About X and Y Variations
X and Y Variations, also known medically as Sex Chromosome Aneuploidy (SCA), involve variations in the typical number and type of sex chromosomes.
The typical number of chromosomes in each human cell is 46. These include 24 pairs of “autosomes” (which refers to all chromosomes that are not sex chromosomes) and a pair of sex chromosomes, either an X and a Y (which makes for a male) or two Xs (female). That means the genetic signature for a person with 46 chromosomes is either 46,XY (males), or 46,XX (females).
Any variation from these typical numbers is referred to scientifically as an “aneuploidy.” If there is one additional X or Y, it is known as a “trisomy” (“tri” denoting the number 3). Genetic conditions that reflect a trisomy include:
Trisomies are the most common X and Y aneuploid conditions. Less common are “tetrasomies,” which denote the presence of four sex chromosomes:
Still rarer are the “pentasomies,” indicating five sex chromosomes:
Typically, 48- and 49-chromosome variations produce more pronounced symptoms than trisomies, although this is not always the case.
At the other end of the spectrum lie the “monosomies,” meaning a person born with only one sex chromosome. The only survivable monosomy SCA is 45,X. This is a female with only one X chromosome, a condition known as “Turner syndrome.”
A fetus with only one Y chromosome and no X chromosome is not viable and cannot survive. AXYS does not include Turner syndrome in its mission. Turner syndrome advocacy and support can be found at www.turnersyndrome.org.
About 20 percent of persons with sex chromosome aneuploidies exhibit “mosaicism,” meaning they have two or more cell lines with different genetic signatures. Examples include 46,XY/47,XXY, and 45,X/47,XXX.
Sex chromosome aneuploidy is not inherited. The extra chromosome is the result of an error that occurs in cell division when the egg or sperm is formed, or occasionally during cell division after fertilization. An extra X can be contributed by either the mother or father. Only the father can contribute an extra Y chromosome.