Linked genes
Introduction
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Each chromosome has ___________________or even ___________________ of genes.
“______________________________” are genes located on the ____________ chromosome that
tend to be inherited ___________________ in genetic crosses.
The results of breeding experiments __________________ from those expected by Mendel’s law
of independent assortment when linked genes are being studied.
How linkage affects inheritance
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The wild type ______________________ had __________ bodies and ________________ wings
___________________ had black bodies and vestigial wings (much smaller).
Allele symbols: ______ = grey, _____= black, ______ = normal wings, _____ = vestigial wings
Mutant alleles were ___________________ and ______ located on a sex chromosome.
A test cross was done between…
true-breeding double-mutant males(_________________) and
dihybrid females (__________________)
…To find out if the genes were located on the ____________ chromosome or _______________
chromosomes.
*Because all of the male’s alleles were recessive, the phenotype of the offspring would depend on
the __________________ alleles. Would the alleles for body color and wing shape stay together
during the formation of the female’s gametes?
Morgan classified the offspring from the testcross matings according to ___________________
He found a much __________________ proportion of parental phenotypes than would be
expected if the genes assorted independently.
Body color and wing size must usually be inherited ________________, and therefore be located
on the _________________ chromosome.
However – _______________________ phenotypes were also produced, suggesting that body
color and wing size genes are only __________________________ genetically. (see p.278,279)
Genetic Recombination and linkage
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When offspring inherit a phenotype that matches one of the parental phenotypes, they are called
“________________________.”
When offspring inherit a phenotype that is a new combination of traits, they are called
“recombinant types” or “_______________________.”
When half of the offspring are recombinants, there is a ________% frequency of recombination.
This would result from recombination between ____________________ genes
Due to the ___________ orientation of homologous chromosomes which leads to independent
assortment of alleles (during ______________________ of meiosis).
(see p. 278)
_____________________________ accounts for the recombination of linked genes.
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Some of the female’s gametes had a _____________________ chromosome.
Most of the ova had a b+ vg+ or b vg
But ________________ had b+ vg or b vg+
Linkage Mapping
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One of Morgan’s students began to construct a “______________________” – 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 ___________
at all points along a chromosome.
His prediction: the ______________________________ two genes are, the ____________the
probability that a crossover will occur between them and therefore the higher the recombination
frequency (% of recombinant offspring)
____________________________ = a genetic map based on recombination frequencies
Three genes will be used for illustration:
Body color – ______
Wing size – ______
Cinnabar – ______ (affects eye color… mutants have brighter red)
Recombination between cn and b is ____%, that between cn an vg is _____%, and that between b
and vg is _____%.
Crossovers between cn and b and cn and vg are about _______________________________ as
crossovers between b and vg.
A map would then place _____ midway between b an vg.
Distances between genes are called “______________________.”
One map unit is equal to ____% recombination frequency.
Does __________ correspond to actual _________________ distances, but does show ________
of genes along a chromosome
The ______________ 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 ___________________________, regardless of whether or not they exist on the
same chromosome.
________________________________ – locate genes with respect to chromosome features such
as stain bands (constructed using other methods)
Bozeman video!
https://www.youtube.com/watch?v=rIe7mPXkYhs