Y Chromosome Analysis

In the human, there are normally 46 chromosomes, two sex chromosomes and 22 chromosome pairs for which one copy is inherited from each parent at conception. The sex chromosomes are called the X and the Y chromosome. Everyone needs at least one X chromosome to survive. Females normally have two X chromosomes whereas males typically have one X and one Y chromosome. In the absence of a Y chromosome, babies will develop as females. When the Y chromosome is present, they will develop as males.

The Y chromosome is different from all of the other chromosomes in a couple of different ways. First, it contains the fewest number of genes of any chromosome, far fewer than chromosome 21, the next smallest chromosome. Second, the vast majority of the Y chromosome is composed of heterochro-matin, a form of DNA that does not contain functional genes. Third, the genes that are present on the Y chromosome are critically important in sexual development.

As only males have a Y chromosome, and the presence of the Y chromosome determines male sexual development, the pattern of inheritance is that fathers uniformly transmit the Y chromosome to their sons at conception, and never to their daughters. This allows a tracing of inheritance patterns for genes and other markers on the Y chromosome from father to son down through many generations.

Because the Y chromosome has so much noncoding DNA, there are many different DNA sequence variants that may be identified on the Y chromosome. These non-coding DNA sequences have a very high rate of mutation, and many potentially informative short tandem repeat (STR) sequences that permit a detailed study of paternity and other forensic testing based on DNA sequences.

The Y chromosome has a distinctive pattern of fluorescence (light emission) naturally and also when using certain organic dyes. These properties can be exploited in various ways to identify the presence of semen based on natural fluorescence, or to identify Y-bearing sperm and separate them from X-bearing sperm. Furthermore, chromosomal analysis for the sex chromosomes can be used to predict the sex of a baby prenatally. As of 2005, it is not considered ethical to use chromosome analysis prenatally to facilitate sex selection for parents who desire either a boy or a girl unless there is a sex-linked genetic disease risk.