Lyon hypothesis

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Lyon hypothesis

[′lī·ən hī‚päth·ə·səs]
(genetics)
The concept that mammalian females are X-chromosome mosaics as a result of the random inactivation of one X chromosome in some embryonic cells and their descendant and of the other X chromosome in the rest.
References in periodicals archive ?
However, as women age, one of the two X's starts to predominate; 35% of women over 60 have the same X chromosome silenced in >90% of their cells (termed 'skewed X-inactivation'), and the proportion of women with preferential silencing of the same chromosome X increases with age.
Osteopathia striata cranial sclerosis: non-random X-inactivation suggestive of X-linked dominant inheritance.
The X-inactivation patterns of female carriers were investigated by studying the polymorphic trinucleotide (CAG) repeats in the first exon of the human androgen receptor gene as reported previously [11].
The recurrent risk of the same chromosomal abnormality is difficult to identify because it depends on the type of disjunction and X-inactivation [7].
Milder findings in the female can be attributed to differences in expression of the mutated allele caused by skewed X-inactivation. Somatic mosaicism in the boy was supported by 8-lathosterol accumulation being less than expected in affecteds.
Although X-autosome translocations are frequently associated with streak gonads and clinical features of the Turner syndrome, the majority of X-autosome carriers may present with a variable phenotype, developmental delay, and recognizable X-linked syndrome due to nonrandom X-inactivation. In this article, we describe a healthy 15.5-year-old girl with primary amenorrhea, gonadal dysgenesis, and tall stature without other manifestations of the Turner syndrome.
Researchers named these good genes "EXITS" (Escape from X-Inactivation Tumor Suppressors).
Patient-derived iPS cells are expected to be useful for disease modeling, but the effects of reprogramming by Yamanaka factors on X-inactivation in female iPS cells remain controversial.
(6,15) Historically, skewed X-inactivation was the accepted mechanism for the development of symptoms in carriers.
Due to non-random X-inactivation, the majority of X; autosome carriers that present with abnormal phenotypes include multiple congenital abnormalities, developmental delay, a recognizable X-linked syndrome or gonadal dysgenesis (3).
In a normal female cell, there are two copies of the X chromosome; however, only one copy is active, because the other copy is "silenced" through a process called X-inactivation or Lyonization.