Genetics and heredity

Introduction to genetics and genes

Genetics is a branch of biology help in understanding the way the characteristics (traits) are passed from parents to their offspring. The tendency, percentage, frequency and the pattern of how these traits are transferred from parents to children is generally called heredity. Genetic disorders are becoming a challenge to the doctors and researchers as there is no complete cure for all the conditions nevertheless, genetic therapies mask the negative impact of faulty genes.  The major components of genetics are DNA, genes, genomes, and chromosomes. This article is the elaboration of the terminologies, definitions and the basic concepts of genetics.


Structure of DNA

DNA stands for deoxyribonucleic acid –a type of nucleic acid-containing peptide bonds and bases (purines and pyrimidines). The whole idea of genetics is built around DNA molecules. DNAs provide a special place for the genes to be situated in a precise order and style. Some bases form a sequence of genes. The bases are of 2 types, namely purines (Adenine, Guanine, Thymine, and cytosine) and pyrimidines (Adenine, Guanine, Thymine, Cytosine, and Uracil). A gene is an orderly arranged sequence of DNA, in simple words many genes can make up a molecule of DNA. To transmit the traits from one generation to another, the DNA must replicate in a normal way. 




A gene is a localized segment of a DNA molecule containing genetic information for one specific protein. DNA works on a language called genetic code that defines behaviour or a trait in the form of specific codons (information). Genetic code is formed by the rearrangement of purines (Adenine, Guanine, Thymine, and cytosine) and pyrimidines (Adenine, Guanine, Thymine, Cytosine, and Uracil).





A chromosome is a bunch of tightly packed strands of DNA.  The packing of strands of DNA is aided by the histone proteins to form long strands-like appearance called ‘chromatids’. When 2 chromatids join together then it is called a chromosome. Different organisms have different numbers of chromosome . Normal humans have 46 chromosomes found in 23 pairs, however, there are chances that chromosomes can vary in numbers. There are different types of chromosomes based on the location of centromere in the chromatids as shown in the image below. Based on the placement of centromere in a chromosome, the chromosomes can be metacentric, sub-metacentric, telocentric and acrocentric as shown in the image below.




Genetic traits 

These are the combination of visible and invisible behavioural components of a human being that is formed by the genetic tendency. Some of the very common traits are height, the colour of eyes, colour of hair, physical appearance, intelligence, etc.


Common definitions and terminologies used in genetics

  1. Genetics: It is the study of heredity and the variation of inherited characteristics through generations. The generations are identified by G1( first generationG2( second generation), G3( third generation), etc.

  2. Heredity: The phenomenon of passing on of physical or mental characteristics from one generation to another and the process is mediated by genes.

  3. Phenotype: It is the observable expression of the genetic makeup of an individual. The phenotype may be expressed morphologically or biochemically and may be altered by the environment.

  4. Genotype: It is the genetic makeup of an individual, which may or may not be expressed outside depending upon the dominance.

  5. Gene: It is the basic unit of inheritance responsible for transferring traits from generation to generation.

  6. Allele: Allele is a variation in the form of a single gene. Alteration can be structural, functional,  and may be unrecognizable phenotypically.

  7. Locus: the location on the material of inheritance (chromosomes); the location of the allele, explaining some of its transference.

  8. Autosomal: Referring to the chromosomes that do not impart sexual characteristics.It refers to explain the 22 somatic pair of chromosomes excluding the last pair  in humans.

  9. Sex-linked inheritance: Inheritance because of a recessive gene found in sex chromosomes (the X chromosome found in the XY chromosomes set in humans and drosophila) is called sex-linked inheritance. This is primarily a trait observed in males.  Generally, colour blindness occurs in a male child born to a carrier X′X female and a normal XY male.

  10. Heterozygote: An individual having two different alleles of a particular gene or genes, and so giving rise to varying offspring.

  11. Homozygote: An individual with two of the same alleles responsible for the phenotype.

  12. Pedigree: The formal report of a family tree with distinctions made between 2 or more generations of individuals indicating the percentage of transfer of traits from parent to offspring. They also indicate the nature of illness, age of the descendants, consegeanity etc.

  13. Crossing over: It occurs during meiosis where the homologous pairs of chromosomes may exchange genetic material into sister chromatids leading to new chromosome alignments.

  14. Syndrome: a group of signs, symptoms, and anatomical findings that occur together. A syndrome can cause a particular genetic abnormality.



Read more


Introduction to genetics and heredity


Gene mutation


Human genome project


Disorders and defects of chromosomes    


Mendelian inheritance




  1.  Explain the double-helical model of DNA 

  2. What is a genotype and how it is different from phenotype?

  3. Distinguish between homozygous and heterozygous chromosomes.

  4. What are the types of chromosomes based on the position of the centromere, explain?


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