Question

What is Lyon’s hypothesis?

Answer 1

In nature, a strange imbalance tends to occur: the presence by one sex of two X chromosomes (generally the female) opposed to just one X in the other sex.

Complete answer:
First we should know about Lyon’s hypothesis to answer this question. The theory, named after Mary Lyon, who described it, indicates that inactivation in every but one X chromosome in cells containing over one X chromosome results in dose compensation in mammals. The Barr body, which is noticeable in certain mammalian female cells, is also an inactivated X chromosome. In placental mammals, like humans, the decision over which X chromosome will be inactivated is spontaneous, so if an X chromosome becomes inactivated, it will stay inactive for the life of the cell as well as its descendants in the body. Inactivation of marsupials refers specifically to the paternally-derived X chromosome, unlike spontaneous X-inactivation of placental mammals.

The hypothesis of Lyon notes that in the mammalian female that has two X chromosomes, the phenotypical influence of the X chromosome is the same as in the male that has only a single X chromosome. Early in embryonic development, one in two X chromosomes in females becomes inactivated. It is known as the Barr body. The Barr body is highly condensed and has no phenotypic effect.

Note: Recently, an interesting hypothesis has been formulated to account for this equalisation of expression, and the investigation could strengthen our understanding of X chromosome aberrations, sex variations, and the underlying processes of gene activity in the growing fetus and the maturing organism.