Introduction

Someone might have told you that you resemble your dad, or your brown eyes are same as your brother’s, or maybe you have inherited the same dimples as that of your granny. These things are commonly said and heard. Ever wondered is it just a random thing happening or there is a specific reason for that? Even when we have the same parents two brothers or sisters might not resemble each other. Your parents might have blonde hair and you might have luscious dark locks or your sister might have dark brown eyes and you have hazel eyes. So if these are the questions that you have been thinking we have an answer for you. Before we go further we should know what is heredity?

What is heredity?

Heredity is the process due to which all biological characteristics are transmitted from parents to offspring. It is that mysterious code that decides our biological makeup which includes all the processes from basic functionality of cells to the colour of your eyes and texture of your hair. Heredity is not just passage of characteristics from one generation to another rather it is a complex phenomenon that includes the constant genetic makeup of species from one generation to another and variation among individual traits . Constancy and variation are the key note mechanism of heredity and they work hand in hand. To understand the basic mechanism of heredity we should also know what are genes and how is information transmitted from one generation to another.

How is information transmitted from one generation to another?

All of us know that cell is the basic unit of life. Cells carry the genetic information from one generation to another. Genetic information is contained in nucleus of the cell DNA (de-oxy ribose nucleic acid). DNA is the genetic carrier which stores and transmits the genetic information from one generation to another. Genes are subunits of DNA that carry the hereditary information. The information is transmitted from one generation to another and as genes are inherited from both the parents when sperm and ova meet. The genetic information is carried in form of genotype and phenotype. Genotype is the genetic makeup of the cell and the way the physical traits are carried from one generation to another or the way the traits are expressed physically is the phenotype of the cell.

Genotype is the genetic makeup of the cell and determines all the basic functionality of the organism, the way the organs function and the way the normal physiological processes function also depends on the genetic makeup of the cell. Even the lesser known phenomenon such as ageing and some diseases like Alzheimer’s also depend on the genetic makeup of your cells.

Phenotype is the physical manifestation of the genetic traits. Physical traits though have a genetic predisposition but environmental factors also play an important role in expression of physical characteristics.

Since all of us have evolved from the same the evolutionary process all the species have a major similarity in their genetic make-up yet there are many things that make the DNA a unique identifying feature of every individual. There are similarities in the DNA of identical monozygotic twins but there are differences between the genetic structures of siblings from the same parents. Sometimes we observe a lot of variation in the looks of siblings. The colour of your eyes might be black and your sister might have beautiful hazel eyes. So when we know that we get the same genes from our parents then why is there a lot of disparity in the physical expression of genes. Let’s know it in more detail.

How can environment affect the expression of genes?

In spite of the fact that we acquire genes from the same parents we differ from our siblings in many aspects. This variation in the physical expression of genetic traits can be attributed to the environmental factors affecting the expression of genes. The environmental factors depend on the surroundings where an organism lives, the hormonal factors and the metabolic factors.

For many diseases or traits to be expressed the major requirement is the “X” or “Y” chromosome which determines the sex of the individual. So the major internal environment that regulates the expression of certain sex-linked genes and traits is gender. Likewise certain drugs, chemicals, temperature and light are also seen to have effect on the expression of gene and can be considered as the external environmental factors regulating the expression of genes in individuals.

The best example for the expression of traits and effect of environmental factors is best understood by patterned inherited baldness. Although this gene is not carried on the sexual chromosome but still its expression depends on the presence of hormones testosterone and dihydrotestosterone in high levels. These hormones are generally present in higher levels in males and thus gene expression is stronger in males. But this gene also gets functional in females as during times of high stress testosterone is synthesized in females, later converted to dihydrotestosterone. This results in the same way causing hair loss in females.

Another example for expression of sex-limited trait is lactation in females. Although gene for producing milk is present in both males and females but expression of the trait that is lactation occurs only in nursing females.

Another major factor that is responsible for variation in expression of genes is pleiotropism. Pleiotropism is the mechanism due to which a single gene can affect multiple phenotypic traits in an individual. In the same way there are many physical traits that work in a way opposite to pleiotropism and many genes interact to express as one physical trait. The examples to it are skin colour, height and colour of eyes. There is a wide range of permutation and combination that gives a way to expression of wide range of physical characteristics.

However, it is important to keep in mind that there is a very complex interaction between our genes and our environment that defines our phenotype and who we are. So now after knowing so many things we certainly do have answers to many of our queries and why are we so different but in the end same with one another.