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”?

4. Remember that chromosomes assort independently during metaphase I… What kinds of genes could the gametes get from the following chromosomes? ____________________________ AbC DeF ____________________________ aBC dEf

5. 8 different combinations! AbCDeF AbCDEf AbCdeF AbCdEf aBCDeF aBCDEf aBCdeF aBCdEf

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

7. 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?

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

9. 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

10. 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!

12. 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