24. Sex chromatin (Barr’s body) - nature and medical significance.

Question 1

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Answer 1

Barr body - the inactive X chromosome in a female somatic cell, rendered inactive in a process called ionisation in those species in which sex is determined by the presence of the Y chromosome rather than the diploidy of the X chromosome

In cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis

Question 2

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Answer 2

No of Barr bodies

Female, XX 1

Male, XY 0

Kleinfelter’s, XXY 1

Turner’s, XO 0

Superwoman, XXX 2

Question 3

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Answer 3

• X inactivation occurs in embryogenesis, in the morula and blastula stages
• same chromosome remains inactivated
• either the X inherited from the mother or father can be inactivated
• a cluster of genes on the tip of the short arm of the X chromosome escapes inactivation - lie if or adjacent to region of the X chromosome that pairs with Y chromosome
• X inactivation involves most, NOT ALL genes on the X chromosome
• Counting the number of Barr bodies in somatic cells is a way to test sex chromatin for sex chromosome aneuploidy
• karyotyping is more accurate
• Lyon Hypothesis “in cells with multiple X chromosomes, all but one are inactivated during mammalian embryogenesis” - happens early in embryonic development - day 21
• At interphase you see 1 less X chromosome then there is, the invisible one is the Barr body

Question 4

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Answer 4

• Heterochromatin - Clinical implications:
• if a female carrier of a recessive mutation in the active chromosome with no homologous activity, she will express the mutant phenotype
• if in the inactive chromosome, mutation may not even be detected
• Trisomy for sex chromosomes produces much less severe phenotype than trisomy for any autosome