Thomas Morgan observed that genes on the same chromosome can be linked during meiosis. He conducted test crosses on Drosophila with traits for eye color and wing type. The results showed that some offspring received combinations of traits that were not expected if the genes assorted independently, demonstrating genetic linkage. Genetic linkage occurs because homologous chromosomes occasionally exchange sections during meiosis through the process of crossover. Crossover results in new combinations of genes that can become separated from their original linked combinations on the chromosome. The frequency of crossover between genes determines how far apart they will be on the genetic map of the chromosome.

1. LINKAGE AND CROSS OVER
1

2. Polytene Chromosome
2

3. LINKAGE
3

4. F
eE
cC
bB
aA
f
4

5. F
eE
cC
f
B
A
5

6. Thomas Hunt Morgan
6

7. 7

8. Red - dominant (R)
Purple - recessive (r)
Normal wings - dominant(N)
Vestigial wing - recessive (n)
Eye colour
Type of wing
8

9. 9

10. Normal Vestigial
10

11. RrNn x rrnn
RRNN X rrnn
RrNn
RN rn
Parent
Gamete
F1
Test cross
11

12. Expected results were:
25% Red Normal
25% Red vestigial
25% purple Normal
25% purple vestigial
The actual results were:
47% Red Normal
43% Purple vestigial
5% Red vestigial
5% purple normal
12

13. Types of linkage
1.Complete linkage
Closely associated
Pass to 2/ more generations
2.Incomplete linkage
Widely located
High chance of separation by cross over
13

14. CROSS OVER
14

15. 15

16. 16
Crossover is the process by which two chromosomes paired up during
prophase I of meiosis exchange a distal portion of their DNA
Crossover occurs when two chromosomes, normally two homologous
instances of the same chromosome, break and connect to each other's
ends
If they break at the same locus, this merely results in an exchange of
genes
This is the normal way in which crossover occurs

17. 17
If they break at different loci, the result is a duplication of genes on one
chromosome and a deletion on the other
If they break on opposite sides of the centromere, this results in one
chromosome being lost during cell division
Any pair of homologous chromosomes may be expected to cross over three or
four times during meiosis
This aids evolution by increasing independent assortment, and reducing the
genetic linkage between genes on the same chromosome

18. 18
Dihybrid cross – one point

19. 19
Trihybrid cross – 2 point

20. 20
Types of cross over product
1. Parental/ Non cross over type
Coupling phase
2. Recombinant/ Cross over type
Repulsion phase

21. 21
Genetic map
Any method used for determining the location of and relative
distances between genes on a chromosome
Map distance =
Number of recombinant offspring
Total number of offspring
× 100
Map unit – centiMorgan (cM)
1 Map unit = 1% recombination frequency

22. 22
 Parental phenotypes – most frequently
 Double cross over – least frequently
 Single cross over – Intermediate frequently

24. Variation in the
number of chromosomes

28. Variation in the
arrangement of chromosomes

32. 32
Why it is not possible to use a homozygous dominant organism
(such as TT) in a test cross experiment to determine the
genotype of an organism showing the dominant phenotype.
Illustrate your answer with fully using appropriate genetic
symbol?

33. 33
In maize the following cross between pure breeding plants were made,
coloured shrunken starchy × colourless full waxy. The F1 plants were all
found to be coloured full starchy. The F1 plants were then test crossed
and the following progenies were obtained. Explain the results and
obtain the necessary information regarding the inheritance of these
characters.
Coloured shrunken starchy 2538
Colourless full waxy 2708
Coloured full waxy 116
Colurless shrunken starchy 113
Colured shrunken waxy 601
Colourless full starchy 626
Colured full starchy 4
Colourless shrunken waxy 2