Key symbols include: Squares for males, Circles for females. Shaded symbols indicate individuals affected by the trait. Unshaded symbols represent unaffected individuals. A horizontal line between a male and female indicates a mating. A vertical line descending from a mating line connects to their offspring. Siblings are connected by a horizontal line above them, with vertical lines extending to each sibling. Dizygotic (fraternal) twins are indicated by separate lines from the same parent line, while monozygotic (identical) twins are indicated by a branching line from a single offspring line.

Karyotyping involves obtaining cells (usually from blood), arresting them in metaphase of mitosis, staining the chromosomes, and then photographing them. The chromosomes are then cut out and arranged in a standardized format. Autosomes are numbered 1 through 22 in decreasing order of size. The sex chromosomes (X and Y) are placed last. A normal human male karyotype is 46,XY, and a normal human female karyotype is 46,XX. Deviations from this can indicate conditions like Down syndrome (Trisomy 21, karyotype 47,XX or 47,XY), Turner syndrome (45,X), or Klinefelter syndrome (47,XXY).

The human sex determination system is XY. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome contains the SRY (Sex-determining Region Y) gene, which is responsible for initiating male development by producing the testis-determining factor. In the absence of a functional SRY gene, female development occurs. Sex-linked traits are those determined by genes located on the sex chromosomes. X-linked traits are more common because the X chromosome is larger and carries more genes than the Y chromosome. Examples include color blindness and hemophilia, which are more prevalent in males due to their single X chromosome (hemizygosity).