loci and crossing over a review. humans have approximately 20,000 genes and 46 chromosomes…...
TRANSCRIPT
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Loci and crossing overA Review
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• Humans have approximately 20,000 genes and 46 chromosomes…• What’s the take-home message from this
“Factoid”?
![Page 3: Loci and crossing over A Review. Humans have approximately 20,000 genes and 46 chromosomes… What’s the take-home message from this “Factoid”?](https://reader036.vdocuments.us/reader036/viewer/2022081603/56649e715503460f94b700ac/html5/thumbnails/3.jpg)
![Page 4: Loci and crossing over A Review. Humans have approximately 20,000 genes and 46 chromosomes… What’s the take-home message from this “Factoid”?](https://reader036.vdocuments.us/reader036/viewer/2022081603/56649e715503460f94b700ac/html5/thumbnails/4.jpg)
Remember that chromosomes assort independently during metaphase I…
• What kinds of genes could the gametes get from the following chromosomes?
______________ ____ __________A b C D e F
______________ ____ __________a B C d E f
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8 different combinations!
• AbCDeF• AbCDEf• AbCdeF• AbCdEf
• aBCDeF• aBCDEf• aBCdeF• aBCdEf
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How would your answer change if genes A and D were on the same chromosome?
• These are called “linked” genes!
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Soon, geneticists may be able to sequence your DNA for a reasonable cost…
•What would it mean if a geneticist looked at the gametes you produce, and found that 10% of the time, the AB combination occurred?
• The EF combination 40% of the time?
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Which of the two gene pairs are closer to one another?
• A B or E F?• How do you know?
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• Biologists use these “crossover” frequencies to calculate the relative distance between genes linked on the same chromosome
• If crossovers occur 50% of the time or more, it’s assumed the two genes are on separate chromosomes
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Gene “maps” can be constructed based on these frequencies…
• 3 genes on the same chromosome: b, cn, vg• cn and vg cross over ~9.5% of the time• b and vg cross over ~17% of the time• b and cn cross over ~9% of the time• Create a “map” of the genes!
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Example 2
• sc and vg cross over ~13.6% of the time• sc and ec ~6.4% of the time• ec and vg ~9.1% of the time
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