Seymour's Concept of Modern Gene

Genes is a vast topic to cover in a short period if you're a newbie (duh!). Well, genes not only resembles a unit of heredity but they also play important roles in our body in order to keep us going all day.

Modern concept of gene:

Seymour introduced terms like CistronMuton and Recon in biology. These terms are not as hard as they look like but are pretty easy to understand.


The Cistron is a unit of function. It is also the alternative term for gene but only if cris-trans test is performed. This term was coined in the article "The elementary units of heredity" by Seymour Benzer.

Not like Cistron can be used in place of the word gene everywhere but this word should only be used if the cis-trans test is performed. Cistron is not always alternative for the word gene

All these terms used in this particular topic have been given a responsibility of and cistron is in charge of expression of a trait, meaning the traits of a person which can be observed phenotypically (external features like ears, nose, face, etc).

Cistron contain numbers of muton and recons as cistron is the of function.


Muton is a unit of mutation which a segment of a DNA is prone to undergo mutation. It is the smallest element present in the gene.

Muton sometimes cannot be noticed because of the recombination test, so how can we know mutations have taken place ? Well the answer to this is simple, mutons show change in the sequence of amino acid of the polypeptide chain hence, they are noticed. 

Muton is within cistron and mutons are in recons (yes! they are all linked together). Mution is now known as single nucleotide pair.

Mutations can also be caused if there is an error in replication of a DNA sequence.

Types of mutations:

Mutation is alteration or deletion in a sequence but that's not it, there are 3 more types of mutations that take place.

1. Base substitution:

This can also be called as point mutation where only one part of the nucleotide base is affected and not the whole sequence. They are the most common type of mutation. 

Point mutation
Point mutation

Point mutations are of two types
  • Transition mutation - Here, either the purine nucleotide will change to another purine and the same with pyrimidine nucleotide. Two out of three single nucleotide polymorphisms (SNP's) are a result of transition mutation.
  • Transversion mutation - In transition the purine changes to a purine and so with pyrimidine but in transversion the purine can replace a pyrimidine nucleotide and vise versa.

Transition and transverse mutations
Transition and transverse mutations
Point mutation encodes DNA sequence and these again are of three types:
  • Silent mutation - These do not cause much of a change in the sequence of nucleotide nor phenotypically. These mutations mostly go unnoticed.
Silent mutation
Silent mutation

  • Nonsense mutation
    - This mutation results in the stop codon even before mutation. This shortens the protein product which makes it nonfunctional.
Nonsense mutation
Nonsense mutation

      • Missense mutation - In this mutation a single nucleotide shows a codon which codes for a different amino acid.
      Missense mutation
      Missense mutation
      2. Deletion mutation:

      When a DNA sequence is lost during the replication of DNA results in deletion mutation. There is no specific number of nucleotides which can be deleted, it can even delete a single base to a whole chromosome.

      Deletion can be caused by error in the process of chromosomal crossing over which takes please during meiosis. This can result in some serious genetic diseases like chromosomal deletion syndromedown's syndromeIPEX syndrome, etc.

      Deletion mutation
      Deletion mutation
      3. Insertion mutation:

      Insertion is to add new nucleotide base pairs into DNA segment. It can be one or more than one nucleotide base pairs.

      Insertion is somewhat same like deletion. In deletion either a whole chromosome is deleted or a single base pair, same is with insertion, either a whole chromosome is added or a single base pair.

      It is sometimes also referred to frameshift mutation because it shifts the frame where codon is read.

      Insertion mutation
      Insertion mutation


      This is the unit of recombination. A recon is formed when segment of DNA takes part in recombination which occurs whilst the process of crossing over during meiosis. 

      The base pairs in recon are a few to many of them. It is also the smallest unit of gene to recombine. At first, it was known that recombination can occur only between the genes but after advancing in biology, it was found that it can not only occur between the genes but also within the genes. 

      What is a gene ?

      Just like you all know gene is a unit of heredity which is transferred from our parents to us. It has various functions and mainly helps in protein synthesis. It also is a part of DNA.

      How is gene a part of DNA ?

      DNA is a long double helix strand formed by base pairs attached to a sugar phosphate chain. Genes are present in these double helix strands (DNA). Genes together form chromosomes.


      Every sexually reproducing organism contains DNA in them which makes them look different but have different DNA from each other except their own family.

      What are they made of ?

      Genes are the carriers of A (adenine), C (cytosine), G (guanine) and T (thymine). These nucleotide bases can form many combinations which makes a person unique in himself/herself. 

