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  Section: Genetics » Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
 
 
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Position effect variegation

 
     
 
Content
Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
Chromosome Theory of Sex Determination 
Balance Theory of Sex Determination X/A ratio in Drosophila
Triploid intersexes in Drosophila and genie balance theory
X/A ratio and gynandromorphs in Drosophila
X/A ratio in Coenorhabditis elegans (a free living nematode)
Balance Between Male and Female Factors
- Diploid intersexes in gypsy moth (Lymantria)
- X/A ratio and multiple numerator elements (Drosophila and Coenorhabditis)
Sex Determination in Plants
Methods for determining heterogametic sex in plants
Sex determination in Coccinia and Melandrium
Sex determination in other dioecious plants
Sex Chromosomes in Mammals Including Humans (Homo sapiens)
TDF, ZFY and SRY genes in humans
H-Y antigen and male development in mammals
Single gene control of sex
Sex determination in Asparagus
Tassel seed (ts) and silkless (sk) genes in maize
Transformer gene (tra)in Drosophila
Haploid males in Hymenoptera
Hormonal control of sex
Environmental Sex Determination in Reptiles
Dosage Compensation in Organisms with Heterogametic Males
X-chromosome inactivation in mammals
Position effect variegation
Hyperactivity of X-chromosome in male Drosophila
Lack of Dosage Compensation in Organisms with Heterogametic Females
Genetic imprinting
Position effect variegation (due to inactivation of an autosomal segment attached to X-chromosome)
The inactivation is not confined to the facultative heterochromatin mainly present in X-chromosome but may also extend to autosomal chromosome segments, if attached to X-chromosome. For instance, it has been observed that when a translocation occurs between a euchromatin segment from an autosome to an X-chromosome, inactivation seems to spread from the X-chromosome to the translocated autosomal segment. In mouse also it has been observed that when an autosomal gene is transferred to an X-chromosome, its action may be suppressed in some cells alongwith the inactivation of X-chromosome. In a translocated chromosome, having segments from an autosome and an X chromosome, it has been observed that the inactivation of a chromosomal region depends on its proximity to X chromosome, because the effect has been found more pronounced in regions close to the heterochromatin. The autosomal regions which are located away from heterochromatin may remain active. Variegation has been observed in mice which were heterozygous for a translocation between X chromosome and an autosome, the autosomal segment carrying a gene for coat colour. Due to inactivation of the coat colour gene located on autosomal segment, translocated to the X chromosome, variegation has been observed. This kind of variegation has been described as position effect variegation.

 
     
 
 
     




     
 
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