Features passed down from one generation to the other is Heredity or Inheritance. Genes control heredity. Differences in features of individuals of the same species is variation.

An Austrian monk, Gregor Johann Mendel experimented with peas to find out how the various features get inherited. He postulated laws of inheritance.

Mendel’s Laws: Mendel explained heredity and variation through laws called Mendel’s laws of inheritance.

  1. Law of segregation of characters: Every feature is controlled by a pair of genes (factors) which segregate during gamete formation and go to different cells.
  2. Law of Dominance: A gene of a pair which may express even in the presence of the other is called dominant. The other gene termed recessive expresses only when two recessive genes are present, one each received from either parent. The different forms of a gene are termed alleles.

Chromosomes and Genes: Sutton (1902) saw thread like structures in the dividing cells of grasshopper’s testis. These were the chromosomes. Soon it was evident that genes are present on chromosomes. chromosomes seen only in dividing cells. Jumbled up as chromatin network in the nucleus of non-dividing cells.

Number of chromosomes are fixed in a species, present in homologous pairs (both chromosomes of a pair bearing same genes, but not necessarily same alleles). Hence, the fixed member is termed diploid and designated as 2n (n = haploid). In humans 2n = 46. Of these 44 are termed autosomes and X and Y as sex chromosomes. Male has a long X and a short Y chromosome and female has two X chromosomes.

A chromosome is made of one molecule of DNA or Deoxyribonucleic acid and proteins. Segments of this DNA molecule are genes. DNA is a polynucleotide made of many units of deoxyribonucleotides. Each deoxyribonucleotide has

  • A nitrogenous base
  • A deoxyribose sugar
  • A phosphate

The nitrogenous bases are Adenine A),Guanine (G), Thymine (T), Cytosine (C). A DNA molecule is made of two strands of DNA helically coiled around each other.

DNA Replication: For genes to be inherited, DNA needs copies of itself. This is called DNA replication. The steps are aided by enzymes, DNA unwinds into its two strands. Upon DNA replication, two identical molecules of DNA are formed. These are termed chromatids and remain joined by a centromere.