Genetic Concepts & Terms

  • Heredity is the process of passing the traits and characteristics from parents to their offspring.
  • The offspring cells get their features, characteristics, and genetic information from their mother and father.
  • Heredity and Genetics are the reason organisms look so much like their parents.
  • Genetics is a branch of biology that studies the genes, genetic variation, and heredity in living organisms.
  • During the sexual reproduction process, genetic variation occurs due to some error in DNA Copying. Variation is important because it contributes to evolution and forms the basis of heredity.
  • Variation is caused due to positive gene mutations, the interaction of genes with the environment, and various combinations of genetic material.
  • Variation can occur through the asexual reproduction process too, but these variations are not very noticeable.

Gregor Johann Mendel (The Father of Genetics) :

  • Acquiring characteristics or traits from one generation to the other is nothing but inheritance.
  • Here, both parents contribute equally to the inheritance of traits.
  • It was Gregor Johann Mendel, known as the Father of Genetics, who conducted immense research on garden pea-Pisum sativum and studied this inheritance of traits.
  • Mendel was a priest in the Church of Brunn City (Austria).
  • It was through his research on plant breeding and hybridization that he came up with the laws of inheritance in living organisms.
  • Mendel observed the pattern of inheritance from one generation to the other in pea plants which can be summarized under the following headings-
    • Law of Dominance
    • Law of Segregation
    • Law of Independent Assortment

Genome :

  • A genome is an organism’s complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome – more than 3 billion DNA base pairs – is contained in all cells that have a nucleus.
  • Gene Mapping: Gene Mapping describes the methods used to identify the locus of a gene and the distance between genes. The essence of all genome mapping is to place a collection of molecular markers onto their respective positions on the genome.

Transposable Elements (TEs) :

  • Transposable elements or transposons, also known as jumping genes, are DNA sequences that move from one location on the genome to another. Sometimes TEs create or reverse mutations and alter the cell’s genetic identity and genome size. These elements were first identified by geneticist Barbara McClintock and for this discovery, she was awarded Nobel Prize in 1983.
  • The production of spotted seeds in maize is due to the jumping genes.
  • Almost half of our DNA sequences are made of jumping genes. They jump around the genome in developing sperm and egg cells and are important to evolution. However, their mobilization can also cause new mutations that lead to diseases such as hemophilia and cancer.

Nucleic Acid :

  • The term nucleic acid is the overall name for DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They are composed of nucleotides which are monomers made of three components: a 5-carbon sugar, a phosphate group, and a nitrogenous base.
  • Nitrogenous bases are of two types-
    1. Purine
    2. Pyrimidine
  • Purine bases are adenine and guanine, which are found in both DNA and RNA. Pyrimidine bases are cytosine, thymine, and uracil. Cytosine is found in DNA and RNA both, thymine is present in DNA and uracil is present in RNA.
  • The pentose sugar found in DNA is deoxyribose sugar and the pentose sugar found in RNA is ribose sugar.
  • The copying of DNA into RNA is called transcription. Reverse transcription is the process by which the DNA copy is formed from the RNA. In reverse transcription, a reverse transcriptase (RT) enzyme takes part.
  • It is noteworthy that DNA is the hereditary material in humans and almost all other organisms.
  • DNA was first identified and isolated by Friedrich Miescher in 1869.
  • Its ‘Double Helix Model’ was propounded by Watson and Crick in 1953.
  • DNA carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses.
  • It carries hereditary information from parents to offspring.

Human Chromosomes :

  • Human is a unisexual animal. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs are called autosomes in males and females alike. The 23rd pair, the sex chromosomes, differ between males and females.
  • In males, the chromosomes are 44 + XY, and in females, chromosomes are 44 + XX.
  • The offspring that gets the XY chromosome from the parents is male and the offspring that gets the XX chromosome from the parents is female.

Some Terms About Heredity

  •  Gene: It is the basic unit of inheritance. It consists of a sequence of DNA which is the genetic material. The genes
    can mutate and can take two or more alternative forms.
  • Chromosomes: These are thread-like structures of
    nucleic acids and proteins that are found in the nucleus
    of most living cells. They carry the heredity or genetic
    information in the form of genes. These are made up of
    nucleoproteins. German anatomist Waldeyer is known for
    naming the chromosome. The number of chromosomes
    varies from species to species but it remains constant for a
    particular species. In most animals, however, any change
    from the typical chromosome number for a species may
    be due to genetic disorders.
  • Genotype: It is the complete heritable genetic identity
    of an organism. It is the actual setup of alleles that are
    carried by the organisms.
  • Phenotype: It is the description of the actual physical
    characteristics of an organism, the way the genotype is
  • Alleles: The alternative forms of the genes which arise
    as a result of mutation. They are found in the same place
    on the chromosome and af ect some characteristics or
    traits but in alternative forms.
  • Dominant alleles: When an allele affects the phenotype
    of an organism, then it is a dominant allele. It is denoted
    by a capital letter. For example- ‘T’ to express tallness.
  • Recessive alleles: An allele that affects the genotype in
    the absence of the dominant allele is called a recessive allele. It is denoted by a small letter. For example – ‘t’ for dwarfness.
  • Homozygous: Each organism has two alleles for every
    gene (each chromosome has one each). If both the alleles
    are the same it is called homozygous. If tallness is the trait,
    it is expressed as ‘TT’.
  • Heterozygous: If the two alleles are different from each
    other, then they are heterozygous in nature. If tallness is
    the trait, then it is expressed as ‘Tt’.