1. Mapping Genes
and Linked Genes

2. Vocabulary
Linked genes- genes that are close together on the same chromosome.
Map unit- a unit for measuring genetic linkage
Recombinants- a new combination of genetic material

3. Mapping genes on the chromosome:
The greater the distance between genes the higher the frequency of crossover
A map unit is the distance between genes
1% Frequency of crossing over= 1 map unit

4. Determining the order of the genes on a chromosome
Using the information on the left, determine the order of the genes on the chromosome?
Frequency of crossover is:
Genes A and B 15%
Genes A and E 5%
Genes A and D 20%
Genes B and D 10 %
Genes A and C 10%

5. The order of the genes on the chromosome are:
A, E, C, B, D

6. Linked genes on the X
Linked genes do not follow mendelian genetics

7. Example- If Genes A and B are linked:
What at the possible gametes for the following genotypes
AaBb x AaBb ?
AB, ab
Set up the Punnett square and do the cross to determine the phenotypes of the offspring.

8. Outcome showing linked genes on the chromosomes
A is attached earlobes
a is free earlobes
B is Brown eyes
b is blue eyes
What are the phenotypes of the offspring?
What percent?
If there were 100 offspring produced what percent of each phenotype would you predict?

9. Ex: Do the Punnett square for each trait
Attached ears A Brown eyes B
Free ears a Blue eyes b
Cross: Attached ears x Free ears
Brown eyes Blue eyes
AaBb x aabb

10. Check your work Attached ears X Free ears Brown eyes Blue eyes
AaBb X aabb

11. These are the expected phenotypes
Attached ears X Free ears
Brown eyes Blue eyes
AaBb X aabb
Attached Attached Free Free
Brown Brown Blue Blue
1/ / / /4
If there were 200 offspring, how many would be in each category?

12. Out of 200 offspring this is what we would expect
Attached ears X Free ears
Brown eyes Blue eyes
AaBb X aabb
Attached Attached Free Free
Brown Brown Blue Blue
How does meiosis make the above possible?

13. Possible gametes using medel’s law
Attached ears X Free ears
Brown eyes Blue eyes
AaBb X aabb

14. Possible gametes if the genes are linked
If the genes are linked, then the following happens:
Attached ears X Free ears
Brown eyes Blue eyes

15. Two possible genotypes if the genes are linked
If the genes are linked, then in theory there should only be two phenotypes with 100 in each.
Attached ears X Free ears
Brown eyes Blue eyes
AaBb X aabb
Attached Free
Brown eyes Blue eyes

16. Genes are linked and recombinants are formed:
However, the results may be the following:
Attached ears X Free ears
Brown eyes Blue eyes
AaBb X aabb
Attached Attached Free Free
Brown Brown Blue Blue
Meiosis can account for this by crossing over!!

17. This is how recombinants are formed?
If the genes are linked, then the following happens:
Attached ears X Free ears
Brown eyes Blue eyes

18. Example of Linked genes crossing over producing recombinants:

19. Determining the distance or map units between genes:
A map unit is equal to 1% cross over rate. Percentage of crossing over is = # of cross overs/ total of offspring
20/200 x 100 = 20%
Therefore, 20% of the time crossing occurs between these two genes during meiosis.
The further two genes are on the chromosome, the more likely that crossing over will occur.

20. Example:
Two genes A and B are 17 map units apart. If AaBb x aabb are the genotypes of the parents and 200 offspring are produced, predict how many of each offspring will be recombinants (Ab, aB in phenotypes).
200 x .17 = 34 recombinants
17 Ab and aB

21. Are these genes linked? How many map units are the genes apart?
The following cross is made normal winged and normal length heterozygous for both traits (WwLl) to a dumpy winged fly with short legs (wwll). The following offspring were counted.
Normal wings, normal legs
Normal wings, short legs
Dumpy wings, normal legs
Dumpy wings, short legs

22. To determine if the genes are linked.
1. Do individual Punnett squares for each trait and determine the phenotypic ratio
2. If the outcome is extremely different then we can assume that the genes are linked
Compare your phenotypic ratios with this outcome: Are they linked?
Normal wings, normal legs
Normal wings, short legs
Dumpy wings, normal legs
Dumpy wings, short legs

23. How many map units between the genes?
1. Determine which phenotypes are your recombinants
2. Add the total number of recombinants
3. Divide by 2 (because there are two recombinants)
4. This is the distance (or map units) between the two genes.

24. Answer
Yes they are linked The genes are 15 map units apart