Author: shayanghani

Natural Selection


In the mid 19th century, Charles Darwin (1809 – 1882), formulated the scientific theory of evolution by natural selection, published in his book “On the origin of species” in 1859.

Darwin’s idea were inspired by the observations that he had made during a sea voyage in a sail ship called H.M.S Beagle round the world, from 1831 to 1836.

Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution which involves the change in heritable traits of a population over time.

The concept of fitness is central to natural selection i.e., individuals that are more “fit” have better potentials for survival. Herbert Spencer (1820 –  1903) coined the phrase “survival of the fittest”, in his book “Principles of biology” in 1864, after reading Darwin’s book “On the origin of species”.

Alfred Russel Wallace (1823 – 1913) best known for independently conceiving the theory of evolution through natural selection by working in Malay Archepelago. The concept of natural selection was published by Darwin and Wallace in a “Joint presentation of papers” (1858).


Five basic concepts of Darwinism

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Chromosomal Mutations

The process that produces an alteration in DNA or chromosome’s structure or number is known as mutation.

The term ‘mutation’ was coined by Hugo Marie de Vries in 1901.

Any agent that cause mutation or increase the rate of mutation is known as mutagen.

Chromosomal Mutation

Chromosomal mutation is a missing, extra, or irregular portion of chromosome. It is also known as chromosomal anomaly, chromosomal abnormality, chromosomal aberration or chromosomal disorder.

It can be classified into following three categories:

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  • A chromosome abnormality or a chromosomal disorder is a missing or irregular or extra portion of chromosomal DNA.
  • usually occur when there is an error in cell division following meiosis or mitosis.
  • are generally not inherited but occur as random events during the formation of reproductive cell.
  • an error in cell division called non-disjunction results in the reproductive cells with an abnormal number of chromosome.
  • Non-disjunction – the failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during anaphase (meiosis), usually resulting in  abnormal distribution of chromosomes in the daughter nuclei.
  • examples of chromosomal disorders = Down syndrome, Edwards syndrome, Patau syndrome, Cri du Chat syndrome, Turner syndrome etc
Fig: Non-disjunction (wikipedia)

Down Syndrome:

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  • The study and analysis of inheritance as a result of monohybrid crosses is called one-gene inheritance.
  • The cross between true-breeding pea plants with tall stems and dwarf stems is represent Mendel’s monohybrid crosses.
  • When Mendel crossed true breeding tall plants with dwarf plants, the resulting F1 generation consisted of only tall plants.
  • When members of the F1 generation were selfed cross, Mendel observed that 787 0f 1064 F2 plants were tall while 277 of 1064 were dwarf – a ratio of about 3:1.
  • Mendel made similar crosses between pea plants, exhibiting each of the other pairs of contrasting traits. In every case, the outcome was similar to the tall/dwarf cross.
  • Mendel proposed the existence of what he called ‘unit factors’ for each trait. He suggested that these factors serve as the basic unit of heredity and are passed unchanged from generation to generation.
  • Using the results of monohybrid crosses, Mendel derived the following three postulates, also known as ‘Principles of Inheritance’:

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Liver: At a glance


  • It is the heaviest internal organ and largest gland in the body, weighing about 1.2 – 1.5 kg in an adult human.
  • It is located in the upper right side of the abdominal cavity and is reddish brown in colour.
  • It has two main lobes (larger right lobe & smaller left lobe) and two small lobes (quadrate lobe & caudate lobe) present behind the main lobes.
  • The two main right and left lobes are separated by the falciform ligament.
  • The liver consists of small structural and functional units called hepatic lobules.
  • The lobules are roughly hexagonal, and consist millions of glycogen-rich cells, the hepatocytes (hepatic cells).
  • Each lobule is covered by a thin connective tissue sheath known as Glisson’s capsule.
  • The mammalian liver also contains phagocytic cells known as Kupffer cells. They engulf dead WBCs, RBCs and pathogens.
  • A thin-walled, sac like structure, the gall bladder, lies in the lower surface of the right lobe. It stores and concentrates bile secreted by the liver cells.
  • The duct of gall bladder is called cystic duct.
  • The cystic duct is connected with common hepatic duct to form bile duct which passes downward and is joined by the pancreatic duct to form hepatopancreatic duct.
  • The hepatopancreatic duct is connected with duodenum through a hepatopancreatic ampulla which is guarded by a sphincter called the sphincter of Oddi.
  • There are two distinct sources that supply blood to the liver: oxygenated blood flows in, from the hepatic artery and nutrient-rich (deoxygenated) blood flows in, from the hepatic portal vein.


Fig: Liver and pancreas with their ducts.

Vital Functions

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Histology of Human Gut

The wall of the human alimentary canal consists of four distinct layers: the mucosa, sub-mucosa, muscularis and serosa.


Fig: Transverse section of gut (diagrammatic representation)

Mucosa The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen (open space within the tube). This layer comes in direct contact with digested food. This layer forms rugae in the stomach and villi in the small intestine. The mucosa is made up of epithelium – innermost layer, responsible for most digestive, absorptive and secretory processes. Mucosa also contains goblet cells which produces mucus that protects the epithelial surface.

Sub-mucosa The sub-mucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymph vessels and nerves. In duodenum, glands are also present in it. Sub-mucosa supports the mucosa.

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