Final answer:
Poly-X females arise from having multiple X chromosomes, like XXX. One of these X chromosomes undergoes X inactivation to form a Barr body, compensating for the genetic dosage of the extra X chromosomes.
Step-by-step explanation:
Poly-X Females and Sex Chromosome Nondisjunction
Poly-X females result from a condition known as multiple X chromosomes, where a female has more than two X chromosomes, such as XXX, XXXX, etc. This condition is one of the potential outcomes of sex chromosome nondisjunction during gamete formation, leading to aneuploidy – a deviation from the normal number of chromosomes. Most notably, individuals with three X chromosomes are called triplo-X and are phenotypically female, yet they may exhibit developmental delays and reduced fertility. During early development, one X chromosome in each cell will typically undergo X inactivation, condensing into a structure known as a Barr body. This inactivation is a natural mechanism to compensate for the excess genetic dosage from additional X chromosomes. It is the random inactivation of one of the X chromosomes that ensures cells have only one dose of the X-linked genes, maintaining the balance necessary for normal function.