Transcript AP Bio

• 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.
• Now, we want to
look at an example
where the genes
involved are on
the same
chromosome.
•Doesn’t
seem any
different
so far…
•All F1’s
are Abab
• But note:
not AaBb,
this IS
different
•If you
were to
cross two
F1’s…
what ratio
9 : 3: 3: 1 would we
expect?
•Actually,
this F1
cross
reveals:
3: 1
This is a
“Mendel
NOT”
• 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
HOWEVER…
• The only possible
gametes are AB and ab…
• BUT, wierdly, when
scored… a few
offspring
did
illustrate the
unexpected:
A_bb and aaB_
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
• THIS could
account for the
small number of
the unexpected
A_bb and aaB_
Crossing over
•NOTE: the
unexpected is
known as a
RECOMBINANT
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.
The more likely there is to be
crossing over between
those two genes… those are
the RECOMBINANTS! (they look
different
from either parent)
That means that the more likely a
RECOMBINANT is shown, the
further apart it is on the gene.
When crossing Over happens,
a RECOMBINANT is shown