Evidence of Inheritance – Phases & Significance

Evidence of Inheritance

Introduction

Inheritance: Living organisms reproduce to give rise to their next generation. When these organisms reproduce, they transfer their characters (in the form of DNA) to their offspring. This transfer of characters is known as inheritance. 

Explanation

As we have studied, parents transfer their characters to their offspring; these characters can range from hair colour, eye color, skin color, some disease, etc. The characters are present in the form of genes that are present on the chromosomes. 

Gene is the structural and functional unit of heredity. Our DNA consists of gene codes that encode for our appearance, specific characters, and various functions in our body. These genetic codes are sequences of nucleotides that are arranged in a particular manner. These genetic codes are present at specific positions on the chromosome, which in turn is present inside the nucleus of a cell. A typical chromosome can have thousands of genes. Different gene codes contribute to different characters and functions. 

When we look around ourselves, we can see that all individuals look different from each other. The most apparent differences are in height or hair type. We also show differences in terms of intelligence, blood groups and other inherited characters. These differences are due to variation. Every organism has a different set of genes that is specific for that organism. This leads to genetic variation. Variation can be defined as the degree to which the offsprings differ from their parents. Two offspring from the same parents do not look alike as a result of the variation. 

Several factors cause variations in living organisms. These factors include: 

• New genetic combinations by meiosis 
• Errors occurring during replication 
• Mutations caused by environmental factors 

New genetic combinations are created at the time of cell division during meiosis. As we know, replication is a controlled process, but it is not fool-proof; as a result, errors are bound to happen. These errors lead to mutations that can change the genetic codes. Environmental factors also cause mutations. 

Genetic Recombination:  

It is an important mechanism that introduces variation in a population. The process of recombination happens during meiosis when the genes from both the parents regroup to form gametes. It is a commonly occurring event in meiosis. 

During meiosis, crossing over of homologous chromosomes takes place, which leads to this genetic recombination. 

As we know, organisms reproduce by sexual and asexual methods. In asexual reproduction, cell division occurs by mitosis, whereas in sexual reproduction, gametes are formed by the process of meiosis. 

Every living organism has a fixed number of chromosomes. For example, humans have 46 chromosomes. When the organism reproduces, it only passes on half the number of chromosomes. Meiosis is responsible for the production of gametes containing only half the number of chromosomes as the parent’s body cell. 

Meiosis: 

It is also called reductional division. This is because we have diploid cells in our body, but during the formation of gametes, these diploid cells give rise to haploid gametes (n). In a diploid cell, the two chromosomes of each pair are called homologous chromosomes, and each chromosome of this pair has genes for the same traits in the same order. 

Meiosis consists of two separate divisions. The first division is meiosis I, and the second is meiosis II. Before the start of meiosis, the chromosomes in a cell are copied. Each chromosome consists of two sister chromatids which are connected by a centromere. 

Phases of Meiosis:  

Meiosis I begins after DNA replicates during the interphase of the cell cycle. In both meiosis I and meiosis II, cells go through the same four phases as mitosis – prophase, metaphase, anaphase and telophase. However, there are some differences in meiosis I and meiosis II. The phenomenon of crossing over occurs in meiosis I but is absent in Meiosis II.  

Meiosis I

Prophase I: the nuclear envelope starts to disintegrate, and the chromosomes condense. Centrioles begin to move away, and a spindle begins to form. The homologous chromosomes pair up and create a tetrad. Crossing-over occurs during this phase. 

Metaphase I: Spindle fibers attach to the paired homologous chromosomes. Tetrads align in the center. 

Anaphase I: Spindle fibers become short, and the chromosomes of each homologous pair start separating from each other. 

Telophase I and Cytokinesis: The spindle fibers break, and new nuclear membranes form. The cytoplasm of the cell divides, and two haploid daughter cells are formed. Both of which go on to meiosis II.  DNA does not replicate between meiosis I and meiosis II. 

Meiosis II 

Prophase II: The nuclear envelope breaks down, and spindle formation takes place in both haploid daughter cells from meiosis I. The centrioles start to separate. 

Metaphase II: Spindle fibers attach to the sister chromatids of each chromosome along the cell’s equator. 

Anaphase II: Spindle fibers contract leading to the separation of sister chromatids. Sister chromatids move to the opposite poles. 

Telophase II and Cytokinesis: The spindle fibers break, and new nuclear membranes form. The cytoplasm of the two cells divides and form four haploid cells. As a result, all the cells have a unique combination of chromosomes. 

Crossing over: 

Crossing over takes place during prophase I of meiosis. 

During crossing-over, homologous chromosomes are aligned at the center, and the chromatids of the homologous chromosomes come very close to each other. The homologous chromosomes pair so tightly that sometimes, a piece of chromatid may break and change place with a piece on the adjacent chromatid from the other homologous chromosome. 

This exchange is known as crossing over. It can occur at several places at the same time. This process enables the exchange of genetic material between maternal and paternal chromosomes. Thus, genetic recombination is seen because a new mixture of genetic material is produced. 

Significance of crossing over:  

Crossing over helps exchange the genetic material between two non-sister chromatids of homologous chromosomes. This exchange leads to the formation of a new genetic combination with each cell division. 

Genetic recombination leads to the formation of new and unique offspring every time the organism reproduces. As a result, tremendous genetic variation is seen among the population of that particular species. 

For example: in humans, no two siblings look alike unless they are twins. 

Crossing over can be described as a potential genetic mechanism for creating variability, which is essential for improving genotypes through selection. 

Summary

• Living organisms reproduce to form their offspring.
• Reproduction takes place by sexual and asexual means. When organisms reproduce, they transmit their characters to their offspring.
• This transmission of characters is known as inheritance.
  Specific gene codes give rise to different characters seen in an organism.
• Our DNA is made up of gene codes and is present in the chromosomes, which are present
  inside the nucleus of a cell.
• Gene is the structural and functional unit of heredity.
• Variation is the degree to which offsprings differ from their parents.
• Variation is caused by new genetic combinations through meiosis, errors that take place
  during replication and mutations.
• Genetic recombination occurs during the process of meiosis and is responsible for the
  genetic diversity that we see between that same species.
• Meiosis is a process of cell division that divides a single cell two times to give four haploid
  daughter cells.
• Meiosis happens in two stages, meiosis I and II. Both the stages include prophase,
  metaphase, anaphase, telophase and cytokinesis.
• Crossing over takes place during prophase I.
• In crossing over, non-sister chromatids of homologous chromosomes exchange genetic
  material. This exchange leads to variation.