1. 11-5 Linkage & Gene Maps VOCABULARY: KEY CONCEPT: Gene map
What structures actually assort independently?

2. Remember
Genes are located on chromosomes

3. The Big Question…
It may be easy to see that genes located on different chromosomes assort independently, but what about genes located on the same chromosome?

4. Thomas Morgan’s Research
Morgan identified more than 50 genes on Drosophila’s (fruit fly) 4 pairs of chromosomes.
He discovered that many genes seemed to be “linked” together
They are almost always inherited together & only rarely become separated
Grouped genes into 4 linkage groups

5. Morgan’s Conclusion
Each chromosome is actually a group of linked genes
It is the chromosome that assorts independently!!
Mendel missed this because 6 of the 7 traits he studied were on different chromosomes.

6. Gene Linkage Morgan saw that linkage groups assort independently
Each chromosomes is actually a group of linked genes
Definition
Linked genes: genes located on the same chromosome that may be inherited together
Chromosomes assort independently, NOT individual genes
BUT…If 2 genes are found on the same chromosome are they linked forever?
NO!!

7. Crossing Over Crossing over is random
Crossing over during Meiosis (Prophase I) can separate genes
Crossing over sometimes separates linked genes and so there is a new combination of alleles!
CROSSING OVER increases genetic diversity by producing new combinations of alleles

9. Review
What are linked genes?
How can linked genes be separated?

10. Gene Maps
Alfred Sturtevant was a graduate student working in Morgan’s lab part-time in 1911
He hypothesized: the farther apart 2 genes are on a chromosome the more likely they are to be separated by crossing-over
How often linked genes are separated can be used to produce a “map” of distances between genes
Alfred Sturtevant

11. Gene Maps…in other words
Frequency of crossing over: how often crossing over occurs between 2 genes.
If two genes are close together, they have a LOW cross over frequency
Knowing this helps estimate the positions of genes (where genes are found) on chromosomes.

13. Gene Maps
This map shows the relative locations of each known gene on a chromosome
We have used this technique to help construct maps of the Human Genome

14. Review
What information do gene maps give us?