2. Incomplete dominance

3. Polygenetic
Ex. Human skin color

4. KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

5. Objectives
Students will be able to describe the discovery of gene linkage.
Students will be able to explain how linkage maps can be used to estimate distances between genes.

6. Gene linkage was explained through fruit flies.
Mendel’s Work
Crossed pea plants; determined 9:3:3:1 ratio in dihybrid crosses
Conclusions: two alleles for each trait; alleles assort independently
Punnett and Bateson’s Work
Crossed pea plants; dihybrid cross ratios differed from 9:3:3:1
Conclusions: Suggested that some traits were linked

7. Gene linkage was explained through fruit flies.
Morgan found that linked traits are on the same chromosome.
Chromosomes, not genes, assort independently during meiosis.
Wild type
Mutant

8. Linked genes are not inherited together every time.
Chromosomes exchange homologous genes during meiosis.

9. Linkage maps estimate distances between genes.
Sturtevant’s Hypothesis:
The closer together two genes are, the more likely they will be inherited together.
Cross-over frequencies are related to distances between genes.
Linkage maps show the relative locations of genes.

10. Sturtevant’s Experiments
Studied linked traits in fruit flies;
recorded the percentage of times crossing over occurred;
used cross-over frequencies to make linkage maps
Wild type
Mutant

11. Cross-over frequencies can be converted into map units.
gene A and gene B cross over 6.0 percent of the time
gene B and gene C cross over 12.5 percent of the time
gene A and gene C cross over 18.5 percent of the time

12. Calculate at your table the order of these genes.
The crossover frequencies between linked genes A and B is 40%; between B and C, 20%; between C and D, 10%; between C and A, 20% and between D and B, 10%.