Gene
A chromosome is made up of large molecule of deoxyribonucleic acid (DNA). A small section of DNA is called gene. Genes are functional units of heredity as they are made of DNA. Therefore, a large DNA molecule that makes up a chromosome contains thousands of genes. Every gene comprises of set of instruction for a particular function called genetic code. This is passed from the parent to offspring. Scientists estimate that human have as many as 25000 genes. Genes exist in more than one form. These alternate forms are called alleles. Alleles determine distinct traits that can passed on from parents to offspring. The human cell contains 23 pairs of chromosomes. The trait is determined by one gene or more genes. Abnormal genes and genes that are formed due to new mutations. Genes come in pairs in the same way as the chromosomes. Each parent of human being carries two copies of their genes, and each parent passes one copy of genes to their child. This is the reason why the child has many characteristics of both the parents like hair colour, same eyes etc. The said genes are first transcribed into mRNA and then get converted into a polypeptide chain. A polypeptide is then converted to a protein. All the hidden code inside our genes emerged as our physical traits, which are known as gene expression.