7/23/2019 12-2_gene_linkage.doc
1/4
Chapter 12.2 - Gene Linkage
New alleles arise by mutation
Different alleles exist because any gene is subect to mutation!
or change! to a stable! heritable
new form
"lleles can ran#omly mutate to become a #ifferent allele
#epen#ing on DN" se$uence changes.
Wild typeis a term use# for the most common allele in the
population.
%ther alleles! often calle# mutant alleles! may pro#uce a
phenotype #ifferent from that of thewil#-type allele.
"n alternate form of #esignating alleles. "lleles that are wil#
type are expresse# with a &
'x. (e# eye color )w&* is #ominant to white eye color )w*.
+he re# eye is the wil# type. Don,t let
this confuse you! its ust a #ifferent way to express
alleles.
Linkage Groups
o far! your stu#ies an# practice with crosses hae inole# alleles
locate# on separate chromosomes! an#crosses follow /en#el,s Law of
0n#epen#ent "ssortment
0n actuality! many genes are locate# on the same chromosome! an#
they #o not assort in#epen#ently!
instea#! they are inherite# together! they won,t follow the
normal rules of punnet s$uares! an# the ratiosobtaine# from crosses
#o not hae the normal ratios. Consi#er the following chromosome map
of the fruitfly "ll the alleles are locate# on chromosome 2 of the
fruit fly! an# are inherite# together.
hen performing crosses with linkage groups! 0 fin# it best to
#raw a little picture of the chromosomes to
show how they are inherite#.
'xample " fly that is hetero3ygous for long wings )Ll* an#
heteroy3ygous for long aristae )"a* is crosse#with another fly of
the same type. "aLl x "aLl. 0n both cases the #ominant allele is
locate# on the same
chromosome.
4efore you set about making a 5x5 s$uare! you nee# to consi#er
the linkage groups. ketch them6
7/23/2019 12-2_gene_linkage.doc
2/4
+he results of the cross woul# change consi#ering the arrangment
of alleles. how the cross that woul#
occur if the #ominant alleles were on D077'('N+ chromosomes. )ee
answer*
+he expecte# ratios are not always correct! because remember
also that #uring meiosis! chromosomes
oerlap an# exchange genes. 0n the case aboe! you may get an
8o##8 long wings! short aristae becausethe genes were exchange
#uring meiosis. Confusing! huh9
:ow Chromosome /aps "re Determine#
peaking of crossing oer! it is this fre$uency )the o##s of the
genes exchanging #uring meiosis* that#etermines how far apart the
alleles are on a chromosome. "lleles that are farther apart! like
the aristae
allele an# the brown eye color allele are more likely to
exchange than one closer together! like the aristaeallele an# the
long wing allele. 0n other wor#s! alleles that are close together
ten# to stick together.
+hat being sai#! biologists use the percent of crossing oer to
#etermine the locus of alleles on achromosome. +he #istance between
alleles is measures in MAP UNITS! or /;. %n the #iagram aboe
the
long wing allele is 1
