Difference Between X and Y Chromosome

Main Difference – X vs Y Chromosome

X and Y chromosome are the two sex chromosomes in humans. The genes on the sex chromosomes and their downstream factors such as sex hormones act on the tissues of the body and produce the sex difference. Typically, the human genome is composed of twenty three pairs of chromosomes. Twenty two of them are autosomal chromosome pairs and the remaining one is the sex chromosome pair. The sex chromosome pair is composed of a combination of X and Y chromosomes. The main difference between X and Y chromosome is that X chromosome is the female sex determining chromosome whereas the Y chromosome is the male sex determining chromosome.

Key Areas Covered

1. What is X Chromosome
      – Definition, Features, Function
2. What is Y Chromosome
      – Definition, Features, Function
3. What are the Similarities Between X and Y Chromosome
      – Outline of Common Features
4. What is the Difference Between X and Y Chromosome
      – Comparison of Key Differences

Key Terms: Pseudochromosomal Region, Sex Chromosome, Sex Determination, SYR Gene, X Chromosome, Y Chromosome

What is X Chromosome

X chromosome is one of the two sex chromosomes in humans. It occurs as pairs in females, but only a single chromosome can be found in males. Therefore, females inherit one X chromosome from both parents and males inherit an X chromosome from the mother. The size of the X chromosome is 155 million base pairs. X chromosome is more than 5 times larger than the Y chromosome. It represents 5% of the whole genetic material in humans. It contains around 1000 genes. X chromosome contains ten times the amount of genes than Y chromosome.

Figure 1: Human X Chromosome

During early embryonic development, one of the two X chromosomes of a female is permanently inactivated in a process called X-inactivation. The X-inactivation occurs through DNA methylation. Since X-inactivation is a random process, either the X chromosome from the mother or the father can be inactivated in the daughter cell. However, some genes on the X chromosome, especially those located in the pseudochromosomal regions, can escape from X-inactivation. The pseudochromosomal region contains genes which are homologous with the Y chromosome. The genes in the pseudochromosomal regions are required by the normal development of the individual. Therefore, these genes occur in pairs. The human X chromosome is shown in figure 1.

What is Y Chromosome

Y chromosome is the other sex chromosome in humans. There is only one Y chromosome in males. The size of the Y chromosome is about 59 million base pairs. Y chromosome represents 2% of the entire human genome. It contains about 70 protein-coding genes. The Y chromosome contains a gene called SYR, which develops the fetus into a male. The other genes in the Y chromosome are involved in male fertility.

Figure 2: Human Y Chromosome

The pseudochromosomal regions of the Y chromosome are homologous with the X chromosome. The genes in the pseudochromosomal region are essential for the normal development of the individual. The human Y chromosome is shown in figure 2.

Similarities Between X and Y Chromosome

X and Y chromosomes are involved in the sex determination in humans.
Both X and Y chromosomes are composed of p arm, q arm and a centromere.
Both X and Y chromosomes contain pseudoautosomal regions at both termini, which act as an autosome.
The pseudochromosomal regions in both X and Y chromosomes contain homologous chromosomes, which can be genetically recombined during the formation of gametes.

Definition

X Chromosome: X chromosome is a sex chromosome that occurs paired in the female and single in the male.

Y Chromosome: Y chromosome is a sex chromosome which is normally present only in male cells.

Sex Determination

X Chromosome: X chromosomes contain genes for female sex determination.

Y Chromosome: Y chromosome contain genes for male sex determination.

Size

X Chromosome: The X chromosome is bigger (about 155 million base pairs).

Y Chromosome: The Y chromosome is smaller (about 59 million base pairs).

Represent

X Chromosome: The X chromosome represents 5% of the entire human genome.

Y Chromosome: The Y chromosome represents 2% of the entire human genome.

Number of Genes

X Chromosome: X chromosome contains more genes (about 1000 genes) than the Y chromosome.

Y Chromosome: Y chromosome contains fewer genes (about 70 genes) than the X chromosome.

Male/Female

X Chromosome: Female contains the XX genotype.

Y Chromosome: Male contains the XY genotype.

SYR Gene

X Chromosome: X chromosome does not contain the SYR gene.

Y Chromosome: Y chromosome contains the SYR gene, which is involved in the development of the testes.

Conclusion

X chromosome and Y chromosome are the two sex determining chromosomes in the human genome. In females, two X chromosomes can be found. In males, both X and Y chromosome can be identified. The main difference between X and Y chromosome is their involvement of sex determination in humans. One of the two X chromosomes in females is randomly and permanently inactivated by DNA methylation. The SYR gene in the Y chromosome is involved in the development of the fetus into a male. Both X and Y chromosomes contain pseudochromosomal regions, which consist of genes for the normal development of the individual. The pseudochromosomal regions of the X and Y chromosome are homologous.

Reference:

1. “X chromosome – Genetics Home Reference.” U.S. National Library of Medicine. National Institutes of Health, n.d. Web. Available here. 11 Aug. 2017.
2. “Y chromosome – Genetics Home Reference.” U.S. National Library of Medicine. National Institutes of Health, n.d. Web. Available here. 11 Aug. 2017.
3. Quintana-Murci, Lluís, and Marc Fellous. “The human Y Chromosome: the biological role of a “functional wasteland”.” Journal of Biomedicine and Biotechnology. Hindawi Publishing Corporation, 2001. Web. Available here. 11 Aug. 2017.

Image Courtesy:

1. “Human chromosome X with ASD genes from IJMS-16-06464” By Merlin G. Butler, Syed K. Rafi, Ann M. Manzardo and Lorie Gavulic (illustration) – “High-Resolution Chromosome Ideogram Representation of Currently Recognized Genes for Autism Spectrum Disorders.” Int. J. Mol. Sci. 2015, 16(3), 6464-6495; doi:10.3390/ijms16036464 PMC 4394543 (CC BY 4.0) via Commons Wikimedia
2. “YChromShowingSRY2” By Je_at_uwo (talk) (Uploads) – Je_at_uwo (talk) (Uploads) (Public Domain) via Commons Wikimedia