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Gene Linkage
Chapter 19.2
Page 635-641
Gene Linkage and Crossover
Back to Morgan and the fruit flies
 first Morgan crossed wild-type body colour (AA) and
straight wings (BB) with black body colour (aa) and curved
wings (bb)
AABB x aabb
 the F1 generation is heterozygous for both traits AaBb
 Morgan then crossed the F1 generation and expected the
Mendelian ratio of 9:3:3:1 for a dihybrid cross.
• instead all the individuals with wild-type body colour
had straight wings and all those with black body colour
had curved wings.
Gene Linkage and Crossover

Morgan concluded that the two genes:
• did not undergo independent segregation
• for this to happen genes would have to be on the
same chromosome
• the genes are linked
Gene Linkage and Crossover


because the genes are linked
the two gametes form an
individual that is heterozygous
for both traits.
Morgan predicted that crossing
the F1 generation would
produce an F2 generation what
would have a 3:1 phenotypic
ratio (3 flies with wildtype
body type and straight wings)
Linked genes
Trait
Dominant/
Recessive
Location
wingless (wg)
recessive lethal
chromosome 2
curly wings (Cy)
dmoninant
chromosome 2
purple eyes (pr)
recessive nonlethal
chromosome 2
stubble bristles (Sb)
dominant
chromosome 3
ebony body (e)
recessive nonlethal
chromosome 3
miniature wings (m)
sex-linked recessive
chromosome 4
cut wings (ct)
sex-linked recessive
chromosome 4
white eyes (w)
sex-linked recessive
chromosome 4
vermillion eyes (v)
sex-linked recessive
chromosome 4
Crossing over

Morgan observed that in a small number of
dihybrid crosses the offspring had different
combinations of traits than the parents
Data:
Phenotype
Number
Possible Genotypes
wild-type body colour,
straight wings
290
AABB
black body colour,
curved wings
92
Aabb
wild-type body colour,
curved wings
9
AAbb or Aabb
(recombinant types)
black body colour,
straight wings
9
AaBB or aaBb
(recombinant types)
Crossing Over

the new combinations of
alleles that Morgan
observed came about by
DNA crossing over during
meiosis.
Crossing Over and Chromosome
Mapping



there are groups of linked genes on a chromosome.
•
these are called a linkage group
particular genes are always found on the same location
(locus) on a chromosome.
Morgan showed that the frequency of crossovers
between any two genes in a linkage group is always the
same.
•
the frequency of crossing over between any two genes can be
stated as a percent.
crossover percentage = (number of recombinations/total number of offspring) x 100%
using the previous table’s data:
crossover % = 18/400 x 100%
crossover % = 4.5%

geneticists use this number to say the two alleles are 4.5
map units apart.
probability = (genotype looking for)/
(total number of possible genotypes)
probability = (phenotype looking for)/
(total number of possible phenotype)
Homework!


Review sample question page 638
Complete Lab Exercise 19.B page 639640 – Due next class!