Crossing over

Crossing over: Concept, Types and theories

1. Crossing over: Concept, Types and theories Vaishali S.Patil Assosiate Professor, Department of Botany Shri Shivaji College of Arts, Commerce & Science Akola

2. Crossing over refers to the interchange of parts between non-sister chromatids of homologus chromosomes during meiotic prophase ( pachytene ). In other words, crossing over results from exchange of genetic material between non-sister chromatids involving breakage and reunion at precise point . The term crossing over was first used by Morgan and Cattell in 1912.

3. Crossing over takes place during meiotic prophase , i.e., during pachytene . Each pair of chromosome has four chromatids at that time . 2. Crossing over occurs between non-sister chromatids . Thus one chromatid from each of the two homologus chromosomes is involved in crossing over . 3. It is universally accepted that crossing over takes place at four strand stage . 4. Each crossing over involves only two of the four chromatids of two homologus chromosomes . However, double or multiple crossing over may involve all four, three or two of the four chromatids , which is very rare . 5 . Crossing over leads to re-combinations or new combinations between linked genes .

4. 6 . Crossing over generally leads to exchange of equal segments or genes and recombination is always reciprocal. However, unequal crossing over has also been reported. 7 . The value of crossover or recombinants may vary from 0-50 %. 8. The frequency of recombinants can be worked out from the test cross progeny. It is expressed as the percentage ratio of recombinants to the total population (recombinants + parental types). Thus , Cases of two strand crossing over, somatic crossing over, sister strand crossing over and unequal crossing over are also known. The point of exchange of segments between non-sister chromatids of homologous chromosomes during meiotic prophase is called chiasma (pleural chiasmata ). It is thought to be the place where crossing over takes place. Crossing over generally yields two recombinant types or crossover types and two parental types or non-crossover types.

5. Depending on the position, chiasma is of two types, viz., terminal and interstitial . When the chiasma is located at the end of the pairing chromatids , it is known as terminal chiasma and when it is located in the middle part of non-sister chromatids , it is referred to as interstitial chiasma .

6. Later on interstitial chiasma is changed to terminal position by the process of chiasma terminalization .(The movement of chiasma away from the centromere and towards the end of tetrads is called terminalization .) The number of chiasma per bivalent may vary from one to more than one depending upon the length of chromatids . When two chiasmata are formed, they may involve two, three or all the four chromatids .

7. Types of Crossing Over: Depending upon the number of chiasmata involved , crossing over may be of three types, viz., single, double and multiple as described below: i . Single Crossing Over: It refers to formation of a single chiasma between non-sister chromatids of homologous chromosomes. Such cross over involves only two chromatids out of four .

8. ii. Double Crossing Over: It refers to formation of two chiasmata between non-sister chromatids of homologous chromosomes. Double crossovers may involve either two strands or three or all the four strands. The ratio of recombinants and parental types under these three situations are observed as 2:2:3:1 and 4 : 0, respectively.

9. iii. Multiple Crossing Over: Presence of more than two crossovers between non-sister chromatids of homologous chromosomes is referred to as multiple crossing over. Frequency of such type of crossing over is extremely low.

10. Theories of Crossing Over: ( i ) Contact First Theory (by Serebrovsky ): According to this theory the inner two chromatids of the homologous chromosomes undergoing crossing over first touch each other and then cross over. At the point of contact breakage occurs. The broken segments again unite to form new combinations (Fig. 5.11A).

11. (ii) The Breakage-First Theory (By Muller): According to this theory the chromatids under-going crossing over first of all break into two without any crossing over and after that the broken segments reunite to form the new combinations (Fig. 5.11 B). (iii) Strain Theory (by Darlington): According to this theory the breakage in chromosomes or chromatids is due to strain caused by pairing and later the breakage parts again reunite.(16.5)

12. Factors Influencing Crossing Over: 1 . Sex: In Drosophila, crossing over is completely suppressed in male but very high in female , also there is a tendency of reduction of crossing over in male mammals. 2.Mutation : Gwen first discovered that mutation reduces crossing over in all the chromosomes of Drosophila . 3 . Inversion: Inversion is an intersegmental change in the chromosome. In a given segment of chromosome crossing over is suppressed due to inversions. 4 . Temperature: Plough has experimentally shown that when Drosophila is subjected to high and low temperature variations, the percentage of crossing over in certain parts of the chromosome is increased. 5. X-ray Effect: Muller demonstrated that X-ray irradiations increase crossing over near centromere . Similarly Hanson has shown that radium increases crossing over. 6. Age : Bridges has demonstrated that the age also influences the rate of crossing over in Drosophila . When the female becomes older the rate of crossing over increases. 7. Nutrition : High calcium diet in young Drosophila decreases crossing over rate where as diet deficient of metallic ions increases crossing over. 8. The frequency of crossing over is less at the ends of the chromosome and also near the centromere in comparison to other parts.

13. Significance of Crossing Over: Crossing over provides direct proof for the linear arrangement of genes. 2. Through crossing over segments of homologous chromosomes are interchanged and hence provide origin of new characters and genetic variations. 3. Crossing over has led to the construction of linkage map or genetic maps of chromosomes. 4. Linkage group and linear order of the genes help to reveal the mechanism and nature of the genes. 5. Crossing over plays a very important role in the field of breeding to improve the varieties of plants and animals.