Question

Name the phenomenon that leads to situations like ‘XO’ abnormality in humans. Also name this genetic disorder. How are individuals with an XO chromosomal abnormality affected ? Write its symptoms as well as karyotype.

Hint:

Turner's syndrome ($\text{XO}$) represents an example of chromosomal monosomy, yielding a total count of 45 chromosomes instead of the standard 46.

Answer 1

Concept: The loss or gain of a chromosome happens due to an error during cell division cycles. When sex chromosomes fail to segregate cleanly, individual gametes end up with missing components.

Step 1: Identification of Phenomenon and Disorder

• Phenomenon: The primary mechanical cause is Nondisjunction. This occurs when homologous chromosomes (during Meiosis I) or sister chromatids (during Meiosis II) fail to separate properly during gamete formation.
• Genetic Disorder: An individual having a single X chromosome with the absence of the second sex chromosome ($\text{XO}$ configuration) suffers from Turner's Syndrome.

Step 2: Chromosomal Structure and Karyotype
Normal individuals carry 46 chromosomes ($44 \text{ autosomes} + \text{XX or XY}$). Individuals affected by Turner's syndrome are missing one sex chromosome entirely.
• Total Chromosome Count: $45$ chromosomes
• Karyotype Formula: $44 \text{ Autosomes} + \text{XO}$

Step 3: Clinical Effects and Symptoms
Such individuals are phenotypically female but exhibit distinct structural and physiological deficits:
• Sterility: The ovaries are rudimentary (underdeveloped and streak-like), leading to a complete lack of secondary sexual character development.
• Stature: Noticeably short stature.
• Physical Traits: Webbed neck design, low-set ears, and a broad shield-like chest with widely spaced nipples.
• Menstrual Profile: Amenorrhea (absence of menstrual cycles).
• Systemic Complications: Cardiovascular defects (such as coarctation of the aorta) and mild cognitive delays or spatial learning difficulties.