A specific recessive X-linked mutation prevents the formation of sweat glands within the skin. Individuals heterozygous for this mutation exhibit a mosaic phenotype, with some patches of skin lacking sweat glands and other patches containing a normal distribution of sweat glands. Which of the following statements best explains the genetic mechanism underlying this mosaic phenotype in heterozygous individuals?

Question

A specific recessive X-linked mutation prevents the formation of sweat glands within the skin.  Individuals heterozygous for this mutation exhibit a mosaic phenotype, with some patches of skin lacking sweat glands and other patches containing a normal distribution of sweat glands.  Which of the following statements best explains the genetic mechanism underlying this mosaic phenotype in heterozygous individuals?

Quizplus · Accepted Answer

11ecba2d_1033_3a5a_a3bf_cb120d364093_MD0008_00 In mammals, sex is determined by an inherited combination of sex chromosomes. Females (XX) inherit one X chromosome from their mother and one from their father. Males (XY) inherit an X chromosome from their mother and a Y chromosome from their father. Sex-linked genes, located on either the X or Y chromosome, exhibit different inheritance and expression patterns between males and females. Specifically, females will inherit two copies of X-linked genes, whereas males will inherit only one. The question describes a recessive X-linked mutation that prevents sweat gland formation within the skin. Heterozygous individuals (ie, having one mutant and one wild-type gene copy) exhibit a mosaic phenotype in which some skin patches lack sweat glands and other patches have a normal distribution of sweat glands. Because this gene is X-linked, only females (XX) carry two copies of this gene and can be heterozygous. The mosaic phenotype in these heterozygous females indicates that some skin patches express the mutant gene and other skin patches express the wild-type gene. This is best explained by random inactivation of one X chromosome in embryonic cells during development, which generally occurs in mammalian females. Depending on which X chromosome is inactivated, some embryonic cells will contain an active copy of the wild-type X chromosome and some will contain an active copy of the X chromosome with the mutation. Accordingly, some of these cells will divide and differentiate into patches of skin cells exhibiting either wild-type expression (normal sweat gland formation) or expression of the mutation (no sweat gland formation). (Choice A) Because the individuals described in the question are heterozygous for the X-linked gene, they must be female (have two X chromosomes) and would not carry a Y chromosome (ie, be male). (Choice B) Silencing of paternally inherited X chromosome genes would cause expression of only maternal X chromosome genes in all cells of the female organism. For females heterozygous for the mutation, this would cause expression of a single (not mosaic) phenotype. (Choice C) If no mutant genes on X chromosomes were expressed, the individuals with this mutation would express only the wild-type gene and exhibit normal sweat gland formation. Educational objective: The inheritance and expression of sex-linked genes (located on either the X or Y chromosome) differ between males and females.

A Specific Recessive X-Linked Mutation Prevents the Formation of Sweat