Understand causes of genetic variation

Crossing over is a process that happens between homologous chromosomes in order to increase genetic diversity. During crossing over, part of one chromosome is exchanged with another. The result is a hybrid chromosome with a unique pattern of genetic material. Gametes gain the ability to be genetically different from their neighboring gametes after crossing over occurs. This allows for genetic diversity, which will help cells participate in survival of the fittest and evolution.

The tetrad, which divides into non-sister chromatids, exchanges genetic information in order to make the genetic pool more variant, and result in combinations of phenotypic traits that can occur outside of linked genotypic coding.

During prophase I homologous chromosomes will line up with one another, forming tetrads. During this lining up, DNA sequences can be exchanged between the homologous chromosomes. This type of genetic recombination is called crossing over, and allows the daughter cells of meiosis to be genetically unique from one another.

Crossing over can only occur between homologous chromosomes. Cells become haploid after meiosis I, and can no longer perform crossing over.

When chromatids "cross over," homologous chromosomes trade pieces of genetic material, resulting in novel combinations of alleles, though the same genes are still present. Crossing over occurs during prophase I of meiosis before tetrads are aligned along the equator in metaphase I.

By meiosis II, only sister chromatids remain and homologous chromosomes have been moved to separate cells. Recall that the point of crossing over is to increase genetic diversity. If crossing over did not occur until sometime during meiosis II, sister chromatids, which are identical, would be exchanging alleles. Since these chromatids are identical, this swap of material would not actually change the alleles of the chromatids.

During crossing over, homologous chromosomes come together in order to form a tetrad. This close contact allows the nonsister chromatids from homolgous chromosomes to attach to one another and exchange nucleotide sequences. The word "nonsister" implies that the chromatids have the same genes, but are not exact copies of one another, as they come from separate chromosomes.

The crossing over of homologous chromosomes occurs in prophase I of meiosis. Prophase I of meiosis is characterized by the lining up of homologous chromosomes close together to form a structure known as a tetrad. A tetrad is composed of four chromatids.

Anaphase I is marked by the separation of homologous chromosomes, whereas in anaphase II there is the separation of sister chromatids. In anaphase I sister chromatids are still intact and connected at the centromere. Prophase II is similar to prophase in mitosis in that there is the break down of the nuclear membrane and the formation of spindle fibers in preparation for the separation of sister chromatids.

The recombination frequency depends upon the distance between the genes; a larger distance between the genes increases the probability of crossing over occurring.

During prophase I, homologous chromosomes pair with each other and exchange genetic material in a process called chromosomal crossover. The exchange occurs in segments over a small region of homology (similarity in sequence, ie., the same alleles). The new combinations of DNA created during crossover provide a significant source of genetic variation.

Crossing over occurs when chromosomal homologs exchange information during metaphase of Meiosis I. During this stage, homologous chromosomes line up on the metaphase plate and exchange genetic information.

Crossing over occurs during prophase I when parts of the homologous chromosomes overlap and switch their genes.