1. Several genes that contribute to human height appear to be located in the ter
ID: 146595 • Letter: 1
Question
1. Several genes that contribute to human height appear to be located in the terminal region of the p arm of the X chromosome.
(a) Discuss why these regions on the tips of the X chromosome are considered to be pseudo-autosomal. [8 marks]
(b) Discuss whether these genes are inactivated on the X chromosome that undergoes X inactivation.
(c) Explain why people with the karotype 45,X are usually much shorter than average.
(d) Explain why people with the karyotype 47,XXY or 47,XYY are usually much taller than average.
Explanation / Answer
a). The region on the tips of the X-chromosome ( along with Y-chromosome) are considered to be pseudo-autosomal because these regions exhibits autosomal inheritance and not sex-linked inheritance. This is due to the sequence identity shared between X and Y chromosomes. These regions are classified into two,
1). PAR1- 2.6 megabase region located in the short arm tip of both X and Y chromosome.
2). PAR2- 320 kilobase region located in the long arm tip of both X and Y chromosome.
PAR1 and PAR2 undergoes recombination similar to the autosomes. This pairing and crossing over of X and Y chromosomes in their pseudo autosomal region is needed for the male meiosis to undergo normally.
b). No, these genes are not inactivated on the X-chromosome that undergoes X-inactivation. Pseudo-autosomal region are homologous and are present in both X and Y chromosome and thus no dosage compensation is required for females.
c). People with Turner's syndrome have karyotype 45,X (absence of X-chromosome, either partial or complete).
SHOX gene, which stands for short stature homeobox containing, located on the short arm tip of the X-chromosome (Xp22) in PAR1 region. Mutation in this gene is associated with the short stature of the people suffering from Turner's syndrome (45,X). Also if this gene is not present in two copies it leads to the defect in the growth.
d). Although the mutation or deletion of SHOX causes short stature, increased copy number (presence of multiple copies of the gene) can cause tallness. This occurs in case of 47,XXY or 47,XYY where there is an extracopy comparing to normal male(XY) or Female (XX). This is the reason behind the tall phenotype in these conditions.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.