Crossing Over.pptx

Genetics

1. CROSSING OVER Dr. Gayatri D. Hande Assistant Professor, Shri. Shivaji Science College Amravati.

2. Genetics is the study of heredity and variations. Linkage and Crossing over are the two main principles of inheritance and variations. The concept of linkage deals with the detailed study of gene location on the chromosome and their inheritance that reflect in the phenotype of an organism. Linkage keeps the two or more linked genes together over generations during inheritance.

3. Linkage can be complete or incomplete. The early experiments to understand the phenomenon of linkage were studied using Drosophila and sweet pea. Crossing over is the phenomenon of the exchange of genes through the recombination of sister chromatids. It is of great importance due as it leads to variations in the organisms.

4. Crossing over – The exchange of chromosomal segments between two non-sister chromatids of non homologous chromosomes after crossing over. F . Janssens was the first person to discover chiasma formation and the related process of crossing over. Morgan found the phenomenon of linkage and recombination.

5. Alleles-Different versions (sequences) of a gene. Mutant-Newly created allele made by mutagenesis. Genotype-The complete set of alleles for all genes carried by an individual. Wild type-Standard reference genotype. Most common allele for a certain trait. Phenotype-Observable trait specified by the genotype. Point mutation-A change in a single base pair. Silent mutation-A point mutation in a codon that does not change the specified amino acid. Genetics Terms

6. Crossing-over is a physical exchange between chromatids in a pair of homologous chromosomes. It results in a new association of genes in the same chromosome. The role of crossing-over is important for evolution to take place. In fact, crossing-over and independent assortment are mechanisms that produce new combinations of genes. Natural selection can then act to preserve those combinations that produce organisms with maximum fitness, that is, maximum probability of perpetuation of the genotype

7. The Concept of Crossing-Over Following are the important features of crossing-over: A gene is located on chromosomes at a particular site called a locus (plural-loci). The loci of the genes on a chromosomes are arranged in a linear sequence. In a heterozygote, the two alleles of a gene occupy corresponding positions in the homologous chromosomes, that is, allele A occupies the same position in homolog 1 that allele a occupies of a species is fixed or constant .

8. iv) Crossing-over involves the breakage and rejoining of two chromatids (of homologous chromosomes), resulting in reciprocal exchange of equal and corresponding segment between them v) Chromosomes with recombined or new combinations of genes are formed by the occurrence of crossing-over. vi) Crossing-over occurs more or less at random along the length of a chromosome pair. Thus, the probability of its occurrence between two genes increases with increasing physical separation of the genes along the chromosome.

9. Crossing over refers to the exchange of genetic material or chromosome segments between non-sister chromatids in meiosis. This genetic process occurs between homologous regions of matching chromosomes and the interchange of homologous chromosomes. Depending upon the number of chiasmata that appeared, crossing-over can be categorised into three types: Single cross-over Double cross-over Multiple cross-over Significance of crossing-over are:

10. Crossing over produces a new combination of genes Crossing over plays an essential role in the process of evolution Crossing over frequency helps in the construction of genetic maps Crossing over provides the evidence for a linear arrangement of linked genes in a chromosome Crossing over provides an inexhaustible store of gene variability in sexually reproducing organisms

11. Crossing over can put new alleles together in combination on the same chromosome, causing them to go into the same gamete. When genes are far apart, crossing over happens often enough that all types of gametes are produced with 25% frequency.

12. Theories of Crossing Over There are two theories to explain the relationship between crossing over and chiasma formation: Classical theory : It is also called a two-plane theory. This theory was proposed by L.W. Sharp. This theory stated that chiasma is the cause of crossing over but not the result of crossing over. The formation of chiasma occurs before the genetic crossing over.

13. 3. Chiasma type theory : It is also called one plane theory. It was proposed by Jannsen and later developed by Belling and Darlington. According to this theory, chiasma is the result of crossing over, and crossing over precedes the chiasma formation. This theory is widely accepted.

14. Mechanism of Crossing Over The mechanism of crossing over involves the following steps: 1. Synapsis : The pairing between the homologous chromosomes (synapsis) takes place during zygotene. This pair of homologous chromosomes is called synapsis.

15. 2. Tetrad formation : The two chromatids of a chromosome are referred to as dyads. A group of four homologous chromatids (two dyads) of two synapsed homologous chromosomes is known as a tetrad. The two chromatids of the same chromosome are called sister chromatids. The two chromatids, one of the one chromosome and the other of its homologue, are termed non-sister chromatids. A highly organized structure of filaments is formed between the paired homologous chromosomes at the zygotene stage of meiosis-I called synaptonemal complex. It helps in keeping the homologous chromosome in a closely paired state.

16. 3. Exchange of Chromatid segments : The two non-sister chromatids come in contact at certain points. This is the region where the exchange of genes between the two non-sister chromatids of a tetrad takes place. The places where homologous chromosomes are held together and exchange bits of chromatids are known as chiasma. In synapsis, the non-sister chromatids of homologous chromosomes break and recombine. This leads to the formation of chiasmata. The exchange of fragments is stimulated by the development of recombination nodules during the pachytene stage. The unchanged part of the chromatid is called non-crossover, and the changed parts are called recombinants.

17. 4. Terminalization : The chromatids separate progressively from the centromere towards the chiasma and get separated from each other. It is called terminalization. Terminalization of chiasma begins in the diplotene stage after crossing over, and completion takes place in the diakinesis stage.

18. Significance of Crossing Over The phenomenon of crossing over is of great significance that can be discussed as follows: 1. It provides an inexhaustible store of genetic variability in sexually reproducing organisms. 2. Since crossing over helps in the development of new characteristics. Therefore it is of paramount importance in plant breeding. 3. The new gene combination produced during crossing over plays an important role in microevolution. 4. The frequency of crossing over is helpful in the mapping of chromosomes. 5. Crossing over also justifies the linear arrangement of genes.