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

2. Gene linkage was discovered by deviations from Mendel’s rules
Mendel crossed pea plants with different discrete traits
Alleles assort independently with 9:3:3:1 ratio

3. R.C. Punnett and William Bateson
Developed Punnett squares. Crazy chicken breeder.
Found that rates often did not follow 9:3:3:1 proportions
Therefore, some genes are linked
Cream Legbar chicken.
Originally bred by Punnett

4. Gene linkage was explained using fruit flies.
Thomas Hunt Morgan studied fruit flies
Found some traits were inherited together
Linked traits fell into four groups
Determined fruit flies have four pairs of chromosomes
Chromosomes, not genes, assort independently during meiosis.
Wild type
Mutant

5. Linked genes are not inherited together every time.
Chromosomes exchange homologous genes during meiosis.
Genes that are closer together will more often cross over together.

6. Alfred Sturtevant (student of Morgan)
Hypothesis:
Frequency of genes crossing-over together depends on distance between the genes on the chromosomes
Experiments:
Found 3 linked traits in fruit flies: body color, eye color, wing size.
Recorded how many times the genes crossed over together. Number out of 100 (i.e. percent)

7. Alfred Sturtevant (student of Morgan)
Conclusions:
Determined rates of crossover between each of the three genes.
Linkage Maps show relative distance between genes on a chromosome
If you know crossover rates of three or more genes, you can determine their relative locations.

8. Linkage maps estimate distances between genes.
The closer together two genes are, the more likely they will be inherited together.
Linkage maps show the relative locations of genes.

9. 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. Quiz Question 5:
gene A and gene B cross over 9.5 % of the time
gene B and gene C cross over 14.5 % of the time
gene A and gene C cross over 5 % of the time B C A B C