DNA is the genetic material i.e contains the instruction to direct all function of cells it perform its rule by given instructions for the synthesis of specific proteins some proteins perform structural role wile the other acts as enzyme to control all biochemical reaction of cell in this way what ever a cell does is actually controlled by its DNA in other words DNA makes the characteristics or trait of cell or organisms let us see how DNA is responsible for this

we studied that traits are made by specific proteins specific proteins have specific number and sequence of their amino acids DNA controls this sequence of amino acids by the sequence  of its nucleotide during protein synthesis the sequence of DNA nucleotide decides that what will be the sequence of amino acids for this purpose the specific sequence of DNA nucleotide is copied in the form of messenger RNA(mRNA)nucleotide this process is called transcription .the mRNA carries the sequence of its nucleotide to ribosome the ribosome reads this sequence and join specific of its amino acid according to it to form protein this steps is know as translation The segment of DNA (sequence of nucleotide) that contains the instructions for the synthesis of a particular protein is known as a gene.The DNA of each chromosome contains thousands of genes. Positions located in a pair on a homologous chromosome Genes on a chromosome are known as loci (single locus).each gene determine a particular trait in an organisms each individual carries at last one pair of gene of each trait for convenience pair of gene are represented by a latter or symbol both member of a gene pair may be the same in some individual (a condition which we may represented as AA or aa or BB)and difference in other (Aa or Bb)it means that a gene exists in more than one alternate forms in the above example ,`A' and `a' are the two alternate forms of a gene and `B'and `b' are the alternate form of other gene the alternate form of a gene are called alleles . if an individual has Aa gene pair `A' and `a' are the alleles of one another in this individual alleles `A' is located on one of the two homologous chromosome and the allele  `a' is on the other chromosome shown in figer  
                                                                                                                                                                                                                                  when chromosomes sprat during meiosis , alleles also sprat and each gamete gets one of two alleles also sprat and each amalgamate gets one of two alleles when gamest of both parents unite the zygote (and the offspring also) revive one alleles from each parent