Genes are individual segments of DNA and chromosomes are structures which contain many genes packed together. Each chromosome contains one DNA molecule and each DNA molecule contains several genes or individual strands.
Genes are the basic unit in the study of genetics. Each gene contains the specific code relating to the function of a single protein. Genes combine to form specific DNA molecules. The nucleus of the cell of an organism contains chromatin, which is complex combination of proteins and DNA. When cell division occurs during reproduction, the chromatin forms with the DNA into a chromosome.
In a chromosome, the DNA is tightly packed together with histone proteins. If all the DNA strands in the human body were laid end to end, they would form a very thin strand about 6 billion miles long. The complexity of the organism usually predicts the number of genes and chromosomes. A frog, for example, has 26 chromosomes, or 13 pairs, while a human being has 46 chromosomes, or 23 pairs. Human chromosomes range from 300 genes to 8,000 genes in size.
Chromosomes are paired as the offspring receives one from each parent. Chromosomes and sometimes their combination in the pair determine all inherited characteristics, including sex. For instance, two X chromosomes will produce a female baby while an XY combination will produce a male baby.