Question

Incontinentia pigmenti displays at birth as a patchy pigmentation pattern in female; it does not appear in males. Which explanation best explains the appearance of this disease?

  

The disease is a sex-limited trait that is female-specific

   

The allele responsible is X-linked recessive and shows up in homozygous females, but can not show up in males because males can not be homozygous for X-linked genes

   

Incontinentia pigmenti is an X–linked trait that shows a patchy pattern due to X-inactivation; it is embryonic lethal in males

   

Estrogens cause the trait, so males do not show it

Two of the genes involved in X-inactivation are called XIST and TSIX; is there a logical explanation for why TSIX is XIST spelled backwards?

  

No – there is no special reason for this coincidence

   

the proteins produced by these two genes have opposing functions – the XIST protein promotes X-inactivation while the TSIX protein prevents X-inactivation

   

XIST and TSIX are transcribed in opposite directions from the same region of the X chromosome

   

XIST stands for “X inactive specific transcript” and TSIX stands for “transcription-factors included on the X”

Answer 1

1). The correct statement is - Incontinentia pigmenti is an X–linked trait that results in the patchy pattern due to X-inactivation, it is embryonic lethal in the case of males.

Explanation: Incontinentia pigmenti is a dominant X-linked disorder. The gene linked with this condition is situated on the X chromosome, which is one of the two sex chromosomes. Mutations in the IKBKG gene leads to incontinentia pigmenti. The IKBKG gene gives directions for making a protein that helps to regulate nuclear factor-kappa-B. In males, they have only one X chromosome the mutations result in a total loss of the IKBKG protein. A lack of this protein emerges to be lethal early in embryonic development, so only a few males are born with incontinentia pigmenti. The males who survive with this mutation may have an IKBKG mutation with a moderately mild consequence, and IKBKG mutation in only a few of the body cells.

2). The correct statement is - the proteins produced by these two genes have opposing functions the XIST protein promotes X-inactivation while the TSIX protein prevents X-inactivation.

Explanation: Tsix is a non-coding RNA gene that is antisense or opposite to the Xist RNA. Tsix connects to Xist during the inactivation of the X chromosome. The Tsix is the reverse of Xist, which stands for an X-inactive specific transcript.

X inactivation permits for an equivalent dosage of X-linked genes for both females and males by inactivating the additional X chromosome in the females. Tsix prevents the gathering of Xist on the future active female X chromosome to sustain the active euchromatin status of the chosen chromosome. Tsix binds complementary Xist RNA and make it non-functional. After binding it, Xist is completely inactivated through the dicer.