SEX LINKED GENES Dr V P Saini Prof
- Slides: 12
SEX LINKED GENES Dr V. P. Saini Prof. & Dean College of Fisheries, Kishanganj
Qualitative phenotypes may also be controlled by genes located on one of the sex chromosomes and the genes are called sex-linked genes Sex-linked phenotypes have been discovered in relatively few species of fish and most of the information came from the two species guppy and platy.
Contd…. . The sex-linked inheritance in fish was first described by Johannes in 1920 in guppy, Poecilia reticulata. All sex-linked phenotypes that are known in fish are controlled by genes that are located on the X and / or Y chromosome No genes restricted to the W or Z chromosomes have been discovered.
1. Y–Linked Genes Only one copy of a Y- linked gene exists on the single Y chromosome of a normal male. Y- linked phenotypes cannot be fixed in normal females, which do not possess Y chromosome The Y–linked inheritance of colour patterns in male P. reticulata was reported by Winge.
Genes located on the Y–chromosomes are transmitted from father to son, and unless they cross over to the X chromosome, they will never exist in normal females (XX). Thus Y-linked phenotypes are seen in only one sex, males.
The maculatus gene in the guppy, which controls the maculatus pigment pattern (black spot on the dorsal fin and a red spot on the body) will be used to illustrate the inheritance of a Y-linked phenotype. The symbol for the maculatus gene is Yma. The symbol for the allele which produces the wild type or unspotted phenotype is Y.
2. X-linked Genes Sex linked genes can also be located on Xchromosome 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
X-linked alleles follow definite genetic patterns, but the phenotypes do not always follow the expected ratios This is because many X-linked phenotypes (some autosomal phenotypes too) are sex-limited in that the phenotype is expressed in only one sex
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).
3. Sex-limited traits The black strain ‘Nigra’ is inherited as a dominant sex linked allele. a cross between a ‘Nigra’ female and a lightly pigmented male ‘white” produced ‘Nigra’ sons and ‘white’ daughters and F 2 produced approximately equal frequencies of ‘Nigra’ and ‘white’ individuals of each sex.
4. Sex-linked inheritance in the Platy- WZ/ZZ system Wo Zn x Nigra ♀ ↓ Wo Zw White ♀ ↓ Wo Zn : Wo Zw : Nigra ♀ White ♀ Zw Zw White ♂ Zn Zw Nigra ♂ Zn Zw : Nigra ♂ Zw Zw White ♂ Sex linked inheritance in platy
