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
• Thomas 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 IN THE
OFFSPRING?
• GgWw
• WHICH IS WHAT HAPPENED
• Okay Big deal!
• What do you think he did next?
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
http://nortonbooks.com/college/biology/animations/ch11a01.htm
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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
NETTIE STEVENS
Discovered Sex Chromosomes
• She worked with
MEALWORMS
• DISCOVERED they had 20
chromosomes.
• MALES had 19 regular size &
1 small one. The Females all
the same size chromosomes
• SHE SAID “THE 19 that were
the same are AUTOSOMES
(body chromosomes), & the
other set were SEX
CHROMOSOMES”
• FRUIT FLIES same for
male/females
• Males have XY
• Females have XX
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
Short Arm
Bands Represent Genes
Long Arm
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
Chromosomal mutations
Nondisjunction
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/
• Point mutation-single nitrogen base is
misplaced
• A substitution may be okay
• May not be fatal, since there is
redundancy in the amino acid codons
• Deletion/insertion of a point mutation can
cause a frame shift
• Can code for the wrong 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
http://www.pbs.org/wgbh/nova/genome/program.html
13:08 time
ONE WRONG LETTER
JOSEPH KOLREUTER
• 1760-Crossed white rr x red
RR carnations
• He got…Rr which were
pink!
• Phenotype was in between
the parents
• He crossed the Rr hybrid F1 and got red, white and
pink combos
• This shows that R is
incompletely dominant over
the r gene
• r does not code for a protein
but R can not compensate
for this so an intermediate
hybrid is created
INCOMPLETE DOMINANCE
• A PHENOTYPE
THAT IS INTERMEDIATE OF
EITHER GENE
CODOMINANCE
Two genes are expressed equally
• ONE gene is not
DOMINANT over the
other gene
• Two dominant genes
Are expressed
• How does this
happen?
POLYGENIC INHERITANCE
• INHERITANCE
OF SEVERAL
GENES TO
EXPRESS A
SINGLE TRAIT
LIKE SKIN COLOR
Concept map scientists in this chapter