8 Main Criticisms for Lyon’s Hypothesis | Genetics

The following points highlight the eight main criticisms for Lyon’s hypothesis.

Criticism # 1.

That XO, XXX, XY, XX etc. individu­al differ among themselves suggesting that the entire X chromosome is not genetically inac­tive. If it were so, an XX and XXX female would have had no difference at all.

From various chemical stigmata it seems that a part of the X, if not the entire length, is inactive.

Criticism # 2.

In fact, autoradiography shows late replication:

i) In long arm of the two Xs in human.

ii) In the short arm of cow and

iii) In half X’s in hamster.

Criticism # 3.

The XO tumer indicatingly suggest that the second X at least in its minor seg­ments, is required for feminization and fertility.

Criticism # 4.

The inactivation of all but one X’s has minimized the drastic effect not only in X polysomics but also in minor deletion or even omition of an X as compared to the severity of cells autosomal trisomic or deletion.

Criticism # 5.

Natural selection mostly remove the abnormal cell lines from a mosaic individual in course of time as cited by Whittinghill (1970) in individuals mosaic in relation to Duchenne muscular dystrophy.

Criticism # 6.

The fact that structurally abnormal X, be it a ring-X or an iso-X, is invariably late replicating, goes against random differentia­tion concept.

Criticism # 7.

Ghosal (1975) contemplated phe­nomenon of Y chromosome differentiation.

In adult male, somatic cell of Chinese hamster (XY male), Taylor (1960), in human males (Y) (Lima-de-Faria, 1961), in mouse and hamster male (Y) — Y chromosome is late replicating.

Criticism # 8.

Mimicry between X and Y chromo­some differentiation: A striking similarity exists in between the second X (this would be late X), of the female and the Y chromosome of the male in being both —

i) Late replicating in adult somatic cell,

ii) Early replicating in the mediotic gonadal cells.

iii) Earliness in pre-implanted embryo during post-zygotic mitosis and

iv) Lateness in post-implanted embryos continuing in adults and

v) With the postulation that X and Y chromosome evolved from an ances­tral homologous pair, followed by

vi) Differential heterochromatinization thereby restricting the pairing seg­ments, their behavioural mimicry pal­pable.

Sex chromatin differentiation is neither a monopoly of the mammalian female nor restricted to the X-chromosome. But such a phenomenon does exist in case of mammalian females, and, hence, for Y chromosomes too.

From the different biochemical and cyto­genetic data so far established it can be inferred that:

(i) A major portion not the entire length of the X-chromosome is inactivated.

(ii) The Y chromosome mimics the inac­tive X in several aspects.

(iii) The specific fractions complexed with heterochromatic segments of inactive X may interfere with the transcription process.