Download Balancer Chromosomes – An Optional Minitutorial What follows is a

Balancer Chromosomes – An Optional Minitutorial
What follows is a little optional material on Balancer Chromosomes and how they
can be used to maintain deleterious mutations in living stocks. You are not required
to know these details, but its neat to know some of the cool things you can do when
you have a good genetic system like Drosophila. If you like this sort of thing, be sure
to take Genetics (BMB 222 General Genetics) and think about a
follow-up with BIOL 401/BMB497E (Genetic analysis).
1. balancer chromosomes allow experiments to observe effect of mutation on fitness
a. Balancer chromosomes in Drosophila– a trick to maintain deleterious
mutations in living stocks.
b. Balancer chromosome is a chromosome with: multiple overlapping
inversions, a recessive lethal mutation, and a visible dominant phenotype.
c. Inversions built into the balancer chromosome prevent the chromosome from
recombining with the other homologous chromosome, and the lethal mutation
prevents the culture from becoming homozygous for the balancer
chromosome. Result is that lines that contain mutations can always be
maintained in heterozygous form as long as the mutation is recessive or (if
some degree of dominance) not too deleterious.
2.
Diagram of how a balancer works
a.
Balancers contain a lethal recessive allele, so the balancer chromosome can
only be recovered as a heterozygote.
A fly with the Curly mutation has a visible dominant phenotype (ie, curly
wings); this mutation is also lethal when homozygous. Cy shows the position
of the curly mutation on the SM5 balancer chromosome, while + shows wild
type genotypes. Both parents are heterozygotes, and so have curly wings.
Parent 1
Cy
+
------------------------------------+
+
Parent 2
x
Cy
+
------------------------------------+
+
Live offspring of this cross are all either heterozygous for Cy, or
they are wild type (the homozygotes for Cy chromosome all die).
You can choose the balancer flies by looking for curly wings.
b.
if a mutation occurs in a wild-type fly, the mutant chromosome (m) can be
maintained as a heterozygote with the balancer chromosome. Here’s why:
Parent 1
Cy
+
------------------------------------+
m
Parent 2
x
Cy
+
------------------------------------+
m
This cross would yield three possible genotypes, Cy/Cy (all die), Cy/m (live
unless m is a dominant lethal; these flies have the curly wings) and m/m (might survive; but
won’t have the curly wings). The fact that there are inversions built into the balancer
chromosome means that crossover cannot occur between the balancer and the other
chromosome. Hence, if the researcher selects curly winged flies for further work, s/he is
guaranteed to have obtained a heterozygote with one copy of the mutation. Cool.
c. other balancer chromosomes have also been made using other lethal recessive
alleles. Furthermore, additional or different phenotypic dominant marker can be used. Small
modifications of this basic approach can be used to generate flies that are homozygous for a
given chromosome (or homozygous for every chromosome if enough balancer chromosomes are
available).