Question

Nondisjunction is the failure of homologous chromosomes to separate properly during meiosis. Below is a karyotype of a person that is the product of the nondisjunction event.
Which of the following statements below could predict the events that occurred to produce the individual shown in this karyotype?
(a)As this person was a female, one X chromosome was inactivated. This inactivation prevents the appearance of two X chromosomes on the karyotype.
(b)When the cells that gave rise to this individual were forming, one of the gametes became cancerous. Errors in cell division caused by cancer produced the karyotyped individual with an abnormal chromosomal number.
(c)During meiosis, two X chromosomes combined to make one larger-than-average X chromosome. Because the amount of DNA in the cell is unchanged, this had no effect on the development of the karyotyped individual.
(d)A mistake was made during meiosis, resulting in either the sperm or the egg missing its sex chromosomes. Fertilization of this unusual gamete produced an individual with only 45 chromosomes.

Answer 1

Hint: The given karyotype has only 45 chromosomes instead of the usual 46 in a human body. Additionally, it does not have a second sex chromosome. This karyotype is indicative of a syndrome that has symptoms of sterility, abnormal menstrual cycle, and short stature.

Complete answer: - The given karyotype shows 44 X+O chromosomes. There are only 45 chromosomes instead of the normal 46 for human beings, with one missing sex chromosome. Hence, this represents females suffering from Turner’s syndrome.
- The syndrome occurs when the nondisjunction of the X chromosome during meiosis, resulting in the formation of only 45 chromosomes. This causes aneuploidy or an abnormal number of chromosomes in the cell.
- This disorder is monosomy in females. Symptoms of the syndrome include short stature, short and webbed neck, sterility, issues with puberty (abnormal periods and development of breasts), and hormonal issues.

Additional information: - Other sex chromosome aneuploidies rising from nondisjunction are Klinefelter syndrome, XYY Male syndrome, and Trisomy X.
- Nondisjunction may occur during meiosis I or II, and both lead to different results.
- Monosomic human zygotes that have a missing autosome (any chromosome other than a sex chromosome) usually fail to be born.

So, the correct answer is ‘As this person was a female, one X chromosome was inactivated. This inactivation prevents the appearance of two X chromosomes on the karyotype’.

Note: - Other names for Turner syndrome are gonadal dysgenesis, Ullrich–Turner syndrome, and 45X.
- People affected with Turner Syndrome usually display normal intelligence.
- Turner syndrome is the only survivable monosomy in humans.