In X-linked recessive inheritance, an affected mother has two defective X chromosomes and will pass one defective X chromosome to all her sons. Since males inherit only one X chromosome, they will always express the trait if they inherit the defective gene. Females are less frequently affected because they require two copies of the defective gene to show the trait.
| List I (Enzyme) | List II (Function) |
|---|---|
| (A) Cytochrome oxidase | (I) Electron transport system |
| (B) Topoisomerases | (II) Change linking number |
| (C) Cohesins | (III) DNA Replication |
| (D) PCNA | (IV) Cell cycle |