The Application of Sex-linked Markers to Forensics
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|✅ Wordcount: 2144 words||✅ Published: 23rd Sep 2019|
The application of sex-linked markers to forensics.
Within the nucleus of every cell, there are thread-like molecules called chromosomes. Each chromosome is an extremely long fragment which contains genetic information. They are each made up of proteins and a single molecule of DNA.
Chromosomes are responsible for determining various things, for example, hair and eye colour, and also the sex of an organism. The absence or presence of a certain sex chromosome decides whether you are a male or a female. The male gametes contain either an X sex chromosome or a Y sex chromosome. Female gametes, however, are homogametic, as they only contain the X sex chromosome. [ThoughtCo, 2019].
Genes which are situated on the X sex chromosomes are known as X-linked genes, and these determine X sex-linked traits. Both males and females are able to inherit X-linked genes, as they both carry X sex chromosomes, whereas as only males inherit Y sex chromosomes, females cannot inherit Y-linked traits. [Www2.palomar.edu, 2019]
As a female carries two X sex chromosomes, she will have two copies of each X-linked gene. [Khan Academy, 2019]
For example, in 1906, Thomas Hunt Morgan discovered that the Drosophila fruit fly, which shares the same sex chromosomes as humans, had a white-eye variant on the X sex chromosome, and a female fruit fly therefore has two copies of this gene. [Www2.palomar.edu, 2019]
If this gene was to come in two different allelles, such as standard red eyes, which is dominant (XW) to the fruit fly, and white eyes, which would therefore be recessive (Xw) to the fruit fly, the female fruit fly could have any of the following three genotypes, red eyes (XW XW), red eyes (XW Xw) and white eyes (Xw Xw). Since a male fruit fly only has one X sex chromosome, he can therefore only inherit one of two genotypes. For example, red eyes (XWY) or white eyes (XwY). Therefore, a male fruit fly has a greater chance of inheriting the X-linked gene of white eyes, as he has no other gene copy.
The same principles can also be applied to human genetics, in the form of conditions such as red-green colour blindness and hemophilia. Males are affected much more frequently than females, as the gene is located on the X sex chromosome.
The identification of sex chromosomes was first identified in beetles by Nettie Stevens, who was born in 1861 in Cavendish, Vermont.
Stevens was one of the first female biological scientists who discovered that sex is determined by a particular configuration of chromosomes. [Encyclopedia Britannica, 2019].
Stevens discovered this by identifying that the female beetle’s gametes had identical pairs of each of their chromosomes, however, the male beetle’s did not. The male beetle’s gametes had one pair of chromosomes, in which they were different from one another.
Although beetle’s and fruit flies share the same sex chromosomes as humans, not all insects are the same. Some insects such as crickets, grasshoppers and cockroaches have a similar method for determining the sex of an organism. Adult males of these insects do not have the Y sex chromosome that male humans have, they only have the X sex chromosome. Therefore, the male gametes will either consist of an X sex chromosome or no sex chromosome at all, which is consequently labelled as O. The females however, are the same as human females, as the female gametes consist of two X sex chromosome.
This is type A of Edmund Beecher Wilson’s sex determination systems which he designed from his observations about chromosome numbers. [Nature.com, 2019].
Type B of Edmund Beecher Wilsons’s sex determination systems was originally known as the system we use for humans, however, future research revealed that males are not always the sex with the abnormality from chromosome pairs. For example, snakes and birds have a ZZ/ZW sex determination system, where the females inherit the non-identical chromosome pair, ZW. [Nature.com, 2008].
The analysis of short tandem repeats is a very extensive application within the forensic science field. Although various articles have studied relevant information about short tandem repeats located on autosomes and Y chromosomal markers, the application of X chromosomal markers has simply only played a minimal part within forensic science so far.
However, X chromosomal genotyping can complement the analysis of autosomal and Y chromosomal markers efficiently, particularly in difficult circumstances of kinship testing. The analysis of short tandem repeats has become a very extensive application within the forensic science field. Whilst various articles have studied relevant information about short tandem repeats located on autosomes and Y chromosomal markers, the application of X chromosomal markers has simply played a minimal part within forensic science so far.
However, X chromosomal genotyping can complement the analysis of autosomal and Y chromosomal markers efficiently, particularly in difficult circumstances of kinship testing. [Szibor et al. 2003]
The Y sex chromosome is more commonly used in forensic DNA analysis in comparison to the X sex chromosome, especially in circumstances where generic autosomal DNA profiling is difficult.
The Y sex chromosome is used for understanding the biological sex of an individual.
If investigating a sexual assault case where there is a mixed profile, from the suspect and the victim, it can be particularly useful to use Y chromosomal short tandem repeat polymorphisms in order to distinguish paternal lineages for the unknown male, which will possibly result in identifying him. [Kayser, 2017]
Other applications for Y sex chromosome within forensics are paternity testing, studying human migration patterns and genealogical research. The discovery of the Y sex chromosome was a huge development in order to understand sexual development. The most important Y sex chromosome gene, which is the sex determining gene, Sry, has been
recommended for a multitude of applications. For example, reprogramming chromatic architecture and the regulation of sex determining genes. [Sciencedirect.com, 2012]
Taxonomy is the science of naming, describing and classifying organisms. For several groups of insects such as the velvet ant, distinguishing between a male and a female can be rather challenging due to their morphological differences. Many species, or even genera, are known from only a single sex. [Jstor.org, 2006].
