Final answer:
A person can have sex chromosomes that differ from their external appearance due to variations in the SRY gene and errors during meiosis such as nondisjunction. Conditions like Klinefelter syndrome (XXY) and Androgen Insensitivity Syndrome (AIS) illustrate how chromosomal makeup doesn't always predict physical sex characteristics.
Step-by-step explanation:
Individuals typically have two sex chromosomes; females possess two X chromosomes (XX), and males one X chromosome and one Y chromosome (XY). The presence of the SRY gene, located on the Y chromosome, triggers the development of male characteristics. Without the SRY gene, the default development is toward female characteristics. During meiosis, which is the process that produces gametes, errors can occur leading to variations such as XXY or XYY in males or XXX in females.
These errors happen due to nondisjunction, where the chromosomes do not separate properly during meiosis. For instance, if a sperm cell with both an X and a Y chromosome fertilizes an egg with an X chromosome, the result might be an XXY karyotype, known as Klinefelter syndrome. In this condition, even though there is a Y chromosome and potentially a functional SRY gene, the individual might exhibit a mix of male and female characteristics because of the extra X chromosome.
Furthermore, genetic and environmental factors might influence the expression of sex-linked genes. There are cases where a person with XY chromosomes does not develop male external characteristics because the SRY gene is not expressed or functional. This condition is known as Androgen Insensitivity Syndrome (AIS).