Download Characteristics of linked genes

THOMAS MORGAN
Morgan’s DISCOVERY of LINKED
GENES
• Characteristics of linked genes
1. WHEN GENES ARE CLOSE TOGETHER ON
A CHROMOSOME THEY TEND TO BE
INHERITED TOGETHER
2. LINKED GENES tend to not SEPARATE from
one another during Crossing Over Prophase I
During Cross Over in Prophase, they tend to
stay together instead of separating and
switching
How Morgan Discovered Linked
Genes
• USED DROSOPHILA Genus name
– common name Fruit flies WHY?
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They Mature in 2 weeks
They Produce large numbers of offspring
They only have 4 pair of chromosomes
One pair are the Sex CHROMOSOMES
The Experiment for Linked Genes
Part I
• crossed PURE BRED fruit flies for two traits
• Homozygous Dominant GRAY BODIES & NORMAL
WING SIZE (GGWW) with Homozygous Recessive flies
that had BLACK BODIES & SMALL WINGS (ggww)
• GG WW X gg ww
• WHAT Genotype did he expect based on Mendelian
genetics?
• GgWw
• WHICH IS WHAT HAPPENED
• Okay Big deal!
• What do you think he did next? Think about Mendel’s
experiments
MORGAN’S Experiment Part II
• He then crossed an F-1
of the GgWw hybrid
offspring w/a recessive
ggww
• What would you expect?
• Fill out this punnett-
• he did not get as
expected…
• ¼ GRAY NORMAL
• ¼ GRAY SHORT
• ¼ BLACK NORMAL
• ¼ BLACK SHORT
• INSTEAD, he got…
GgWw x ggww
GW
gw
gw
gw
gw
gw
Gw
gW
Experiment’s Results for Part II
Gg Ww alleles
Expected
Actual Results
2300 total offspring
gg ww alleles
MORGAN’S RESULTS
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41.5% GRAY body/Normal wings
41.5% BLACK body/small wings
8.5% GRAY body/Small wings
8.5% BLACK body/Normal wings
MORGAN’s Conclusion
The genes for wing size and body color were so
commonly inherited as only two combinations either
gray body/normal wing or black body/small wing that
they had to be …
• on the same chromosome!
• This indicated that the genes for body color and wing
size were…
• LINKED onto one chromosome.
Crossing Over Explains the other 8.5%
combinations of either Black Body/Normal Wing or
Gray body/Small Wing
Homologous
chromosomes
Chromosome combinations
for gametes
This means that even though
genes can be linked,
They can sometimes separate
from one another during
Crossing Over in Meiosis
GENE MAP
• If you know the frequency of how often genes cross
over, you can use the percentage to estimate how far
apart the genes are from on another on a chromosome
• This is called a Gene Map
• So if two genes have an 8% frequency of crossing then
are they far apart or close on a chromosome?
B
c
A
Try this!
Map this chromosome’s genes using these
crossing over frequencies
• AE cross over 5%
• BA cross over 10%
• BE cross over 20%
• DE cross over 50%
• DB cross over 25%
• CB cross over 5%
• CA cross over 20%
How did you do?
D
C
B
A
E
http://www2.edc.org/weblabs/crossingOver/intro.htm
Gene Mapping Tutorial
GENES ON SEX CHROMOSOMES
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structur
e/ge/m3/s2/index.htm
• A gene located on a A SEX CHROMOSOME is
called a SEX-LINKED GENE
• MORGAN DISCOVERED the 1ST SEX-LINKED
gene in fruit flies
• He crossed a PUREBRED Dominant REDEYED FEMALE W+W+ with a PUREBRED
recessive WHITE-EYED WW MALE
• Let’s take a look at the 1st cross.
Morgan’s 1st Sex Linked Cross
All offspring had red
eyes
MORGAN’S 2nd CROSS
• HE crossed from the F-1 generation a heterozygous
FEMALE, W+W RED-EYED with a RED-EYED W+
MALE (note only 1 allele!
• Let’s do the cross
• HE got a 3:1 RATIO of Red eye to White eye, but only
MALES had WHITE EYES! Why?
• Since no FEMALES had WHITE EYES Morgan
hypothesized that EYE COLOR must be a SEX-LINKED
gene
• IT must be on the X CHROMOSOME
The Y chromosome does not carry a gene for EYE
COLOR
• The RECESSIVE TRAIT White eyes, is inherited more
often in males that receive the r allele on their one and
only X CHROMOSOME
MORGAN’S EXPERIMENTAL CROSS
Red eyed female
All females had
red eyes
3:1 ratio red eyes to
white eyes
White eyed male
All males had
red eyes
Only males had
white eyes
Chromosomal mutations
http://en.wikipedia.org/wiki/Chromosomal_translocation
http://www.nature.com/scitable/topicpage/dna-deletion-and-duplication-and-theassociated-331
DUPLICATION, INVERSION, DELETION, TRANSLOCATION
Mutations
http://learn.genetics.utah.edu/content/begin/traits/predictdisorder/
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Point mutation-single nitrogen base is misplaced
Missense mutation –single point mutation
A substitution may be okay
May not be fatal, since there is redundancy in the amino
acid codons
Nonsense mutation, a single codon codes for a stop
codon
Deletion/insertion of a point mutation can cause a frame
shift
Can code for the incorrect amino acid
Could create an incorrect protein
Nondisjunction
http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ge/m3/s3/i
ndex.htm
• Failure to separate chromosomes evenly
during meiosis
• Chromosomal mutations involving whole
or complete pairs of chromosomes
• 3n-triploidy
• 4n-tetraploidy
• Having more then one set of
chromosomes-polyploidy
• Fatal in humans, beneficial in plants
SRY gene
Sex determining Region of the Y
chromosome
• The gene can trigger biochemical,
physiological & anatomical features
because it regulates other genes
• If it is missing embryonic gonads develop
into ovaries
Sex linked genes
• Males have only 1 X gene Hemizygous
also XY heterogametic
• Females XX homogametic
• If a male inherits a recessive allele from
his mom he will express the trait
• Chance of a female inheriting a double
dose is much less
• It is rare for X & Y to cross over
Sex linked disorders
• Duchenne muscular dystrophy- absence
of an X linked gene for a muscle proteindystrophin. Weak muscles, loss of
coordination
• Hemophilia- blood clotting disorder,
absence of a clotting factor.
Barr body, X chromosomes in
females
• In females during development one X
chromosome condenses into a Barr body.
Most of its genes are inactive
• Females therefore are a mosaic of
paternal X and maternal X as to which
genes are activated
• In an X linked mutation for sweat glands,
half her glands produce sweat, the others
do not
X chromosomes
• Methyl groups attach to the inactive X
chromosome
• One of the two X has an active XIST gene
• It produces multiple copies of rna that
covers the X chromosome. This initiates X
inactivation
Tortoise haired cats
• Female cats have patches of orange and
black fur due to cells expressing an
orange allele and others express no
orange allele
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