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  Section: Genetics » Sex Linked, Sex Influenced and Sex Limited Traits
 
 
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Non-disjunction of sex chromosomes in Drosophila

 
     
 
Content
Sex Linked, Sex Influenced and Sex Limited Traits
Sex linked traits 
Sex linkage in Drosophila
Sex linked lethals in Drosophila
Sex linkage in human beings
Sex linkage in poultry
Non-disjunction of sex chromosomes in Drosophila
Sex Influenced Traits (horned character in sheep)
Sex Limited Traits (cock feathering in poultry and baldness in man).

Primary non-disjunction. In 1916, C.B. Bridges reported that in rare cases (one in 2,000 or 3,000), results contrary to expectations could be obtained in inheritance experiments on sex linked characters. For instance, when red eyed males are crossed with white eyed females, one expects white eyed males and red eyed females in F1 generation. However, Bridges found that rarely in such a cross red eyed males and white eyed females were observed. Bridges explained this instance on the basis of non-disjunction of X-chromosomes in female individual. The phenomenon of non-disjunction means absence of separation of two homologous X-chromosomes during anaphase I of meiosis. Instead, both X-chromosomes go together to same pole. Other pole obviously will receive no X-chromosome. Results obtained due to such an event are shown in Figure 16.14. Such a non-disjunction with normal female parent would be called primary non-disjunction in order to distinguish it from secondary non-disjunction discussed in the next section.
 
Inheritance of eye colour in Drosophila due to primary non-disjunction of sex chromosome.
Fig. 16.14. Inheritance of eye colour in Drosophila due to primary non-disjunction of sex chromosome.

Secondary non-disjunction. The XXY females obtained due to primary non-disjunction (Fig. 16.14) were again used in further experiments and secondary non-disjunction was shown. When white eyed XXY females were crossed with red eyed normal males, 96% female progeny had red eyes and only 4% females had white eyes, indicating that there was 4% secondary non-disjuncion. XXY would normally disjoin into X and XY, but secondary non-disjunction will result into gametes carrying XX and Y. Results obtained in such an experiment involving XXY white eyed females and XY red eyed males are shown in Figure 16.15.
 
Inheritance of eye colour in Drosophila from an XXY fly resulting from primary non-disjunction and exhibiting secondary non-disjunction.
Fig. 16.15. Inheritance of eye colour in Drosophila from an XXY fly resulting from primary non-disjunction and exhibiting secondary non-disjunction.


Non-disjunction and chromosome theory of inheritance. The results obtained due to non-disjunction were not expected on the basis of normal segregation patterns. However, these could be easily explained on an assumption that two X-chromosomes in female individual are not disjoining. If such an assumption of non disjunction was correct, XXY female individuals would obtained in the progeny. When exceptional females (white eyed) were cytologically studied, these were found to carry two X-chromosomes and one Y-chromosome (XXY). Such cytological results definitely suggested and, therefore, were put forward as an evidence to show that the genes or Mendelian factors were carried on chromosomes. Therefore, non-disjunction of sex chromosomes (primary and secondary) is considered as a strong evidence in favour of the chromosome theory of inheritance, already discussed earlier in Physical Basis of Heredity 4. The Chromosome Theory of Inheritance.

 
     
 
 
     




     
 
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