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Loci and crossing over

Loci and crossing over. A Review. Humans have approximately 20,000 genes and 46 chromosomes… What’s the take-home message from this “Factoid”?. Remember that chromosomes assort independently during metaphase I…. What kinds of genes could the gametes get from the following chromosomes?

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Loci and crossing over

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  1. Loci and crossing over A Review

  2. Humans have approximately 20,000 genes and 46 chromosomes… • What’s the take-home message from this “Factoid”?

  3. 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

  4. 8 different combinations! • AbCDeF • AbCDEf • AbCdeF • AbCdEf • aBCDeF • aBCDEf • aBCdeF • aBCdEf

  5. How would your answer change if genes A and D were on the same chromosome? • These are called “linked” genes!

  6. 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?

  7. Which of the two gene pairs are closer to one another? • A B or E F? • How do you know?

  8. 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

  9. 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!

  10. 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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