Question

Describe how nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring following fertilization by a normal male gamete.

Short answer

Answer: Nondisjunction in human female gametes can result in offspring with Klinefelter syndrome (XXY) or Turner syndrome (X0), due to an abnormal number of sex chromosomes in the zygote. Klinefelter syndrome, which affects males, presents symptoms such as reduced fertility and small testicles, while Turner syndrome affects females with features like short stature and infertility.

Step-by-step solution

Understanding nondisjunction

Nondisjunction is an error in meiosis, the process that creates gametes (egg and sperm cells). During meiosis, the chromosomes should separate properly, resulting in gametes with the right number of chromosomes. However, in nondisjunction, chromosomes fail to separate correctly, producing gametes with an abnormal number of chromosomes. In this exercise, we'll focus on nondisjunction in human female gametes (egg cells).

Normal chromosome distribution

In humans, each cell contains 46 chromosomes in 23 pairs. One chromosome of each pair is inherited from the mother, and the other from the father. Sex chromosomes determine an individual's sex: females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During normal meiosis in females, each egg cell should receive one X chromosome.

Nondisjunction in human female gametes

In the case of nondisjunction, an error occurs during meiosis in the egg cell formation. The X chromosomes fail to separate properly, leading to egg cells with either two X chromosomes or no X chromosome instead of just one.

Fertilization with normal male gamete

A normal male gamete contains either an X or a Y chromosome. When a male gamete (sperm) with an X chromosome fertilizes an egg cell with two X chromosomes (due to nondisjunction), the resulting zygote will have three sex chromosomes (XXY). This condition is known as Klinefelter syndrome. On the other hand, if a male gamete with a Y chromosome fertilizes an egg cell without an X chromosome (also due to nondisjunction), the resulting zygote will have only one sex chromosome (X0). This condition is called Turner syndrome.

Klinefelter syndrome and Turner syndrome offspring

The offspring resulting from the aforementioned cases of nondisjunction will have the Klinefelter or Turner syndrome. Klinefelter syndrome (XXY) affects males, causing symptoms like reduced fertility, small testicles, and enlarged breast tissue. Turner syndrome (X0) affects females and is characterized by short stature, infertility, and heart defects, among other features. In summary, nondisjunction in human female gametes can give rise to Klinefelter and Turner syndrome offspring when these abnormal egg cells are fertilized by a normal male gamete. This is because the chromosomes fail to separate properly during meiosis, which leads to zygotes with an unusual number of sex chromosomes.

Key concepts

Klinefelter syndrome

Klinefelter syndrome is a genetic condition affecting males. It occurs when a boy is born with an extra X chromosome, resulting in a 47,XXY karyotype. This syndrome is a direct consequence of nondisjunction, a process where chromosomes fail to separate properly during meiosis.
For those with Klinefelter syndrome, the addition of an extra X chromosome can lead to a range of symptoms, including:

• Reduced testosterone levels
• Small testicular size
• Gynecomastia, or enlarged breast tissue
• Increased height
• Possible learning or social challenges

While Klinefelter syndrome is relatively common, affecting about 1 in 500 to 1 in 1,000 newborn males, it often goes undiagnosed until later in life.
Early diagnosis and treatment can help manage symptoms, and often include testosterone replacement therapy to help develop secondary sex characteristics.

Turner syndrome

Turner syndrome affects females and results from the absence or structural alteration of one X chromosome, leading to a 45,X0 karyotype. This condition is also caused by nondisjunction, typically occurring in the formation of eggs or during early fetal development.
Girls with Turner syndrome usually experience a variety of physical and developmental features, such as:

• Short stature
• Ovarian insufficiency, leading to infertility
• Heart defects
• Broad or "webbed" neck
• Low hairline at the back of the neck

Turner syndrome is less common than Klinefelter, affecting about 1 in 2,500 female births.
Many symptoms can be managed successfully with early treatment, including growth hormone therapy to increase height and hormone replacement to induce pubertal development.

Meiosis

Meiosis is a specialized type of cell division that results in four gametes, each with half the number of chromosomes compared to the parent cell.
In humans, meiosis is crucial for sexual reproduction, reducing the chromosome number from 46 (in somatic cells) to 23 in egg or sperm cells.
There are two main stages of meiosis:

• Meiosis I - Homologous chromosomes separate.
• Meiosis II - Sister chromatids separate.

Errors like nondisjunction can occur in either stage and lead to disorders such as Klinefelter and Turner syndrome.
Meiosis ensures genetic diversity. By shuffling alleles and allowing homologous chromosomes to exchange genetic material, meiosis results in unique combinations of genes that contribute to the genetic variability of offspring.

Human chromosomes

Human cells typically contain 46 chromosomes, arranged in 23 pairs. Out of these, 22 pairs are autosomes, and the final pair is the sex chromosomes, which determine the biological sex of an individual.
The sex chromosome pair is XX in females and XY in males. Each parent contributes one chromosome to each pair.
During the formation of gametes through meiosis, these chromosomes should segregate so that each gamete receives one chromosome from each pair, totaling 23 chromosomes.
Chromosomal anomalies such as nondisjunction disrupt this balance, leading to gametes with abnormal chromosome numbers. Upon fertilization, this can cause genetic syndromes such as Klinefelter and Turner syndromes.
Understanding how chromosomes behave during cell division is key to grasping the complexities of genetic inheritance and the impact of chromosomal disorders.