Crossing over occurs during meiosis when non-sister chromatids of homologous chromosomes exchange segments. This creates new combinations of genes in gametes and contributes to genetic diversity. Morgan's experiment with fruit flies provided early evidence of crossing over and genetic linkage. Recombinant frequency is a measure of how many new combinations are formed in offspring and indicates the distance between genes - a higher frequency means genes are farther apart. In an experiment with beetles, two traits - body texture and color - showed a recombinant frequency of 30%, indicating the genes are linked but some crossing over occurs between them.

2. CROSSING OVER
Presented By:
Muhammad Anas Rasool
AGL-15-07
B.Sc. (Hons.) Agriculture
College of Agriculture
BZU Bahadur Sub Campus Layyah

3. What is crossing over..?
• Crossing over, or recombination, is the
exchange of chromosome segments between
non-sister chromatids of homologous
chromosomes in meiosis.
• Crossing over creates new combinations of
genes in the gametes , contributing to
genetic diversity.

4. Morgan’s Experiment
EXAMPLES

5. According to Mendel’s Laws:
Allele pairs assort independently
during gamete formation.
Therefore, each gamete receives
only one copy of an allele.

6. But when Genes are linked
They are transmitted to
offspring as they are present
in parents.

7. Continue…!
Alfred Henry Sturtevant

8. 151+154
151+154+1195+1339
100
Recombination frequency (RF) = ​Recombinants
​Total offspring
100
10.7%
=
Recombination frequency (RF) =
Let’s Find Recombinant Frequency

9. What does recombinant frequency mean…?
• Recombinant Frequency (RF) means that how many new recombinants are
formed in F2 & it represents that how far the two genes are located.
• Higher the RF, more is the distance between both genes & vice versa.
• If RF of two genes (present on the same chromosome) is 20% it means that
the distance between them is 20 map units or 20 centimorgan.
• Therefore, RF is useful for Gene Mapping.

10. BATESON & PUNNETT’s Experiment

12. 19+27
19+27+296+85
100
Recombination frequency (RF) = ​Recombinants
​Total offspring
100
10.7%
=
Recombination frequency (RF) =
Recombinant Frequency

13. 0.1% frequency of recombination is observed in:
a) Only in sex chromosomes
b) Only in unlinked chromosomes
c) In genes on two different chromosomes
d) In genes located very close on same chromosome
Now, it’s your turn…!

14. A scientist is studying two traits in a type of beetle: body texture and color.
Smooth body (B) is dominant to rough body (b), and gray (G) is dominant to
white (g).
• A BbGg and a bbgg beetle mate and produce the following offspring:
Phenotype Number of Offspring
• Smooth, gray 345
• Rough, white 355
• Smooth, white 148
• Rough, gray 152
Now try to tell that either the genes B & G are linked or not…?
Genes are linked (on the same
chromosomes) but due to
crossing over new recombinants
(30 %) are formed…!

15. What is the distance
between
genes B & G
that were discussed
in last slide…?

16. THANK YOU

17. References
https://www.khanacademy.org/science/biology/classical-
genetics/chromosomal-basis-of-genetics/e/recombination-frequency-
and-gene-mapping
https://www.khanacademy.org/science/biology/classical-
genetics/chromosomal-basis-of-genetics/a/linkage-mapping
https://en.wikipedia.org/wiki/Chromosomal_crossover
http://nfs.unipv.it/nfs/minf/dispense/geneticaumana/10-
%20linkage.pdf
http://www.encyclopedia.com/science-and-technology/biology-and-
genetics/genetics-and-genetic-engineering/crossing-over