- NETTIE STEVENS | AMERICAN BIOLOGIST AND GENETICIST
- In-text: [Encyclopedia Britannia, 2019] Encyclopedia Britannica. (2019). Nettie Stevens | American biologist and geneticist. [online] Available at: https://www.britannica.com/biography/Nettie-Stevens [Accessed 18 Jan. 2019].
- SEX CHROMOSOMES AND SEX DETERMINATION | LEARN S CIENCE AT SCITABLE
- In-text: [Nature.com, 2008] Nature.com. (2019). Sex Chromosomes and Sex Determination | Learn Science at Scitable. [online] Available at: https://www.nature.com/scitable/topicpage/sex-chromosomes-and-sex-determination-44565 [Accessed 18 Jan. 2019].
- MALE OR FEMALE? HOW CHROMOSOMES PLAY A ROLE IN SEX DETERMINATION
- In-text: [ThoughtCo, 2019] ThoughtCo. (2019). Male or Female? How Chromosomes Play a Role in Sex Determination. [online] Available at: https://www.thoughtco.com/how-chromosomes-determine-sex-373288 [Accessed 18 Jan. 2019].
- BIOLOGICAL BASIS OF HEREDITY: SEX LINKED GENES
- In-text: [Www2.palomar.edu, 2019] Www2.palomar.edu. (2019). Biological Basis of Heredity: Sex Linked Genes. [online] Available at: https://www2.palomar.edu/anthro/biobasis/bio_4.htm [Accessed 18 Jan. 2019].
- X-LINKED INHERITANCE
- In-text: [Khan Academy, 2019] Khan Academy. (2019). X-linked inheritance. [online] Available at: https://www.khanacademy.org/science/high-school-biology/hs-classical-genetics/hs-sex-linkage/a/sex-linkage-sex-determination-and-x-inactivation [Accessed 18 Jan. 2019].
- USE OF X-LINKED MARKERS FOR FORENSIC PURPOSES
- In-text: [Szibor et al. 2003] Szibor, R., Krawczak, M., Hering, S., Edelmann, J., Kuhlisch, E. and Krause, D. (2003). Use of X-linked markers for forensic purposes. [online] Biotype.de. Available at: http://www.biotype.de/fileadmin/user/Paper/SziborR_2003_IJLM.pdf [Accessed 18 Jan. 2019].
- FORENSIC USE OF Y-CHROMOSOME DNA: A GENERAL OVERVIEW
- In-text: [Kayser, 2017] Kayser, M. (2017). Forensic use of Y-chromosome DNA: a general overview. [online] Available at: https://link.springer.com/article/10.1007/s00439-017-1776-9 [Accessed 18 Jan. 2019].
- WHAT IS TAXONOMY?
- In-text: [Cbd.int, 2007] Cbd.int. (2007). What is Taxonomy?. [online] Available at: https://www.cbd.int/gti/taxonomy.shtml [Accessed 18 Jan. 2019].
- A MOLECULAR METHOD FOR ASSOCIATING THE DIMORPHIC SEXES OF VELVET ANTS (HYMENOPTERA: MUTILLIDAE) ON JSTOR
- In-text: [Jstor.org, 2006] Jstor.org. (2006). A Molecular Method for Associating the Dimorphic Sexes of Velvet Ants (Hymenoptera: Mutillidae) on JSTOR. [online] Available at: https://www.jstor.org/stable/25086326?seq=1#page_scan_tab_contents [Accessed 18 Jan. 2019].
- SEX CHROMOSOMES – BIOLOGY ENCYCLOPEDIA – CELLS, BODY, HUMAN, DIFFERENT, ORGANISMS, LIFE, USED, TYPES, PRODUCE
- In-text: [Biologyreference.com, 1996] Biologyreference.com. (1996). Sex Chromosomes – Biology Encyclopedia – cells, body, human, different, organisms, life, used, types, produce. [online] Available at: http://www.biologyreference.com/Re-Se/Sex-Chromosomes.html [Accessed 18 Jan. 2019].
- SEX-LINKED MARKERS IN THE NORTH AMERICAN GREEN FROG
- In-text: [Lambert, Skelly and Ezaz, 2016] Lambert, M., Skelly, D. and Ezaz, T. (2016). Sex-linked markers in the North American green frog (Rana clamitans) developed using DArTseq provide early insight into sex chromosome evolution. https://bmcgenomics.biomedcentral.com/articles/10.1186/s12864-016-3209-x#Sec1 [Accessed 18 Jan. 2019].
- Y CHROMOSOME – AN OVERVIEW
- In-text: [Sciencedirect.com, 2012] Sciencedirect.com. (2012). Y Chromosome – an overview | ScienceDirect Topics. [online] Available at: https://www.sciencedirect.com/topics/medicine-and-dentistry/y-chromosome [Accessed 18 Jan. 2019].
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