Senogles 4

Senogles 4: Color Blindness

She expects us to know her learning objectives, and her tests are very straightforward. She shows references in her lectures, but we DO NOT have to read them.

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Next 3 lectures are about the molecular basis of 3 different diseases. Humans are trichromats, and our brains mix the colors to produce what we see.
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Opsins function as a tandem array.
Ancestor opsin duplicated and changed to produce color opsins.
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Green deficiency-muddy brown. Cats and dogs are dichromatic.
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Number cannot be perceived by colorblind person.
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Rhodopsin allows dim-light vision. Cones are for color. Photoreceptors are named for color opsins they carry.
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Bottom graph- different opsins are sensitive to different wavelengths.
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Eye perceives light by rods and cones. Rods mediate light intensity. Cones carry distinct opsins for color receptors.
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Rhodopsin is a G-protein-coupled receptor with 7 transmembrane spanning domains. Light is agonist causing isomerization of retinal. Cis-trans conformational change is the activation signal for the receptor.
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Nathan Used rhodopsin as probe in eye library. Little squares indicate 6 exons for each protein he found.
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He cloned them out and looked at protein similarity. Did stepwise comparison of proteins sequences for amino acid similarity. Amino acid identity shown in white. Differences are shown in black. Highly related proteins.
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Probably result of gene duplication event.
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3 reasons for tandem arrays in the human genome:
1. Histone tandem array (lots of copies of the genes for histone subunits arranged end to end) on genome because you need a lot of histones at certain points in growth. Get a lot made in a short period from master promoter.
2. BetaGlobin locus gene family: this protein has different fetus, embryo, adult forms. Region can be under coordinate regulation- region under master switch.
3. diversity in organisms. Evolutionary pressure to increase color vision for viability.
Unequal crossing over event creates 2 genes on one chromosome, none on other. The one with two may have a survival advantage.
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Selection pressure leads to gene being maintained in original form if it is necessary for viability. If selection pressure is not so great, mutations may result in a gene with a related function or a pseudogene. There are lots of human pseudogenes. Slow and rapid are still millions of years. What was thought to happen in opsins was that rhodopsin maintained its original function and color opsins came later (18). Know this is how proteins with closely related functions are thought to be generated.
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Photopigments underly both normal and color-deficient vision.
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XLR=x-linked recessive.
AD=autosomal dominant
AR= autosomal recessive
Blue opsin is on chromosome 6, red and green on X.
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Remember that affected males are related through mother- and which X male gets determines whether his vision is normal or not.
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Nathan says take color normal individual and look a red and green opsins. He made probes for green and red opsins (at bottom). Used shorthand nomenclature as way to express data. Arrow is entire locus. Probes are A,B,C,D. Ag is the A probe for green.
He normalized DNA on the Southern blot. Intensity of bands is different. That means more copies of G on blot than R (For example, look at blot A. The bands for Ag are much darker than those for Ar. Another clue is that some color normal individuals are missing Dr. This is a key to the arrangement of the tandem array. They have to be in order on the chromosome. Dr was lost during recombination events that duplicated the green opsin gene. R gene is 5’. If it were at 3’ end you would sometimes lose parts of green.
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Nathan speculated . Did densitometry scan on Southern blot and found it was common to have multiple copies of green. Had one red, 1 to 3 copies green.
A is color normal males. B shows mechanism for changing number of copies of green.
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Looking at dichromats, genotypes shown across top of blot. Genomic probes shown upper left. This is a more complex analysis. Some individuals have complete loss of parts of opsins probed. You do ratio of green to red.
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G-R+ had 2 phenotypes: missing entire Green opsin, or missing Dg. Just missing 3’ end probe.
G+R- more complex- variety phenotypes listed. All have Dg. 14 is color normal. Compare the others in this graph to him. These are “blown out” slides, so you cannot tell intensity easily.
Conclusions are based on Southern-blot data.
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remember arrow represents entire gene region. Colored in to represent what is present.
A- color normal individual.
B- Recombination resulting in G+R-
C- Examples of how recombination results in unequal crossing over events with unusual arrays.


The data is complicated- go back and compare the slides and data. Slide 27 is a model to explain what is sitting in the genome to produce the Southern blot data. He hypothesized these structures to account for dichromatic vision. Look at analysis and blots to get it to make sense. She will not ask details of this. Understnd how got idea of 1 red to multiple copies of green and how to look at pathology of cases. How do you get the model from the data?
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Achromatopsia is rare. No red or green.
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X-linked trait.
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genomic map red-green opsins. Added Z probe upstream of red locus.
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Southern analysis again. 9 and 10 are color normal. Z missing or shortened in people with BCM. They have intact fragments for the opsins mostly, but missing 5’Z region (See data in G). Z region appears to be essential. Major control region just upstream is necessary.
Go back and look. It will make sense.