LINKED GENES
INTRODUCTION
• Each chromosome has hundreds or even thousands of
genes.
• “Linked genes” are genes located on the same
chromosome that tend to be inherited together in genetic
crosses.
• The results of breeding experiments deviate from those
expected by Mendel’s law of independent assortment
when linked genes are being studied.
HOW LINKAGE AFFECTS
INHERITANCE
• The wild type Drosophila had gray bodies and normal-sized
wings.
• Mutants had black bodies and vestigial wings (much
smaller).
• Allele symbols: b+ = grey, b = black, vg+ = normal wings, vg
= vestigial wings
• Mutant alleles were recessive and not located on a sex
chromosome.
HOW LINKAGE AFFECTS
INHERITANCE
• A test cross was done between…
• true-breeding double-mutant males(b b vg vg) and
• dihybrid females (b+ b vg+ vg)
…To find out if the genes were located on the same
chromosome or different chromosomes.
*Because all of the male’s alleles were recessive, the
phenotype of the offspring would depend on the female’s
alleles. Would the alleles for body color and wing shape stay
together during the formation of the female’s gametes?
HOW LINKAGE AFFECTS
INHERITANCE
• Morgan classified the offspring from the testcross matings
according to phenotype
• He found a much higher proportion of parental phenotypes
than would be expected if the genes assorted
independently.
• Body color and wing size must usually be inherited together,
and therefore be located on the same chromosome.
• However – nonparental phenotypes were also produced,
suggesting that body color and wing size genes are only
partially linked genetically. (see p.278,279)
GENETIC RECOMBINATION AND
LINKAGE
• When offspring inherit a phenotype that matches one of the parental
phenotypes, they are called “parental types.”
• When offspring inherit a phenotype that is a new combination of traits,
they are called “recombinant types” or “recombinants.”
• When half of the offspring are recombinants, there is a 50% frequency
of recombination.
• This would result from recombination between unlinked genes
• Due to the random orientation of homologous chromosomes
which leads to independent assortment of alleles (during
metaphase 1 of meiosis).
• (see p. 278)
GENETIC RECOMBINATION AND
LINKAGE
• Crossing over accounts for the recombination of linked
genes.
• Some of the female’s gametes had a recombinant
chromosome.
• Most of the ova had a b+ vg+ or b vg
• But some had b+ vg or b vg+
LINKAGE MAPPING
• One of Morgan’s students began to construct a “genetic
map” – an ordered list of the genetic loci along a particular
chromosome.
• Since crossing over is a random event, the chance of it
happening is approximately equal at all points along a
chromosome.
• His prediction: the further apart two genes are, the higher
the probability that a crossover will occur between them
and therefore the higher the recombination frequency (% of
recombinant offspring)
LINKAGE MAPPING
• Linkage map = a genetic map based on recombination frequencies
• Three genes will be used for illustration:
• Body color – b
• Wing size – vg
• Cinnabar – cn (affects eye color… mutants have brighter red)
• Recombination between cn and b is 9%, that between cn an vg is 9.5%, and
that between b and vg is 17%.
• Crossovers between cn and b and cn and vg are about half as frequent as
crossovers between b and vg.
• A map would then place cn midway between b an vg.
LINKAGE MAPPING
• Distances between genes are called “map units.”
• One map unit is equal to 1% recombination frequency.
• Does not correspond to actual physical distances, but does show order of genes
along a chromosome
• The highest value is 50%... This percentage is true of genes located on
different chromosomes as well as genes on the same chromosome that are
so far apart that a crossover is virtually certain.
• Such genes are genetically unlinked, regardless of whether or not they exist
on the same chromosome.
• Cytogenetic maps – locate genes with respect to chromosome features
such as stain bands (constructed using other methods)
BOZEMAN VIDEO!
https://www.youtube.com/watch?v=rIe7mPXkYhs