Loci and crossing overA 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?

______________ ____ __________A b C D e F

______________ ____ __________a B C d E f

8 different combinations!

• AbCDeF• AbCDEf• AbCdeF• AbCdEf

• aBCDeF• aBCDEf• aBCdeF• aBCdEf

How would your answer change if genes A and D were on the same chromosome?

• These are called “linked” genes!

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?

Which of the two gene pairs are closer to one another?

• A B or E F?• How do you know?

• 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

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!

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