?

An allele is considered dominant if the trait it encodes is expressed when only one copy of the allele is present. A recessive allele will only be expressed if there are two copies of that allele.

What are the two general types of chromosomes found in animal cells?

Animal cells, such as humans and flies, have autosomes and sex chromosomes. Most of the chromosomes are autosomes, while the single pair of sex chromosomes determine the sex of the individual.

What is a sex-linked gene?

A sex-linked gene is a gene that encodes a product that has nothing to do with sex determination, but is found on one of the sex chromosomes. This is most frequently the X chromosome.

Why are males more frequently affected with a sex-linked trait?

Males have one X chromosome, while females have two. If a mutation is present in a gene contained on the single X chromosome a male has, then he will express the mutant trait. A female is less likely to inherit the two copies of the allele necessary to express the trait. This is not impossible, just less frequent in a population.