Final answer:
Sex chromosomes, such as X and Y, determine an organism's sex, with XX being female and XY male. Nondisjunction of these chromosomes can cause mild effects due to X inactivation, which balances gene expression in females.
Step-by-step explanation:
The role of sex chromosomes is fundamental to sex determination in humans and many other organisms. The pioneering work of Nettie Stevens, using mealworms, laid the groundwork for understanding how chromosomes carry information that determines an organism's sex. In individuals with the typical chromosomal arrangement, females possess two X chromosomes (XX) and males have one X and one Y chromosome (XY). The Y chromosome is crucial for the development of male characteristics.
Sex chromosome nondisjunction occurs when there is an abnormal number of sex chromosomes, usually with milder effects compared to autosomal aneuploidies. This is due in part to the phenomenon of X inactivation, where one of the X chromosomes becomes inactive, forming a Barr body. This allows for dosage compensation among females. Some X-linked genes, however, remain active even on the inactivated X, and reactivation of the X chromosomes is necessary for female ovary maturation. Abnormalities involving X chromosomes can lead to various phenotypic effects, including mild mental and physical defects and sterility.
Sex-linked traits, such as color variegation in tortoiseshell cats, are seen when different X chromosomes inactivate in different regions of the body. This randomness in X inactivation can result in a mottled coat where each patch of color corresponds to the active X chromosome in that region's cell progenitors. Understanding these principles helps explain how sex chromosome anomalies can result in specific physical and genetic outcomes.