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  Section: Genetics » Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
 
 
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Chromosome Theory of Sex Determination

 
     
 
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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
According to chromosome theory of sex determination, male and female individuals would differ in their chromosome constitution (Table 17.1). There may be two types of chromosomes present in such individuals : (i) autosomes and (ii) sex-chromosomes. In a diploid individual, there are 2n - 2 autosomes and two sex-chromosomes. While in one sex (generally female), two sex-chromosomes are homomorphic (XX), in the other sex (generally male) these are heteromorphic (XY). In birds, usually female is designated as ZW, being heterogametic and male is designated as ZZ being homogametic. Different symbols in birds are used to distinguish the female heterogametic in birds (ZW) from male heterogametic sex (XY) in Drosophila and man.

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Role of sex-chromosomes in determination of sex became evident for the first time due to discovery of sex linked genes, whose inheritance did not follow the expected segregation patterns in both sexes, but instead followed a criss-cross type of inheritance. The experiments of C.B. Bridges on non-disjunction of X-chromosomes proved that a diploid set of autosomes with two X-chromosomes will always give rise to a female individual irrespective of whether the two X-chromosomes came from same parent or from different parents (Fig. 17.2). This example suggested that in Drosophila, Y-chromosome does not carry any sex determining factor. Presence of one or two X-chromosomes is more important than the presence or absence of Y-chromosome.
 
Non-disjunction of X chromosomes in a female Drosophila leading to the transfer of both X's to the daughter.
Fig. 17.2. Non-disjunction of X chromosomes in a female Drosophila leading to the transfer of both X's to the daughter.

 
     
 
 
     




     
 
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