      Ever wondered why one person has brown eyes or blue eyes ? It's because of these nucleotide bases which make various combination in itself. Eg, combination of ATCGTT can make you have blue eyes but if this combination is different like ATCGCT the person can have brown eyes

      Nucleotide combination in a gene
      Nucleotide combination in a gene

      Mutational sites: 

      Mutational sites are where mutations occur especially during the time of meiosis. Mutations are alteration or deletion of a nucleotide sequence in the gene of an organism permanently. 

      Mutation are also associated with in-born diseases. They may or mat not be transferred from your parents to you. Sometimes these mutations can occur in physical ways as well, such as, radiations of the UV or X-rays, extreme heat or chemically

      Chemically, when molecules disturb the helical structure of DNA it can cause mutations. Not like mutations cannot be repaired, they can but sometimes when they fail to repair that can be harmful as it might stop the body from functioning completely or partially

      Who introduced the modern concept of gene ?

      Seymour Benzer was the one to come up with the concept of modern gene. He made it easy to  understand the concept of gene than the classical and neoclassical period of genetics.

      He studied gene with a fruit fly named Drosophila and discovered a gene called "Methuselah".

      Cis-trans test:

      Cis trans test or also known as complementation test it is a to find out if the genes present are different and when mated or crossed the mutant shows normal appearance in the offspring once born.

      We all have seen drosophila which is a common fruit fly and it has red eyes cause a fly with white eyes would look weird and most importantly it would be rare. To get this red pigment in the eyes the genes have to complement each other and hence complementation takes place.

      Complementation is where it's necessary for two genes to complement each other in order to get the natural (wild-type) phenotype in an offspring. The "A" and "a" show dominant trait on the "B" allele and "a" and "a" show recessive trait on "b" allele.

      In order for the red pigment to form there are two gene "A" and "B", the A gene codes for white colour and the B gene codes for red. Now, if the genes go in the combination like aaBB or AAbb you will get only white pigment cause there is no complementation taking place. We need the genes to produce red colour and not white. So, what combination is required for them to complement each other so that we can get red pigment ?

      It's simple we need AaBb type of combination. Here, A and B both are complementing each other cause mutation and is dominant and most importantly it in is heterozygous condition. In homozygous condition the combinations would be AAbb or aaBB.

      Other works of Seymour Benzer:

      Benzer is the pioneer for the world of genetics. He made genes so easy to understand for us humans. He was a physicist, molecular biologist and behavioural geneticist. He did not only introduce the modern concept of gene but he also gave a detailed map of the structure of the gene and discovered a new gene "Methuselah" in a common fruit fly.

      Methuselah is a type of mutation which when occurred has chances of extending your life span by 30-40%. This gene expresses the IGF1 which means insulin-like growth factor 1. IGF1 has anabolic effects on adults and also helps in the growth of a child.

      Anabolic effects increases our body functions and makes us fit hence this mutation is gain of function of IGF1. This mutation is responsible for growth and differentiation of body mass. Most of the growth factor is secreted by liver and simulated by GH (growth hormone).


      As we come to the conclusion, so far we came to know about what a gene is and most importantly Seymour's modern concept of gene.

      Genes not only play a role of transferring from our parents to us but also helps us in giving a unique look from others hence they are called as unit of inheritance and it's not just genes that give us a unique look mutations too play a role in that.

      Seymour made it easy to understand genes by first experimenting with drosophila which is a common fruit fly. He worked on the genes which were similar to us humans as well and whilst he was researching about genes he crossed paths with Methuselah the mutation which extended the lifespan of a mice by 30-40%.

      The word cistron can only be used if cis trans (complementation) test is done. In cis trans test the genes on 2 strains to complement each other to get the wild type offspring which means that it is in it's normal appearance in the nature.


      What are base pairs ?
      Ans: Base pairs are two nucleobases which are the building blocks for DNA sequence. These are bound to each other by hydrogen bond.

      How many base pairs (bp) are there in a gene ?
      Ans: We cannot accurately tell how many base pairs are there but these can range from 27,000 to 2 million base pairs.

      What are chromosomes ?
      Ans: Chromosomes are a thread like structure present in every living organism. It contains our genetic information and genes.


      Where are chromosomes located ?
      Ans: Chromosomes are located inside the nucleus of animal or plant cell.

       Nucleus of animal or plant cell
      Nucleus of animal or plant cell

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