Hello everyone and welcome to this week's blog post. In this blog post, I will be discussing chromosome 23. I will first explain what the X and Y chromosomes are and how they differ. I will then explore their evolutionary history, before examining how the differences between the X and Y chromosome determine sex. Finally, I will illustrate some of the diseases associated with this chromosome.
What are the X and Y chromosomes?
Human cells have a diploid number of 46, which means that they contain 23 pairs of chromosomes. 22 of these pairs (called autosomes) have the same length, size and gene order in males and females. The 23rd pair, however, differs between males and females - women have two X chromosomes and men have one X chromosome and one Y chromosome. This is because females inherit an X chromosome from their mother and their father, while males inherit an X chromosome from their mother and a Y chromosome from their father.
Interestingly, all mammals share this method of determining sex. Whilst the sex of birds is also determined by the inheritance of sex chromosomes, they use different chromosomes to mammals. In birds, males have ZZ chromosomes while females have WZ chromosomes (the W chromosome is the shorter chromosome).
What are the structural differences between X and Y chromosomes?
There are some key structural differences between X and Y chromosomes. One of the biggest differences is in their length. X chromosomes are around 155 Mb (155 million bases) long, which means they account for around 5% of the human genome's genetic makeup. Meanwhile, the Y chromosome is around 58 Mb long, which includes over 200 genes that code for at least 72 proteins.
How did X and Y chromosomes develop?
According to the National Human Genome Research Institute, the X and Y chromosome, both originated from a pair of autosomes (chromosomes which have the same length and gene structure). Around 200-300 million years ago, the X and Y chromosomes began to evolve separately, causing the chromosomes to no longer have the same structure. In addition, the Y chromosome also slowly lost the ability to exchange genetic information with the X chromosome. This resulted in the Y chromosome rapidly deteriorating, thus becoming much shorter than the X chromosome.
Some experts theorise that this rapid deterioration will continue in the future. Currently, the Y chromosome contains less than 80 genes. If this deterioration continues, it is possible that the Y chromosome will eventually become useless, or even decay completely.
How do the X and Y chromosomes determine sex?
According to the World Health Organisation, the biological differences between men and women result from the differences in the genes the X and Y chromosomes carry. If the Y chromosome is present, then these genes cause a male sexual differentiation pathway to be followed, causing the offspring to be a man. On the other hand, if the Y chromosome is absent, then the female sexual differentiation pathway is followed and the offspring will be a woman.
There is an abundance of evidence that the Y chromosome is responsible for male characteristics. For example, more than 95% of the genes on the Y chromosome are male-specific, suggesting that these genes are used for this purpose. In addition, in rare cases, some people have four X chromosomes and one Y chromosome. In these cases, the Y chromosome is still dominant, and they still display male anatomy.
During early development, embryos have a group of cells called the embryonic gonads. These cells are stem cells, so can differentiate to form many different types of cell. When present, the Y chromosome causes these cells to differentiate into testes. These testes will eventually produce and secrete a hormone called testosterone, which induces male characteristics throughout the body. Conversely, in the absence of the Y chromosome, the embryonic gonads differentiate to form ovaries
Interestingly, some animals have the ability to change sex during their lifetimes. For example, some clownfish live in groups consisting of one female and several males. If the female is killed, then one of the males will change into a female - their gonad tissue disintegrates and they form female tissue.
What are the drawbacks of this method of sex determination?
One of the major problems of this method of sex determination is that it makes male recessive diseases more common. This is because men only have one copy of each of the genes on the X and Y chromosome. This means that if they receive a recessive gene on these chromosomes from one of their parents, it will always be expressed.
This is called X-linked recessive inheritance, as the gene responsible for the disease is found on the X chromosome. These diseases include haemophilia, Duchenne muscular dystrophy and fragile X syndrome (a disorder that causes intellectual disability).
Meanwhile, females inherit two X chromosomes. Consequently, they have to inherit two versions of the faulty gene to be affected by the recessive disease. While women can still inherit X-linked recessive diseases, they have a lower chance of doing so than men.
Are there any other diseases linked to chromosome 23?
While the majority of people inherit one chromosome 23 from each of their parents as so have two chromosome 23s, there are disorders in which this doesn't occur. For example, Turner syndrome is a disease which affects development in females. It occurs when one of their X chromosomes gets altered or deleted so that they only have one functional X chromosome.
Symptoms of Turner syndrome include premature ovarian failure and infertility, further supporting the fact that sex is determined by chromosome 23. Fortunately, the prognosis for people suffering from the disease is very good. While the life expectancy of those suffering from the disease is slightly lower than average, this can be improved through good medical treatment. In addition, assisted reproductive treatments can also be used to allow sufferers to have children.
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