"Genetics: Testing Hypotheses about Inheritance
Fruit Fly Background and Life Cycle
In this exercise, you will perform reciprocal crosses to determine the inheritance pattern of various traits in Drosophila
melanogaster. This organism has been used in genetic experiments since 1910. Wild type or normal traits are red
eyes, wings longer than the body and smooth, and tan or brown body color. An enormous number of mutants
spontaneous and induced, have been discovered in Drosophila, including mutations that affect external anatomy (eye
color, body size, body color, and wing shape). The most interesting mutations are those that affect behavior. Mutants
have been found that go into shock at loud noises, forget recently learned things and show excessive libido.
Drosophila are useful for laboratory experiments, because they have a short generation time, produce many offspring
and require no special care or equipment. The life cycle of Drosophila (see Figure 1) is usually completed in 14 days if
incubated at 21C, or in 10 days at 24C. The life cycle includes four stages: egg, larva, pupa and adult. The eggs are 2
mm long, sausage-shaped, and white, bearing a pair of filaments at one end, which keep the eggs from sinking into the
soft food on which they are always laid. One day after being laid, the eggs hatch into first instar larvae (little white
maggots), which feed voraciously. After several days, the larvae molt and begin the second instar developing tracheae.
One more molt occurs to produce third instar larvae, which crawl onto a hard dry surface and transform into pupae in
small dark cocoons. Within the pupae, the third instar larvae go through metamorphosis and eventually emerge as
adult flies. A few hours after emergence, the adult is at first light in color with crumpled wings. Within a few hours, the
flies wings expand and harden, enabling them to fly.
Figure 1. Diagram of Drosophila life cycle.
How we talk about genetic information
Each cell in living organisms contains DNA, which is made of nucleotide subunits arranged in very long strands. By
winding around structural proteins, the strands become condensed into compact units called chromosomes. Regions
in the DNA, known as genes, carry specific instructions for making proteins. Genes represent unique combinations of
nucleotides that provide information for the genetically-based traits that organisms display.
While all humans have the same genes (that is, each human has a gene that codes for eye color, etc), humans are not
genetically identical. Think of all the variation in the human population. We have different colors of hair, eyes
different heights, different tendencies towards disease, etc. One reason for the variation we see is that organisms, like
humans, might have different forms of genes, which we term alleles. These alleles code for different proteins, and
therefore result in the expression of different phenotypes.
A dominant allele is expressed phenotypically when present on either a single chromosome or on both homologous
chromosomes (that is, when its present on either the chromosome inherited from the mother, or the father, or both).
A recessive allele is masked by a dominant allele and is expressed only when paired with another recessive allele on the
Genetics: Testing Hypotheses about Inheritance
homologous chromosomethat is, it is only expressed when the chromosomes inherited from both parents have the
recessive allele. A pair of identical alleles (AA or aa) at a genes locus represents the homozygous condition. Two
different alleles (Aa) at a genes locus represent the heterozygous condition. For example, eye color in humans is
determined by a single gene locus (although other loci can modify its effects). If the alleles at that locus are
homozygous dominant (AA) or heterozygous (Aa), the eyes will be brown. If the alleles are homozygous recessive (aa)
the eyes will be blue. Alleles for brown eyes are said to be dominant over alleles for blue eyes.
How is genetic material passed on from generation to generation?
Alleles are passed on from parents to their offspring through reproduction, specifically through gametes (via meiosis)
and fertilization. Meiosis is the process of cell division that happens only in sex cells and is responsible for creating
gametes (sperm and egg). Sperm and egg cells are unique from other body (somatic) cells in that each sperm or egg cell
contains exactly half of the genetic material from each parent. Why do you think this is the case?
To learn more about meiosis, please visit the Cells Alive site (http://www.cellsalive.com/meiosis.htm). Watch the
animations for the cell and as you do, track the locations of the chromosomes as they move through the phases of
meiosis. In addition, compare the beginning cells to the end products of meiosis. How many cells does meiosis begin
with? How many cells does meiosis end with? Are the numbers and types of chromosomes the same in both cases?
