Why do we use the letter A,C,G, and T for DNA?

Louise:: With me this month is Dr. Robin Hesketh from the University of Cambridge. Hopefully, we have already answered that first part of that question, A, C, G, and T are for adenine, cytosine, guanine, and thymine - the four bases that make up DNA. But Dr. Hesketh, why are the chromosomes called X and Y?

Robin:: Let's just recap for a moment by thinking about human DNA and reminding ourselves that it's in fact divided into 46 different chromosomes. There are 22 pairs numbered 1 to 22, so the chromosome 1 is the biggest, with the most basepairs and chromosomes 22 and 21 are the shortest. And then in addition to them, there are of course the two sex chromosomes designated X and Y. In men for example, we have an X chromosome and a Y chromosome and ladies of course have 2 X chromosomes. Now the X chromosome was named following the general rule of algebra that the symbol X stands for the unknown quantity. X is nothing to do with shape. If you recall those chromosome pictures, most of them actually look like wiggly Xs, apart of course from the shrivelled Y chromosome, and its name just followed on from X.