Download Natural Selection with Drosophila Introduction: Genetic variation

Natural Selection with Drosophila
Introduction: Genetic variation exists within populations of organisms. The variation is due to
mutations, changes in the DNA sequence of an organism. Harmful mutations that hind an individual’s
fitness (its ability to survive and reproduce) are less likely to be passed on to the next generation.
Mutations that confer advantageous traits are more likely to be passed on. A mutation that is passed
along because it confers a reproductive advantage is described as being “selected for”. A mutation for a
trait that confers a reproductive disadvantage is described as “selected against”. Traits that are selected
for begin to become more prevalent in the population, and traits that are selected against become less
prevalent. In this lab, you will investigate the selection of white versus red eyes in Drosophila, fruit flies.
First, you should know the basics of genetic inheritance. Through Gregor Mendel’s studies of pea plants,
he developed the law of gene segregation and independent assortment. These two laws state that (1)
when organisms breed and produce offspring, each offspring has two sets of genetic information—one
from each parent for each trait and that (2) traits are inherited independently of one another (although
modern genetics has found that some genes are linked). Mendel saw these laws at work in his pea
plants as he observed mathematical patterns in the visible traits among the different offspring from a
parental cross. Today, these laws are used to explain heredity (what traits are passed from one
generation to the next) and variation (the differences between parents and their offspring).
Through his experiments, Mendel discovered that many genes occur in multiple forms. These alternate
forms, called alleles, code for different phenotypic expressions of a genetic trait (e.g., a gene for color in
flower petals might have an allele for pink or an allele for white). He saw that some alleles were
dominant and others, recessive. When a dominant allele was combined with a recessive allele, only the
dominant allele was expressed in the offspring’s phenotype. In current genetics, as in Mendel’s records,
a dominant allele is represented with an uppercase letter, and a recessive allele is represented with a
lowercase letter.
Drosophila is an ideal model organism for genetics. Fruit flies are small yet readily visible, and they are
inexpensive to support. Drosophila has a short generation time (about 12 days) and has a genome
packages into only four chromosomes. Furthermore, Drosophila is safe to work with and is easily and
harmlessly anesthetized for observation and sorting. Also, the extent of genomic homology between
Drosophila and humans makes them valuable in disease research.
In the early 20th century, the geneticist Thomas Hunt Morgan used Drosophila to provide the first
evidence supporting the chromosome theory of inheritance, which claims that chromosomes carry
genes and that all genes on a single chromosome tend to be inherited together. Morgan observed an
unusual pattern in the inheritance of a mutation for white eyes; the phenotypic ratios between males
and females were different. He discovered an alternate mode of inheritance. We know that females
have two X chromosomes and males have one X and one Y. These sex chromosomes carry genetic
information just as somatic chromosomes do; however, an allele on the Y chromosome will be inherited
only by males. Further, any recessive allele carried on the X chromosome for a trait that is not
represented at all on the Y chromosome is sure to be expressed in a male, whereas a female may have
the dominant allele on her other X chromosome. The notation for alleles on sex chromosomes differs
from that for alleles on somatic chromosomes. Alleles are still given uppercase letters for dominant
traits and lowercase letter for recessive traits, but this notation is written as a superscript above the X or
Y. In this lab, you will study the white-eye and red-eye traits. Morgan found that the white-eye trait is
sex-linked recessive in Drosophila.
When setting up crosses, the first two lines of flies that are crossed are called the parental (P)
generation. The offspring of the parental generation are called the first filial (F1) generation. The
offspring of the F1 generation are the second filial (F2) generation, and so on. When labeling crosses,
the female description always comes first. For example, if we were crossing red-eyed females with
white-eyed males, we would label the culture vial “red x white”. This standard notation eliminates the
need to distinguish between males and females when labeling crosses.
Purpose: You will use Drosophila to help you investigate and understand the principles of inheritance
and natural selection. You will observe the interactions between males and females of different eye
colors and observe the eye-color ratios of their offspring. During your observation, you will be
responsible for keeping accurate records of the traits of the flies; this is called “scoring” the flies.
Materials: Your laboratory work begins at Week 2 in the following Procedure section. To begin, your
group needs the flies of the parental generation:
1 Vial labeled “Experimental Males” containing five white-eyed and one red-eyed male
1 Vial labeled (and containing) “White-eyed Females”
Procedure:
Week 1
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Mrs. Rector and Mrs. M. have removed the adult flies to ensure that the female flies that you
use have never mated previously.
Week 2
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**Mrs. Rector and Mrs. M. have set up a control cross for the entire class. The control group
crosses white-eyed females with white-eyed males.
Transfer one vial of females into the ”Experimental Cross” vial containing five white-eyed males
and one red-eyed male
o Tap the two vials upright simultaneously on a hard surface.
o Quickly pull both plugs off the vials, and tap once more.
o Invert the female vial such that the mouth of the vial is against the mouth of the male
vial.
o Tap two more times gently and quickly place the plug into the lower vial now housing all
the flies.
Observe the flies inside the vial for mating behavior between the male and female of same and
different colored eyes. The most accurate and conspicuous sex-specific traits are the shape and
color of the posterior abdomen. Mature males have darker, blunter posteriors than females.
Males also have sex combs located on the uppermost joint of their forelegs, while females do
not. The sex combs can be tough to spot; it takes a bit of practice to become proficient at
spotting them. Male flies typically area smaller than female flies of the same line, but size is not
a reliable indicator of gender. Males will chase females around the vial when courting.
Week 3
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Anesthetize the P generation according to Mrs. Rector’s and Mrs. M.’s instruction (They created
the F1 generation.)
Remove the P generation adults from your experimental cross. This is known as “clearing” the
vials. If the parent flies are not removed, there will be generational crossover, with parental flies
mating with their offspring.
Dispose of the anesthetized flies into one of the prepared morgues.
The F1 generation is in the form of eggs and larvae in the vials. Observe the trails in the medium
left by the larvae. You may also be able to see the black mouthparts of the larvae as they move
through the medium.
Week 4
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Anesthetize all the flies from the F1 generation according to the FlyNap directions.
Using the sorting grid, microscope and sorting brushes, take turns observing and sexing the flies.
Record the information you found from sorting the flies in the F1 Data Table.
To calculate the percentage of red-eyed females in one vial, divide the number of red-eyed
females counted by the total number of flies in the vial. Then multiply the quotient by 100.
Compare your findings with those of other groups by filling in the class chart.
Record the F1 class data for red-eyed flies in the data table.
Use your data to answer the analysis questions.