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| Figure 1: Genetic Inheritance of X Chromosome Sex Linked Conditions (Source: Boundless, 2019) |
Today’s blog post is dedicated to expanding on previously discussed Mendelian genetic content, specifically addressing sex-linked genetic inheritance. Prior to reading this post it is important that you have the appropriate background to understand this post’s content. You will need an understanding of Mendelian genetics, dominant and recessive alleles, as well as how genetic inheritance is understood through Punnett Squares. If you do not already understand these terms or have the required background please click on the hyperlinks provided in the previous sentence so you can read and catch up on these topics.
Sex linked traits are genes present on a sex chromosome (typically the X or Y chromosome). These chromosomes are often referred to as the sex chromosomes because they determine the sex of individuals. Males carry one X and one Y chromosome, whereas females carry two X chromosomes. These chromosomes not only determine biological sex of the individual, but they also genetically affect the individual in other ways since both chromosomes carry genetic information. The X chromosome, which is larger, carries approximately 1098 genes, while the smaller Y chromosome carries 26 genes, which largely relate to sperm production and other nominal activities.
There are a variety of X linked genetic conditions out there. These include but are not limited to hemophilia, red-green color blindness, congenital night blindness, some high blood pressure genes, Duchenne muscular dystrophy, and also Fragile X syndrome. Women can either display/present with these conditions or they can be carriers, the latter of which results in them having (or carrying) one recessive allele (or copy) of the gene, but men will always express the X chromosome genetic condition since they only carry one X chromosome.
Males receive their X chromosome from their mothers (a result of the fact that be born a male their father’s must provide the Y chromosome), which in turn explains why X chromosome conditions in males are often blamed on the mother. Since males only carry one X chromosome they are forced to express whichever allele of the gene (be it dominant or recessive) is present on the X chromosome. This means that males have a 50/50 chance of expressing either a dominant or recessive allele for X chromosome conditions, while women do not (Figure 1). Due to the presence of a Y chromosome in males they are the only individuals who can carry and pass on Y linked genes. Y chromosome conditions can only be present in females if and when they carry the Y chromosome, which is typically a third copy of a chromosome.
References
Boundless. (2019, November 19). 12.2E: Sex-Linked Traits . Retrieved from Biology: Libre Texts: https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/12%3A_Mendel%27s_Experiments_and_Heredity/12.2%3A__Patterns_of_Inheritance/12.2E%3A_Sex-Linked_Traits
Easter, C. (No Date). Sex Linked. Retrieved from National Human Genome Research Institute: https://www.genome.gov/genetics-glossary/Sex-Linked
O'Neil, D. (2012). Sex Linked Genes. Retrieved from Palomar Anthropology Biology Basics: https://www2.palomar.edu/anthro/biobasis/bio_4.htm
10 comments:
I learned this in Biology in high school. Boys only have one X chromosome that's why its more likely for them to be color blind. I never understood why the men blames Woman when he wants a boy its the mans fault.
Does this goes into how Dr. can figure out the sex of a baby before it is born?
-Jesse Logan
No. A genetic test taken in utero is only done when absolutely necessary because it endangers the viability of the fetus. A doctor simply looks for the presence of a penis in an ultrasound to identify sex of a fetus.
I found this article very interesting because I never really understood how certain genes were given. How are eye colors determined? I know brown eyes are dominant, is there any certain reason why?
-Alexa Holzschuh
Eye color is a trait covered by various genes, not a single one. Regarding why certain traits are dominant over others I cannot say. It's one of those diversity factors that help provide species with enough genetic diversity to ensure its survival.
This was always fun to me all throughout school. I loved learning about punnet squares, chromosomes, and alleles; basically anything that had to do with traits. I just wonder which traits are dominant when it comes to hair color, eye color, hairlines, etc.
Mya Bursey
I am currently in a Genetics class as well and we have talked about this gene expression rather in depth. A point that there are much more diseases passed from Father to daughter then an other expression because of the different and lack of genes so that the the fathers mother will give through the X chromosome which is then passed to the daughter, I thought it was neat and interesting fact because of the only one X chromosome in males.
Qwynn Marquez
Informative blog about linked traits. It never occurred to me that men carry more genes than women due to the X chromosome. Also it was informative to know that men are more prone to be color blind.
I never knew that men were more likely to be color blind than women. Informative Read!
-Jasmine Hill
It's interesting to find out that men are more likely to be colorblind than women. I never knew that.
- Jada Watkins
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