The human cell contains 46 chromosomes in 23 pairs, one set of 23 from the egg and one set from the sperm. The genes on each chromosome in a pair match each other except for the sex chromosomes called X and Y. Females have two X chromosomes and males have one X and one Y. Each pair of chromosomes carries two copies (alleles) of each gene. Both alleles may be normal, both abnormal or one normal and one abnormal.

Some men carry abnormal chromosomes or genes that affect their fertility. For example, deletion of a portion of the Y chromosome leads to poor (“oligospermia”) or absent (“azoospermia”) sperm production. Similarly, men with one abnormal allele for cystic fibrosis are often found to have structural abnormalities of the internal reproductive organs. Consequently, when we discover a sperm count of <5 million sperm/cc or a significant physical abnormality on exam, we may advocate blood testing for chromosomes (“karyotype”) or, when possible, specific gene abnormalities.