biology.clc.uc/courses/bio105/sex-link.htm must use this site!
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http://biology.clc.uc.edu/Courses/bio105/sex-link.htm MUST use this site!. AP Bio. Mendel NOT: Linked Genes. The dihybrid crosses we previously did assumed the genes were on different pairs of chromosomes. - PowerPoint PPT PresentationTRANSCRIPT
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• http://biology.clc.uc.edu/Courses/bio105/sex-link.htm
• MUST use this site!
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AP Bio
Mendel NOT:Linked Genes
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• The dihybrid crosses we previously did assumed the genes were on different pairs of chromosomes.
• Now, we want to look at an example where the genes involved are on the same chromosome.
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•Doesn’t seem any different so far…
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•All F1’s are Abab
•But note:not AaBb,
this IS different
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•If you were to cross two F1’s… what ratio would we expect?
9 : 3: 3: 1
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•Actually, this F1 cross reveals:3: 1
This is a “Mendel
NOT”
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• The only possible gamete combinations are: AB and ab (like a monohybrid cross)
• not AB Ab aB ab (like a
dihybrid cross)
• b/c the genes are on the SAME chromosome
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HOWEVER…• The only possible
gametes are AB and ab…
• BUT, wierdly, when scored… a few offspring
did illustrate theunexpected:
A_bb and aaB_
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Crossing over
• This is due to the fact that occasionally, during synapsis in meiosis I, while the homologous chromosomes are paired up, sister chromatids from the homologous chromosomes exchange equal segments
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Crossing over
• THIS could account for the small number of the unexpected A_bb and aaB_
•NOTE: the unexpected is known as a RECOMBINANTRECOMBINANT
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T. H. Morgan found that the further apart two genes are on a
chromosome, the more likely there is to be crossing over between
those two genes.
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The more likely there is to be crossing over between those two genes… those are the
RECOMBINANTS! (they look different
from either parent)
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That means that the more likely a RECOMBINANT is shown, the further apart it is on the gene.
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When crossing Over happens, a RECOMBINANT is shown