Sex Chromosome: Videos & Practice Problems

Sex determination varies across organisms, with two main types: homogametic (e.g., XX in females) and heterogametic (e.g., XY in males). Systems include XX/XY, XX/X0, and ZZ/ZW. Nondisjunction during meiosis can lead to disorders like Klinefelter syndrome (47,XXY) and Turner syndrome (45,X). In females, one X chromosome is inactivated, forming a Barr body, a process described by the Lyon hypothesis. This can result in mosaic phenotypes, as seen in calico cats, where different X alleles express distinct traits.

Sex Determination

Sex determination is a fascinating biological concept that varies across different organisms. It primarily involves two types of sexes: the homogametic sex and the heterogametic sex. The homogametic sex possesses only one type of sex chromosome, which can be multiple copies of the same chromosome. For example, human females are homogametic with two X chromosomes (XX). In contrast, the heterogametic sex has two different types of chromosomes, such as X and Y in humans, where males are classified as heterogametic (XY).

Different organisms utilize various sex determination systems. One common system is the XX-XY system, where the presence of two X chromosomes results in a female, while one X and one Y chromosome result in a male. Another system is the XX-X0 system, often found in insects, where females have two X chromosomes (XX) and males have one X chromosome (X0). Additionally, the ZZ-ZW system is unique, as it reverses the roles: males are homogametic (ZZ) and females are heterogametic (ZW).

Organisms can also be classified based on their reproductive structures. Monoecious organisms possess both male and female reproductive organs, often referred to as hermaphrodites. In contrast, dioecious organisms have distinct male or female individuals, but not both within the same organism.

To summarize the sex determination systems: in the XX-XY system, females are XX and males are XY; in the XX-X0 system, females are XX and males are X0; and in the ZZ-ZW system, females are ZW and males are ZZ. Understanding these systems is crucial for studying genetics and reproductive biology across various species.

Human Sex Chromosomes

In humans, sex determination is primarily governed by the X and Y chromosomes, with males having an XY configuration and females having XX. The formation of these gametes occurs through meiosis, a process where chromosomes line up and separate into haploid daughter cells. However, during meiosis, nondisjunction can occur, leading to improper separation of chromosomes. This results in gametes that may carry an extra chromosome or none at all, which can lead to genetic disorders.

One notable disorder caused by nondisjunction is Klinefelter syndrome, characterized by the presence of an extra X chromosome, resulting in a karyotype of 47,XXY. Individuals with this condition typically present as male but may experience infertility and other phenotypic traits such as longer limbs and breast development. In some cases, additional X chromosomes can lead to more severe symptoms, such as 48,XXXY.

Another significant condition is Turner syndrome, which occurs when there is only one X chromosome, resulting in a karyotype of 45,X. Affected individuals usually have female genitalia but may face cognitive impairments and infertility. Rarely, individuals may exhibit mosaicism, where some cells have different chromosomal configurations, such as a mix of 45,X and 46,XY cells.

The Y chromosome plays a crucial role in male development due to the presence of the sex-determining region Y (SRY) gene, which produces the testes determining factor (TDF) protein. This protein is essential for male differentiation. In contrast, females possess two X chromosomes, but only one is active in each cell due to a process called X-inactivation, which leads to the formation of Barr bodies. This inactivation occurs early in development, as described by the Lyon hypothesis, and results in a mosaic phenotype if the two X chromosomes carry different alleles.

An illustrative example of this mosaicism is seen in calico cats, which exhibit a patchy coat color due to the random inactivation of one of their two X chromosomes. The gene for fur color is located on the X chromosome, leading to a mix of orange and black fur depending on which X chromosome is inactivated in different cells. Notably, all calico cats are female, as males possess only one X chromosome and do not undergo this inactivation process.

Understanding these genetic mechanisms is crucial for recognizing how chromosomal abnormalities can lead to various disorders and phenotypes, highlighting the intricate relationship between genetics and development.

Which of the follow sex chromosomes can be describes as homogametic?

• A. XX/XY
• B. ZZ/ZW
• C. XX/XO

Answer: (C)

In the ZZ/ZW sex determination system, the male is…?

• A. Heterogametic
• B. Homogametic
• C. ZW
• D. XY

Answer: (B)

In humans, which region on the Y chromosome determines maleness?

• A. Sperm producing factor (SPF)
• B. Testis determining region (TDR)
• C. Sex Determining Region (SRY)
• D. Barr Body (BB)

Answer: (C)

During meiosis the XY sex chromosomes segregate independently. Which of the following represents the gametes chromosomes after meiosis?

• A. Two haploid cells that each contain X or Y
• B. Two diploid cells that both contain an XY
• C. A single diploid cell that contains XY
• D. Four haploid cells – two contain X and two contain Y

Answer: (D)

Here’s what students ask on this topic:

What are the different sex determination systems in organisms?

Sex determination systems vary across organisms. The most familiar system is the XX/XY system, where females are XX (homogametic) and males are XY (heterogametic). Another system is the XX/X0 system, common in insects, where females are XX and males are X0 (one X chromosome). The ZZ/ZW system is found in birds and some reptiles, where males are ZZ (homogametic) and females are ZW (heterogametic). These systems determine the sex of the organism based on the combination of sex chromosomes they inherit.

What is nondisjunction and how does it lead to disorders like Klinefelter and Turner syndrome?

Nondisjunction is the failure of chromosomes to separate properly during meiosis. This results in gametes with an abnormal number of chromosomes. When such gametes fuse with normal gametes, they can produce offspring with disorders. Klinefelter syndrome (47,XXY) occurs when an individual has an extra X chromosome, leading to male genitalia but often infertility and other physical traits. Turner syndrome (45,X) occurs when an individual has only one X chromosome, leading to female genitalia but often infertility and cognitive impairments.

What is the Lyon hypothesis and how does it relate to X chromosome inactivation?

The Lyon hypothesis states that in females, one of the two X chromosomes in each cell is randomly inactivated during early development. This inactivation forms a Barr body, which is a condensed, inactive X chromosome. This process ensures that females, like males, have one functional X chromosome per cell. The inactivation can lead to mosaic phenotypes, where different cells express different traits depending on which X chromosome is inactivated, as seen in calico cats.

How do calico cats demonstrate X chromosome inactivation?

Calico cats exhibit a mosaic phenotype due to X chromosome inactivation. The gene for fur color is located on the X chromosome. In early development, one of the two X chromosomes in each cell is randomly inactivated. If one X chromosome carries the allele for black fur and the other for orange fur, the inactivation of different X chromosomes in different cells results in patches of black and orange fur. This phenomenon occurs only in females, as males have only one X chromosome.

What are Barr bodies and how are they formed?

Barr bodies are inactivated X chromosomes found in female cells. They are formed through a process called X chromosome inactivation, which occurs early in development. The DNA and protein of one X chromosome condense tightly, preventing the genes on that chromosome from being transcribed into proteins. This ensures that females, who have two X chromosomes, do not produce double the amount of X-linked gene products compared to males, who have only one X chromosome.