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BIOL 2000 Lecture Notes - Karyotype, Mendelian Inheritance, Centimorgan

Course Code
BIOL 2000
Joseph Rasmussen

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Chapter 4
Linked alleles tend to be inherited together
Morgan suggested that crossing over produces allelic combinations that are
different from the meiotic input (recombinants)
Morgan suggested that chromosomes break and re-join during pairing, and
that chiasmata are visual evidence of that event.
If we do not know that genes are linked separate gene pairs with “.”
o pr/pr+ . vg/vg+
If we know genes are independently assorting (unlinked), separate gene
pairs with “;”
o a/a+ ; b/b+
Crossing over occurs during prophase
Morgan proposed that crossing over between chromatids, not chromosomes
A second division segregation pattern in a fungal octad demonstrates that
crossing over must be between chromatids
Multiple crossovers can include more than two chromatids to generate
multiple types of linear tetrads
o Centrosome decides where the crossing over occurs
o Can occur on both sides of centromere
SUMMARY: Crossovers generate recombinant meiotic products
Mapping by recombinant frequency
Morgan’s result: if test crosses were done involving different pairs of genes,
saw different ratios of parental: recombinant
Morgan’s hypothesis: frequency of recombinant products may be related to
distance between genes
Alfred Sturtevant: If Morgan’s hypothesis is true, should be bale to use
recombinant frequency to reestablish spatial relationship between genes on
o A chromosome map
Alfred Sturtevant- data from Morgan’s lab generated formula
o Map distance= percent recombinants
Predicted that if calculated distance reflect linear position on chromosome,
map distances should be additive
o The further apart the chromosomes are, the better chance of
o The closer together the chromosomes are the less chance of
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