Principles of Inheritance & Variation Test 7

Mendel had published his work back in 1865. The communication methods were not well developed and for people to recognize the importance of his work took more than half a decade after it had been published.

Mendel’s laws of inheritance were based on the assumptions of factors/genes being discrete. However, most of the changes observed in nature exhibited the blending of characters. This led to the unacceptance of Mendel’s studies.

The variation observed in nature is mostly continuous. This arises from the complex genetic interactions which Mendel failed to observed in his experiments on pea plants.

People had performed crosses on garden pea plants before Mendel. But no one had applied a statistical and probabilistic approach to unveil the underlying phenomena. Mendel was the first to do so.

Mendel’s work suggested the existence of stable and discrete factors. This was completely shocking for the contemporary scientific world. A major drawback that lied was the inability to discover the factors and prove their existence.

Mendel’s work was laid in papers before people could recognize it after 1900. This was achieved by the rediscovery of his work by three scientists: de Vries, Correns, and Tschermark.

Microscopy came into the picture around the 1900s. This allowed scientists to visualize cells and processes like cell division.

Chromosomes means colored bodies/objects. The staining procedures allowed biologists to visualize the nucleus and chromosomes during the process of cell division.

Sutton and Boveri, along with other scientists, had uncovered the complete movement of chromosomes during the process of cell division. They observed that these movements correlate with Mendel’s findings.

Theodore Boveri and Walter Sutton had unraveled the mechanisms underlying the chromosome movement during cell division. These movements were correlated with Mende’s laws.

These three scientists helped in the rediscovery of Mendel’s work in the 1900s. Mendel’s work was published in 1865 and long forgotten.

The chromosome is composed of many genes. Hence the behavior of gene was similar to the observed movement of the chromosome during the process of cell division.

Mitosis results in the equal division of the chromosomal content of the cell. Hence it is also called equational division. The mitosis results in the production of two daughter cells from one parent cell with the exact chromosomal content as in the parent cell. This is ensured by a duplication event in the synthesis phase prior to the mitosis phase.

During the process of meiosis, each daughter cell gets only half the original chromosomal content. Hence it is called the reduction division.

A diploid cell has double the set of chromosomal content. Hence there are two copies for each chromosome present. These are called homologous chromosomes. Now, genes are fragments of DNA present on the chromosomes. Hence they also occur in pairs, with one gene present on each homologous chromosomes.

The alleles of a gene compete or support each other for expression. Hence, they have to be present at homologous sites and on homologous chromosomes.

In the eukaryotic cell cycle, chromosome duplication occurs during "S phase" (the phase of DNA synthesis) and chromosome segregation occurs during "M phase" (the mitosis phase).

The segregation of chromosomes occurs when the spindle fibers pull the fibers pull the chromosomes to the poles and away from the metaphase plate. The first such segregation occurs during meiosis I Anaphase.

Meiosis takes place in two phases and involves the separation of chromosomes twice. The second segregation occurs in the Anaphase of Meiosis II.

Genes occur in pairs. At the start of meiosis, there are four copies of each gene. They are segregated first during Anaphase I, where each pole receives a pair of each gene.