Chromosomes are thread-like structures found in the core of all cells (except for red blood cells). The chromosomes contain our genetic map (DNA). Chromosomes come in pairs, a normal human cell contains 46 chromosomes, 22 pairs (autosomes) and one pair of sex chromosomes.
DNA, or Deoxyribonucleic Acid, are the nucleus of a cell that contains all instructions for making a living organism. A DNA nucleotide is made of a sugar molecule, a phosphoric acid molecule, and a molecule called `base`. The bases are the letters that spell out our genetic code. In DNA, the letters A, T, G, and C for the substances are adenine, thymine, guanine, and cytosine. Adenine always accompanies thymine, and guanine always accompanies cytosine.
Genes are constructed from DNA. Most genes contain information for creating a specific feature.
Boy or girl?
As soon as a sperm fuses with an ovum, one chromosome of each pair is given to the new person. This includes Chromosome 23. Women have XX, men have XY. When the man delivers the X chromosome, it becomes a girl; if it gives the Y-chromosome the child will be a boy.
From angelababy In Q&A