Question

List the normal pair of sex chromosomes, and possible sex chromosome aneuploidy for male and female, respectively. And then briefly explain the possible mechanism of XY girls and XX boys.

Answer 1

Sex chromosomes are different than the autosomes and are responsible for the sex determination of a person. X and Y are the sex chromosomes. Males are heterogametic with one X and one Y chromosome, while females are homogametic with two X chromosomes. The genes present on X chromosomes are called X-linked genes and are involved in sex linkage. The genes on Y chromosomes are called holandric genes.

So a normal male's sex chromosomes are represented as XY and a normal female's sex chromosome are represented as XX.

Sex chromosome aneuploidy:

Ploidy term depicts the complete set of chromosomes in an organism. For example: Humans have 46 chromosomes (44 autosomes + 2 sex chromosomes) in total, which means two sets of 23 chromosomes (2N). The autosomal cells are diploid (2N) while gametic cells have one set of chromosomes (N) or haploid.

Chromosomes undergo structural or numerical changes which are termed as chromosomal structural or numerical aberrations. Aneupliody is a type of numerical aberration where an abnormal number of chromosomes are present in an organism, such as increase in chromosome number by one (2n+1) called trisomy, decrease in the chromosome number by 1 (2n-1) celled monosomy, and decrease in chromosome number by 2 (2n-2) called nullisomy.

Aneuploidy conditions:

Turner syndrome: In females, 44 autosomes + 1 sex chromosome (44+XO). Example of monosomy. This syndrome comes under sex linked disorder.

Klinefelter syndrome: In males, 47, XXY condition, with an extra X chromosome. It is a sex linked disorder.

Trisomy X or Triple X syndrome: 47,XXX condition, with an extra X chromosome. It is a sex linked disorder.

Down syndrome: 46+1 chromosome, trisomy condition due to an extra copy of chromosome number 21 due to non disjunction. Represented as 47, XX and 47, XY for female and male respectively. This is an autosomal trisomic condition and disorder.

Patau syndrome: 46+1, autosomal trisomic condition at chromosome number 13. Occurs in both males and females.

Edwards syndrome: 46+1,autosomal trisomic condition at chromosome number 18. Occurs in both males and females, and is lethal, death occurs within few days after birth.

XY girls and XX boys:

XY girl: The disorder can be either complete gonadal dysgenesis or Swyer syndrome and complete androgen insensitive syndrome. In complete gonaldal dysgenesis, there is no testicular development, non functional gonad, and the person has a female phenotype with male genotype. It is represented as 46, XY

In complete androgen insensitive syndrome, there is complete testicular development, with release of androgens but androgen receptors do not bind the androgen molecules. So the body is unable to utilize the androgen hormones even after synthesizing androgen. The person has female phenotypic characters but lack uterus and are infertile, and have the genotype of male.

In both these conditions, the person is identified as a female due to the phenotypic characters.

XX boys: Normal males have XY chromosomes. A sex determining region Y (SRY) gene is present on Y chromosome. But due to translocation of this gene to X chromosome, the person, called SRY positive has karyotype of a female (46, XX male). Such persons have reduced male phenotypic characters and are infertile.

XX male and XY female both are termed as Disorder of sexual development (DSD).