Transcript Slide 1 - ap biology

By the early 20th century…
Chromosome Theory
of Inheritance
Haploid gametes
Mendel
rediscovere
d
Morgans fly lab
Morgan’s Flies
• New generation every two weeks
• Four pairs of chromosomes
• Plenty of visible phenotypic variation
Experiment
Wild-type red-eye female x mutant white-eye male
homozygous XWXW
hemizygous XwY
Figure 15.3
Hypothesis: white
allele is on the Y
Results
XW XW
P
Homo. red x
Hemi. white
F1
= 100% red
Hetero x Hemi red
F2
= F: 100% red
M: 50% white
XwY
Conclusions
• The white-eye allele must
be on the X
aka X-linked
sex-linked
XY
X X’
carrier
• First solid evidence
indicating that a specific
gene is associated with a
specific chromosome
• Applies to humans too
XY
XX
X X’
X‘Y
Affected x Carrier
Takes a
heterozygous
carrier mom…
…and an
affected dad
carrier
healthy
affected affected
to have a 0.25
chance of an
affected
daughter
Royal Hemophilia
Queen Victoria, carrier
Hemophilia is an x-linked recessive condition that
results in too little blood clotting factor produced,
leading to uncontrolled bleeding
Red vs Green
‘Colorblindness’ results
from defects in a cluster of
genes on the X
chromosome.
As long as one normal
copy of a red-absorbing
gene and one normal
green-absorbing gene are
present, vision is normal.
X Inactivation
If males have only 1 allele of each
of the X chromosome, do they
produce less of the protein X-linked
genes produce?
Nope - one X is de-activated,
at random, in cells of females
mosaicism
Observation #2
• Mendel concluded
that pea traits sort
independent of one
another
• Morgan noticed that
some traits do not sort
independently
• He proposed these traits
are linked - located on the
same chromosome – and
that chromosomes
independently assort
a
B
c
D
A
B
C
d
BUT NOT
ALWAYS!
Observation #3
“Chromosome pairs are observed to twist
together…conceivable they could break and trade
pieces”
Crossing
over in M2
Recombinant offspring
are those that show new
combinations of the
parental traits
Recombination
Likelihood of recombination
determined by distance
frequently
recombined
linked
infrequently
recombined
independent
Calculate % recombination to
determine distance apart
1% recomb. = 1 map unit
Recombination
1% of crossover frequency = 1 unit of
distance between two genes
Use to create
chromosome
‘maps’ of
relative locations
of genes