What is the Difference Between XX and XY Chromosomes

The main difference between XX and XY chromosomes is that XX chromosomes are the chromosomes in females whereas XY chromosomes are the chromosomes in males, specifically in organisms with the XY sex-determination system. Furthermore, XX chromosomes occur in homogametic sex while XY chromosomes occur in the heterogametic sex.  

XX and XY chromosomes are two types of sex chromosome combinations in humans, mammals, some snakes, some insects, and some plants. 

Key Areas Covered 

1. What are XX Chromosomes
     – Definition, Formation, Inactivation
2. What are XY Chromosomes
     – Definition, Formation, Inactivation
3. What are the Similarities Between XX and XY Chromosomes
     – Outline of Common Features
4. What is the Difference Between XX and XY Chromosomes
     – Comparison of Key Differences

Key Terms 

Heterogametic Sex, Homogametic Sex, Sex Chromosomes, XX Chromosomes, XY Chromosomes

What are XX Chromosomes 

XX chromosomes are the combination of sex chromosomes that occur in female organisms in the XX sex-determination system. Generally, this type of sex-determination system occurs in humans, mammals, snakes, insects, and some plants. Normally, chromosomes in human females can be identified as 46, XX. Therefore, females have two X chromosomes as their sex chromosomes. Furthermore, X chromosomes are larger in size, of about 153 million base pairs. It also contains about 800 genes.

Figure 1: XX and XY Chromosomes

Furthermore, XX chromosomes have homogametic sex. Therefore, a female embryo forms due to the fusion of two same types of gametes. Meanwhile, one of the two X chromosomes undergoes X chromosome inactivation. Basically, it occurs during the embryonic development through two large non-coding RNAs, Xist and Tsix.  

What are XY Chromosomes 

XY chromosomes are the combination of sex chromosomes occur in males with the XY sex-determination system. However, in human males, the chromosomes in the cells are identified as 46, XY. Therefore, they have one X chromosome and one Y chromosome. Basically, the Y chromosome is smaller than the X chromosome. The size of the Y chromosome is about 58 million base pairs. Moreover, it contains around 72 protein-coding genes.

Figure 2: Human Male Karyotype

Moreover, XY chromosomes have heterogametic sex. The single X chromosome in males also undergoes sex chromosome inactivation. The process of this is known as meiotic sex chromosome inactivation. Generally, it occurs during the late stages of first meiotic prophase in spermatocytes. 

Similarities Between XX and XY Chromosomes 

• XX and XY chromosomes are two types of sex chromosome combinations in organisms with the XY sex-determination system. 
• Moreover, they occur in humans, mammals, some snakes, some insects, and some plants. 
• X and Y chromosomes also show regular Mendelian inheritance. 

Difference Between XX and XY Chromosomes 

Definition

XX chromosomes refer to the sex chromosomes in females while XY chromosomes refer to the sex chromosomes in males. 

Type of Sex 

While XX chromosomes occur in homogametic sex, XY chromosomes occur in the heterogametic sex.  

Inactivation of Chromosomes 

Furthermore, XX chromosomes undergo X chromosome inactivation in embryos while XY chromosomes undergo meiotic sex chromosome inactivation. 

Conclusion 

XX chromosomes are the combination of sex chromosomes in females. Moreover, it represents homogametic sex. In females, one X chromosome undergoes inactivation in the embryo. On the other hand, XY chromosomes are a combination of chromosomes in males. However, it represents heterogametic sex. Sometimes, the X chromosome undergoes a transient inactivation. Therefore, the main difference between XX and XY chromosomes is the type of gender in which they occur. 

References:

1. Arnold, Arthur P et al. “What a difference an X or Y makes: sex chromosomes, gene dose, and epigenetics in sexual differentiation.” Handbook of experimental pharmacology ,214 (2012): 67-88. doi:10.1007/978-3-642-30726-3_4
2. Yan, Wei, and John R McCarrey. “Sex chromosome inactivation in the male.” Epigenetics vol. 4,7 (2009): 452-6. doi:10.4161/epi.4.7.9923.

Image Courtesy:

1. “Mice X Y chromosomes” By Janice Y Ahn, Jeannie T Lee (CC BY 2.0) via Commons Wikimedia   
2. “Human male karyotpe high resolution – Chromosome X” 由 National Human Genome Research Institute (公有領域) via Commons Wikimedia