During fetal development, an XY individual with a mutation causing a nonfunctional SRY (Sex-determining Region Y) gene would experience the following:
Absence of Testes Development: The SRY gene is crucial for initiating the development of testes in an XY embryo. A nonfunctional SRY gene means that the testes, which produce testosterone and other hormones necessary for male development, would not form.
Lack of Androgen Production: Without functioning testes, the embryo would not produce sufficient levels of testosterone and other androgens (male hormones). These hormones are essential for the development of male secondary sexual characteristics and the masculinization of internal and external genitalia.
Development of Female Characteristics: Due to the absence of testosterone, the default developmental pathway is taken, which leads to the development of female sexual characteristics. The Müllerian ducts, which would typically regress in a normal male embryo under the influence of anti-Müllerian hormone produced by the testes, will persist and develop into female internal genital structures (such as the uterus and fallopian tubes).
External Genitalia: The absence of testosterone means that the external genitalia will develop along female lines. The individual would have a typical female external appearance (vulva) rather than male genitalia.
Potential Phenotypic Outcome: The result would be an individual with a typically female phenotype (appearance and internal reproductive structures) despite having an XY chromosomal pattern. This condition is known as Complete Androgen Insensitivity Syndrome (CAIS) when it involves a complete inability to respond to androgens.
Summary: An XY individual with a nonfunctional SRY gene would develop with female sexual characteristics due to the lack of testosterone and inability to develop testes. This would result in the formation of female internal and external genitalia.