This presentation explains the basics of crossing over, theories or mechanisms behind it, types, salient features, factors affecting crossing over and its significance.

1. CROSSING OVER
BY: M. JANANI
III B.SC. ZOOLOGY
212018006

2. DEFINITION OF CROSSING OVER
• Interchange of chromosomal parts or
genes between non-sister chromatids
of a homologous pair of
chromosomes.
• Homologous pair of chromosome:
Paternal chromosome + Maternal
chromosome. Same size and shape.
• Non-sister chromatids: The 2
chromatids from 2 different
chromosomes.

3. SALIENT FEATURES OF CROSSING
OVER
• Discovered by Morgan
• Crossing over results in recombination of genes , thus promoting genetic
diversity.
• Results in the formation of chiasmata ; the point of contact, the physical link,
between two non-sister chromatids belonging to homologous chromosomes. At a
given chiasma, an exchange of genetic material can occur. Absence of a chiasma
generally results in improper segregation and aneuploidy.
• It occurs during meiosis (pachytene).
• It occurs only between non-sister chromatids of homologous chromosomes.

4. SALIENT FEATURES OF CROSSING
OVER
• Number/Frequency/Percentage of crossing over depends on the length of the
chromosome and distance between the chromosomes
• Usually crossing over is less frequent near the centromere and tip of the
chromosome.
• Crossing over at one point prevents another crossing over nearby.

5. TYPES OF CROSSING OVER
• Single Crossing Over: One chiasma is formed.
Only one chromatid of each chromosome is
involved.
• Double Crossing Over: Two chiasmata are formed.
Two , three or all four chromatids are involved.
1. Two strand
2. Three strand
3. Four strand
• Multiple Crossing Over: More than two chiasmata
are formed. Rare.

6. FACTORS AFFECTING CROSSING OVER
• Temperature
• X Rays
• Age
• Inversion of chromosome
• Location
• Interference
• Mutation

7. MECHANISMS OR THEORIES OF
CROSSING OVER
CHIASMA TYPE THEORY BREAKAGE FIRST THEORY

8. STRAIN OR TORSION
THEORY
• CONTACT FIRST THEORY
• DIFFERENTIAL
CONTRACTION THEORY

9. TETRAD ANALYSIS

10. SIGNIFICANCE
• Linear arrangement of genes
• Chromosome map
• Recombination
• Variations

11. THANK YOU