Principles of Genetics and Breeding

4.2.3 X-linked Genes

Sex linked genes can also be located on X-chromosome. X-linked genes are expressed differently in normal males (single copy in XY) and normal females (two copies in XX).

• In females, most X-linked genes exhibit complete dominance. Because males have only a single X chromosome an X-linked allele is expressed in males.
• For example, caudalis pigmentation in guppy is due to X–linked genes. The inheritance of X-linked phenotype follows a criss-cross pattern.
• The father determine his daughter’s phenotype while her mother determine her sons phenotype. This phenotype is produced by the dominant Xcp allele and the recessive Xch allele.
• A single Xcp allele produces caudalis pigmentation in either sex.
• The Xch allele must be present in the homozygous state to produce the transparent phenotype in females (Xch Xch), but a single Xch allele will produce the phenotype in males (Xch Y).
  

Genotype	Phenotype
Xcp Xcp	Caudalis female
Xcp Xch	Caudalis female
Xch Xch	Transparent–tailed female
Xcp Y	Caudalis male
Xch Y	Transparent–tailed male