This document discusses sex-linked genes and traits. It explains that females have two X chromosomes and can be homozygous or heterozygous for genes on the X chromosome, while males only have one X chromosome and will always express the phenotype of the single allele they carry for X-linked genes. It uses haemophilia, a bleeding disorder caused by deficient coagulation factor VIII, as an example of a sex-linked recessive trait. The document presents a Punnett square showing that if a healthy carrier woman and healthy man have children, there is a 25% chance of having a hemophilic son since he would receive the defective allele from his mother.