Drosophila are eukaryotic and diploid, having corresponding sets of genes on paired chromosomes. During each cross
the chromosomes containing the genes are shuffled by meiosis and combined at fertilization. Different mechanisms of
inheriting a trait involve different patterns of chromosomal movement during reproduction. The key to studying
genetics is to be able to predict the chromosomal movements that would result from different models for how a trait
might be inherited.
It is important to know that female Drosophila can store and utilize sperm from one insemination for a large part of
their reproductive lives. Therefore, only virgin females should be used in making the initial parental crosses. Females of
this species can mate six hours after they have emerged from the pupal case. If all adult flies are emptied from the
culture vial and the vial is left for six hours, all females removed the second time should be virgin. Males of any age may
be used in the crosses.
Using experiments to test patterns of inheritance
Modern genetics began with the experiments of Gregor Mendel, an Austrian monk with an inquisitive mind. Mendel
performed crosses between garden pea plants and discovered that certain alleles can mask one another (i.e. dominant
alleles mask recessive ones described earlier). A monohybrid cross is one in which the pattern of inheritance of a single
trait (e.g., pea shape) with a pair of alleles (e.g., round or wrinkled peas) is studied. In contrast, a dihybrid cross
involves parents that are identical except for two independent traits (e.g., pea color and shape).
Morgan built upon Mendels findings through his discovery that traits may be sex-linked, meaning the gene is carried
on a sex-determining chromosome (X or Y). In Drosophila, the X-chromosome carries sex-linked genes. So in flies
females have two copies of the X chromosome, and therefore carry two alleles of sex-linked genes, one on each X
chromosome. Male Drosophila, however, only have one X chromosome and therefore only carry one allele of a sexlinked gene. This means that whatever allele is passed on to males through their single X chromosome will be
expressed regardless of whether it is dominant or recessive. Since female flies inherit two alleles for sex-linked genes
expression of the alleles follows the dominant/recessive pattern described above. Any genes that are present on the
other chromosomesthat is, not on X or Y chromosomesare said to be autosomal. For example, all of Mendels pea
plant traits are inherited on autosomal chromosomes.
Alleles that are naturally common in the wild (e.g., red eyes in fruit flies) are known as wild-type alleles. Less common
alleles (e.g., white eyes in fruit flies) are known as mutant alleles. These are derived from characteristics that are
expressed naturally in the wild. Its important to note that wild-type alleles are not always dominant, nor are mutant
alleles always recessive.
Reciprocal crosses were important to Morgans discovery about sex-linked genes. A reciprocal set of crosses is
composed of a forward cross (where the male parent has the mutant allele and the female parent has the wild-type
allele) and a reverse cross (where the female parent has the mutant allele and the male parent has the wild-type allele).
Genetics: Testing Hypotheses about Inheritance
For example, Morgans forward cross was a red-eyed (wild-type) female with a white-eyed (mutant) male. For the
reverse cross, Morgan used different flies with opposite alleles, in this case, a white-eyed (mutant) female and a redeyed (wild-type) male. The patterns of alleles in the offspring of these reciprocal crosses were markedly different and
as a result, Morgan was able to determine that the white eye-color allele in fruit flies was sex-linked.
In both Mendels and Morgans (who did experiments with fruit flies) experiments, it was important to begin with
parents that are said to be true breeding. This means that the parents are homozygous for the alleles of interest for
the study. Because the genetic makeup is known for the starting organisms, it enables scientists to track the genotypes
(or the genetic makeup of the organisms) through generations
Genetics with Computer Flies
All concepts about genetic information, the process of meiosis and experiments used to test for different patterns of
inheritance are identical to what was presented in the Drosophila. It is important to remember that all parental virtual
flies are true breeding and that there is no co-dominance or epistasis for any alleles being studied.
There are several key differences between living Drosophila and what we will be doing with the computer simulation.
The first difference is using virtual flies instead of live ones. These virtual flies are a new species we are naming
Drosophila spartaniensis. This species was created at MSU. The vast majority of the wild type individuals in the
population are small