Short Answer 
The genetic sex of individuals is determined by their sex chromosomes, with “XX” typically designating females and “XY” males. The “Y” chromosome plays a key role in male physical traits, but exceptions like Klinefelter syndrome (XXY) highlight the complexity of genetic sex beyond this binary classification.
Step 1: Understanding Genetic Sex
The terms “XX” and “XY” are crucial in identifying a person’s genetic sex. In mammals, including humans, genetic sex is determined by the combination of sex chromosomes. Individuals with the “XX” configuration are usually characterized as female, while those with the “XY” configuration are mostly classified as male.
Step 2: Chromosomal Influence on Characteristics
The presence of sex chromosomes significantly influences both genotype and phenotype. Specifically, the “Y” chromosome is essential for developing male physical traits. When someone has the “XY” configuration, the “Y” chromosome typically leads to male characteristics, while the “XX” configuration usually results in female traits.
Step 3: Variations Beyond the Binary Classification
It is important to acknowledge that while “XX” and “XY” typically represent female and male sexes, respectively, there are exceptions. These include conditions such as Klinefelter syndrome (XXY) and others that illustrate the complexity of genetic sex, indicating that not all individuals fit neatly into this binary classification.