8 Jan 2023


Sex Chromosomes: Definition, Function & Types

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Academic level: College

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One pair of the twenty-three chromosomes is made up of the sex chromosomes. These chromosomes; X and Y, are usually contributed by each parent and it is through their coding that the sex of an individual is determined. Females carry XX chromosomes, while a pairing of XY chromosomes are found in males. The Y chromosome depicts the distinction between women and men while the X chromosome enables the differences complement each other. This paper will focus on the Y chromosome, compare the two in terms of size and genetic number and how the Y chromosome determines the maleness of a human; and finalize with a descriptive explanation based on the nature of a calico cat. 

The presence or absence of Y chromosome determines the final sex of the offspring produced. The chromosome contains the SRY gene, which is responsible for the development of the testicles. This chromosome is passed to the son from the father. For many generations, the size of the Y chromosome has always been large and contains many genes called Y-linked genes. In addition, the X chromosomes have genes called X-linked which are termed as being dominant for they have no Y- chromosome counterpart due to their large quantities. Research, however, has pointed out the deteriorating nature of the Y-chromosome to a smaller size and up to only 80 functional genes (Hamilton, 2010). 

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The Y chromosome is the key determinant of the maleness of an offspring (Bainbridge, 2003). The SRY gene; located at the shorter arm's tip of the chromosome usually activates the male genes and inhibits the female genes. In such a case, male gonads like testis will be triggered rather than ovaries. Y chromosome comprises of twenty six genes in which nine of them are for sperm production activity while the remaining sixteen genes are responsible for maintenance of the cells

In cats, the X chromosome is responsible for the fur color. This aspect explains best why most calico cats are female since a cat must have two X chromosomes for it to be calico. There are two alleles in the calico cats; that is B and b. an inactivated B allele gives black fur since only b allele is expressed while an orange fur is as a result of b allele being inactivated while B allele is active. At an earlier level of development, one of the two inactivates (X-inactivation) and supercoils to Barr body structure through a mechanism that regulates the distinction of the X chromosome dose in males and females. This process is referred to as dosage compensation which occurs through the Lyon hypothesis (Graves, 2006) . This would lead to heterozygous feature with a mosaic expression. A male cat can also inherit two X chromosomes, under gorelyonisation to have calico feature, though this is a rare feature (Graves, 2006). The patchwork coat color is usually a result of earlier lyonisation or the X-chromosome inactivation in the opposite cells (Graves, 2006). 

In conclusion, it is clear from the above discussion that the X and Y chromosomes are very vital when it comes to the sexual setup and appearance in mammals. They basically consist of instructions which are genetically related and made up a molecule of DNA and proteins. Sex chromosomes undergo processes due to the pressure of their evolution, such as male meiotic inactivation XY and soma inactivation of the X chromosome in females. Despite the impeccable study research are still unable to explain some cases of infertility like do the blockage of spermatogenic curb mutation of X linked genes at early stages of life in men? 


Bainbridge, D. (2003). How the Y chromosome controls our lives. London: Harvard University Press. 

Graves, M. (2006). Sex chromosome specialization and degeneration in mammals. New Jersey: Transaction Publishers. 

Hamilton, J. (2010). How the gene content of human sex evolved. Washington D.C: ABA Book Publishing. 

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StudyBounty. (2023, September 16). Sex Chromosomes: Definition, Function & Types.


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