How Is Sex Determined In Drosophila And Humans?

2053 words - 8 pages

The protein profile of each cell, in other words the amounts and types of proteins it expresses, determines its role in the development of the organism that it forms part of. Each cell is assigned a fate by the information encoded in the DNA of the organism, and these fates vary according to the location of the cell in relation to the original zygote cell. Protein signals from this central cell control the cell division and specialisation of the various groups of cooperating developmental fields of cells. This fate refinement may be entirely dependent on the lineage of the cell or affected by signals from neighbouring cells. This chemical dialogue between cells ensures that development proceeds in a coherent fashion and prevents the death of one cell interrupting the entire process. Thus totipotent cells that have the potential to develop into any type of specialised cell thus pass through a series of steps that commit them to their eventual fate. In the regulatory pathway that determines sex in an organism is an example where this fate, once decided, is irreversible.The regulatory decisions that determine developmental fates may occur at any level in the processes that transfer information from a gene to its protein product. It can involve the actual structure of the DNA, as is the case in the amplification of genes coding for eggshell proteins in the follicle cells surrounding the Drosophila oocyte, in order to produce the large amounts that are needed. Alternative splicing during transcription is another possible mechanism of gene regulation, as are effects caused by sequences in the mRNA after transcription, or processes involving the actual polypeptides produced through translation.The determination of sex involves these processes in determining the development of an organism into either a male or a female phenotype. In many species, it involves the inheritance of a heteromorphic chromosome pair in one sex. Despite this similarity, the underlying mechanisms are quite distinct and this suggests that the process evolved independently several times, for example in mammals and flies. Both exhibit sexually dimorphic characteristics, such as the shape and pigmentation of the Drosophila abdomen. Females have a striped, larger abdomen, while in males it has a dark tip and is shorter.In Drosophila, sex is determined independently in each cell lineage. This can be seen in XX-XY mosaic flies that show a mixture of the male and female phenotype. Which sex a fly becomes is determined by the ratio of X chromosomes to sets of autosomes. Drosophila have n=4 so there are three pairs of autosomes (A). A fly develops as follows, depending on its X : A ratioChromosome ConstitutionSexual PhenotypeXX AAfemale (X : A ratio = 1)XY AAmale (X : A ratio = 0.5)XO AAsterile maleXXX AAAfemaleXYY AAAmaleXXY AAAintersexesThe ratio of X : A in the early embryo determines the sex of the fly. It initiates differential transcription and the decision is maintained through...

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