linkage in drosophila
Answers
The chromosomes present in the diploid cells of the majority of the sexually reproducing animals are of two types: autosomes bearing genes for somatic characters and sex chromosomes bearing genes for sex.
Sex chromosomes also carry some genes for non-sexual characters such as colour blindness and haemophilia.
Such genes which are always associated with sex chromosomes are called sex-linked genes. In man and Drosophila the sex chromosomes (X and Y) are unequal in size and shape, X being larger and rod shaped whereas Y is small and slightly curved. In birds and butterflies the sex chromosomes (Z and W) are also unequal in shape and size, Z being larger than W.
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In Mendelian pattern of inheritance, the genes for contrasting characters were located on autosomes but not on the sex chromosomes. Secondly, the result of reciprocal cross is same as normal cross which is not the case with sex linked inheritance. There are three types of sex-linked genes depending upon their association with particular chromosome.
They are as follows:
(i) The genes which are located on X-chromosomes are called X-linked genes or sex linked genes.
(ii) The genes which are located on Y chromosomes are called Y-linked genes or holandric genes.
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(iii) Certain genes are found to occur in both X and Y chromosomes. Such genes are called incomplete sex-linked genes.