1. Section 7.3: Gene Linkage & Mapping
Biology

2. Objectives Describe the discovery of gene linkage
How was gene linkage explained through fruit flies?
Explain how linkage maps can be used to estimate distances between genes.

3. Bateson & Punnett
William Bateson and R.C. Punnett (of punnett square fame) were studying dihybrid crosses of pea plants
But, depending on the traits they were studying, they were getting variances of the 9:3:3:1 ratio
How could genes be linked and still follow the Law of Independent Assortment?

4. Thomas Hunt Morgan
Morgan began his work with fruit flies (Drosophila melanogaster) because he was looking for a useful model for genetic research
Quickly & cheaply grow generations of flies
Easily identifiable characteristics (eye color, body color, wing shape)
Morgan and his students coined the term wild type, which is the most common genotype
They crossed the wild type with different, less common phenotypes

5. Morgan’s Experiments
His crossed did not always follow the 9:3:3:1 ratio predicted by Mendel
But, they did follow a noticeable pattern
Some traits appeared to be inherited together
These traits were linked traits, and they appeared to form into four groups
Fruit flies have four chromosomes
Each group of genes matched one chromosome
Linked genes were on the same chromosome

6. Probability of Linkage
Probability that two genes on a chromosome will be inherited together is related to the distance between them
The closer they are together the more likely they will be inherited together

7. Alfred Sturtevant
Hypothesized that the frequency of cross-overs was related to the distance between genes
Identified three linked traits in fruit flies and then crossed them
Body Color
Eye Color
Wing Size
Recorded the percentage of times the phenotypes did not appear together
This percentage represented the frequency of cross-overs

8. Linkage Maps Map of the relative locations of genes on a chromosome
One map unit is equal to 1 percentage point of cross- over
Example:
Gene A and gene B cross over 6% of the time
Gene B and gene C cross over 12.5% of the time
Gene A and gene C cross over 18.% of the time

9. Linkage Maps A and B are 40% B and C are 20% C and D are 10%
C and A are 20%
D and B are 10%
A-C-D-B

10. Linkage Maps
Only show the relative locations of linked genes, not actual distances
Some genes may have the same map linkage, but may actually be physically farther apart
Therefore, there is more to gene linkage than just distance

11. Output Page 211, #4 of the 7.3 Assessment
Draw a linkage map based on the following cross-over percentages for three gene pairs: A-B = 8%, B-C = 10%, and A-C = 2%