What are Genes and Chromosomes?

In short

A gene is a section of DNA that carries the instruction for one protein, and so for one inherited feature. Genes are arranged in a line along thread-like chromosomes.

In simple words

Chromosomes come in pairs, one from each parent. Human cells have $46$ chromosomes in $23$ pairs. A different form of the same gene is an allele. If an organism has two different alleles, the dominant one shows and the recessive one is hidden. Genes make proteins, and the proteins decide features such as blood group or flower colour.

Example

In pea plants, the allele for tall ( $T$ ) is dominant over short ( $t$ ). A plant $Tt$ is tall. Crossing two $Tt$ plants gives tall and short offspring in the ratio $3 : 1$ .