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  Section: Genetics » Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
 
 
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X/A ratio in Coenorhabditis elegans (a free living nematode)

 
     
 
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


Coenorhabditis elegans is a nematode, in which following two types of individuals are found : (i) XX self fertilizing hermaphrodites and (ii) XO males (Fig. 17.6; compare with Drosophila). The hermaphrodites are believed to be female in its soma (somatic tissues) and mixed (male and female) in its germ line. When the effect of X/A ratio was examined in this nematode, a pattern similar to Drosophila with minor variations was observed. In this nematode an X/A ratio less than 0.67 (0.50 in Drosophila)leads to male sex and a ratio greater than 0.75 leads to a hermaphrodite (this ratio is 1.0 for female in Drosophila). Acomparison of these ratios in the nematode and fruitfly (Fig. 17.7) suggests that a threshold is in operation in each case, which is an excellent example of threshold response in developmental biology. One may, of course, like to find out the manner by which the embryo is able to compute this ratio accurately and reliably leading to sex differentiation (see later).
 
Effect of X/A ratio on sex; different X/A ratios were obtained experimentally by varying the number of X chromosomes (2X, 3X, 4X) and the sets of autosomes (2A, 3A, 4A); at ratios too low (< 0.3) or too high (> 1.5) the dosage compensation is unable to prevent death; the threshold range (over which intersex develops) is slightly different in two species i.e. at X/A ratio 0.67, Drosophila is intersex, while C. elegans is male (modified from Hodgkin, 1990).
Fig. 17.7. Effect of X/A ratio on sex; different X/A ratios were obtained experimentally by varying the number of X chromosomes (2X, 3X, 4X) and the sets of autosomes (2A, 3A, 4A); at ratios too low (< 0.3) or too high (> 1.5) the dosage compensation is unable to prevent death; the threshold range (over which intersex develops) is slightly different in two species i.e. at X/A ratio 0.67, Drosophila is intersex, while C. elegans is male (modified from Hodgkin, 1990).
 
     
 
 
     




     
 
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