Principles of Inheritance and Variation

Introduction to Genetics

How is it possible that you look like your mother and have your father’s characteristics? What makes siblings look and perhaps behave similarly? Genetics and heredity!

Genetics is defined in many ways. It is defined as the study of genes, genetic code heredity, and variations. It is that field of biology that describes how characteristics and features pass on from the parents to their offsprings in each successive generation. The unit of heredity is known as genes. Before we understand more about genetics, we need to understand what the genetic materials are.

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Introduction to Genetics

There are two classes of genetic materials that are responsible for the transfer of information from one generation to another in animals:

  • DNA or deoxyribonucleic acid
  • RNA or ribonucleic acid

It is in the DNA or RNA sequences that biological information is stored and passed on.

DNA

Most organisms contain DNA except some viruses which contain RNA as their genetic material. DNA was discovered by two scientists- Watson and Crick and their model of the structure of DNA are called the Watson and Crick model. The structure of DNA is said to be a double- helical structure with two strands of DNA that are wound around each other. Each strand of DNA is made up of a sequence of nucleotide monomers. Each nucleotide is made up of:

  • Deoxyribose sugar
  • One of the four nitrogen bases
  • Phosphate group

The nitrogen bases found in a DNA molecule are Adenine, Thymine, Cytosine, and Guanine. The nucleotides create a chain via covalent bonds that are formed between the phosphate of one nucleotide and the sugar of the adjacent one. The two strands of DNA are held together by hydrogen bonds between complementary nitrogen bases i.e. Adenine with Thymine and Cytosine with Guanine.

A DNA molecule is said to be stable enough to be able to replicate itself. DNA replication involves RNA intermediates. The part of the DNA which codes for specific proteins during the replication is called as the gene.

RNA

Genetics

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Unlike the DNA, RNA is a single-stranded genetic material. The nucleotide bases present in RNA are similar to those in DNA except that thymine is replaced by uracil and pairs with adenine. While DNA is the genetic material in most organisms, RNA is found in a few viruses. RNA is of three types depending on their function:

  • tRNA or transfer RNA- helps transfer the amino acids from the mRNA to the ribosomes.
  • mRNA or messenger RNA- helps to carry the codes for amino acids from the DNA to the ribosomes
  • rRNA or ribosomal RNA- are found on the ribosomes and help in protein synthesis.

Video on Genetics

Introduction to Chromosome

Genetics

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Now that we know what the units of heredity are, let us understand what chromosomes are. The DNA that is found in the nucleus of each cell occurs as a tightly coiled package around proteins called as histones. These thread-like packaged structures of DNA are called as chromosomes.

Humans have 23 pairs of chromosomes(or 46 chromosomes). 22 pairs are called autosomes and one pair is called the sex chromosomes. Females in humans have 2 X(XX) chromosomes whereas males have one X and one Y (XY) chromosomes. The number of chromosomes varies in different animal species.

Each chromosome is said to be divided into two unequal halves by a centromere into two arms. The short arm is called as ‘p arm’ and the longer arm is known as the ‘q arm’.

Genetic Inheritance

Gregor Mendel, a monk, is known as the father of modern genetics. He postulated few laws, known as the Laws of Inheritance. Mendel’s Laws are:

  • Law of Segregation
  • The Law of Independent Assortment
  • Law of Dominance

With the help of experiments performed on a pea plant(Pisum sativum) and the use of Punnett squares, Mendel explained heredity and inheritance of characters and laid the foundation of genetic inheritance as we know it today.

Solved Example for You

Q1: What keeps the two strands of DNA linked to each other?

  1. Hydrogen bond   
  2. Covalent bond
  3. Ionic bond
  4. The bond between the sugar molecule and the phosphate molecule

Sol. The correct answer is the option ”a”. The hydrogen bonds between complementary bases in the two DNA strands keep it together.

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