Question

Unlike trisomy for autosomes, trisomy X is often without serious phenotypic consequences. This is due to the fact that the additional X chromosome is converted to an inactive. .............. in the somatic cells of the adult.

Answer 1

Trisomy X does not produce significant phenotypic consequences because one X chromosome becomes a Barr body during embryonic development, a process called X inactivation, ensuring dosage compensation for the extra X chromosome. This inactivation is random and all progeny cells inherit the same inactivated X chromosome, though some gene expression still occurs, resulting in milder effects compared to autosomal abnormalities.

Step-by-step explanation:

The reason that trisomy X often does not lead to serious phenotypic consequences is due in part to a process known as X inactivation, which occurs early in female embryonic development. During this process, one X chromosome in each cell condenses into an inactive structure called a Barr body, thus compensating for the additional genetic dose of the X chromosome. This process is random in terms of which X chromosome (maternally or paternally derived) is inactivated, and all cells derived from this originally inactivated cell will inherit the same inactivated X chromosome, maintaining dosage compensation throughout the individual's life.

While the inactivated X chromosomes remain largely silent, a few genes may still be expressed. Furthermore, for proper maturation of female ovaries, the inactive X chromosomes must reactivate. As a result, individuals with an abnormal number of X chromosomes may experience mild mental and physical defects and possible sterility, much milder than abnormalities with autosomes, which often lead to more severe effects or lethality.