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Challenge Question 1

Aneuploidy resulting in the loss of an entire chromosome usually results in a non-viable embryo. However, if the chromosome concerned is the X-chromosomes the embryo may live.

Explain why the loss of an entire autosome is almost always lethal but the loss of the X-chromosome may not be lethal.

 Newlands College-  A.K. 19.05.11 4pm

Because in humans only one x chromosome is active in each somatic cell, this still leads to a viable embryo, however there are some differences noted in a person with Turner Syndrome


You are correct in identifying that the key idea here is that only one x-chromosome is active in somatic cells.  The question asks you to explain why the loss is almost always lethal in autosomes, but not when it is the X-chromosome - therefore your answer needs to address why we see this difference in affect.  

Onehunga High School - C.Whiteman and J.Leong 19-05-2011

Losing an X-chromosome is not as lethal as losing an entire autosome because all females already have an inactivated X-chromosome in the Nuclei of all cells except germ cells. Therefore losing an X-chromosome is will not cause as much effect as losing an entire autosome because one of the X-chromosomes is already inactivated (Barr body). An effect of losing an entire autosome is not allowing the embryo to develop because the potentially harmful mutations are all expressed because the other chromosome is not there to mask the effect of the mutations by a dominant allele. Losing the X-chromosome causes Turner syndrome, which causes difficulty with spatial memory, low productive hormones and infantility.

You have correctly identified x-inactivation as the key idea here and explained why an embryo can remain viable with only one x-chromosome. It is interesting to note that Turner syndrome is the only viable monosomy in humans. While in some cases having one copy of a gene only is adequate for normal cellular function, many require both copies, therefore loss of a chromosome (containing thousands of genes and the non-coding but essential components) creates a situation where normal cellular function will not be supported.

 Sacred Heart College - Andrew.M  25/05/2011

Each autosome in the normal human genome has another corresponding homologous chromosome containing another set of alleles for the same genes. For the human to be formed properly, both copies of the autosome must be present, so that the correct amounts of the proteins coded for by the genes is created. Therefore, when aneuploidy occurs in an autosome the resulting cells in the embryo will create insufficient amounts the proteins required for growth. The result is a non-viable embryo, and so this aneuploidy is lethal.
The sex chromosome genotype in males is XY, where the Y results in the ‘male’ phenotype, while females have the genotype XX. In males, both the X and Y chromosomes are switched on in each cell, as the proteins coded for by the genes on both of them are necessary to form a fully functioning human being. This means that aneuploidy of the X chromosome in males will also lead to a non-viable embryo being formed, and so will still be lethal
However, females have two copies of the X chromosome, and so each chromosome contains a full set of genes for the formation of a female. This means that it is only necessary for one of the chromosomes in each somatic cell to be switched on, for a fully functioning female to be formed. Therefore, if one of these X chromosomes is lost through aneuploidy, this will just mean that the same X chromosome must be turned on in each somatic cell, and so a viable embryo can still be formed. While this means that the mutation is technically not lethal, the result is a female with Turners syndrome (XO). Turner’s syndrome patients experience many symptoms, including physical distortions, and have a shortened life expectancy. Therefore, while Turners syndrome may result in a live birth, it is still ultimately lethal, due to the lowered life expectancy.


Thank you Andrew - you have covered the key points clearly and importantly have noted that although not lethal, the lack of the X-chromosome does have some detremental effects on the phenotype.

 Sacred Heart College - GerryT - 28th May 7.30a.m.

In aneuploidy, when an autosome is lost, there is an insufficiency of genetic material required to produce the necessary quantity of protein for the cell to function. In a normal human, as there is a presence of a pair of each autosome, the necessary quantities of protein synthesized are correct and allows for appropriate development. Thus, losing an autosome means that there is insufficient genetic material, leading to insufficient protein content and affecting the development of the embryo, always resulting in a non-viable embryo.

However, loss of an X chromosome does not result in a non-viable embryo, as opposed to the loss of an autosome, which does. If the embryo has only 1 X chromosome (ie. a male) the embryo will fail to develop. If it is a female however, it will have 2 X chromosomes and a loss of either of them will not result in a non-viable embryo solely because of X-inactivation. X-inactivation occurs as a form of dosage compensation so that females, which have twice the number of X chromosomes as compared to males, will not have twice the dosage of phenotypes displayed as compared to males. In essence, one of the X chromosomes will be packed in heterochromatin instead of euchromatin, and forms a discrete condensed body known as the Barr body, and its genes are not expressed, leaving one X chromosome to express its genes. Thus, the loss of an X chromosome will not result in a change to insufficiency in the proteins synthesized based on the X chromosome, as the female originally expresses one of those X chromosomes. Losing one of the X chromosomes will just cause the other X chromosome to step into its place and be expressed. Therefore, the loss of an autosome will result in a non-viable embryo, but the loss of an X chromosome might not affect the embryo, more specifically, if it is a female.


Thank you Gerry - Your answer is clear and covers all key points. Good to see a full explanation of the Barr Body.

Horowhenua College - Student JWilton - 1/6/2011 3:25pm

Sex chromosomes determine the sex of the individual while autosomes are any chromosomes that are not sex-determining. Male and female karyotypes are similar, but the sex chromosomes mark the distinct difference in their karyotypes as females have XX chromosomes and males have XY chromosomes.

When an X chromosome is lost, it is only lethal if the embryo has a Y chromosome (for males) as the Y chromosome doesn't carry as much genetic information as the X chromosome and the embryo will become a non-viable embryo. However, if the embryo has one X chromosome, the result of the loss of the other X chromosome will not be lethal to the embryo. The embryo will develop with Turner syndrome and will be female with distinct differences resulted from the loss of the X chromosome.

In humans, only one X chromosome is needed to be active in somatic cells. Therefore, if the developing embryo were to have been male, because it doesn't have an X chromosome and the Y chromosome is left, it develops into a non-viable embryo and dies. The loss of an autosome, however, is lethal. This is because when protein synthesis occurs, the resulting proteins are not the sufficient amount needed for the embryo to develop and function properly.


Good to see a contribution from Horowhenua. Your introduction to the question offers a clear description of the difference between male and female karyotypes and links your answer to this. Remember that the genes on the Y chromosome are not a subset of the genes on the X chromosome. I am not sure if you meant to imply this?

Rotorua Boys' High School - HGosai - 1st June 8.53pm

An autosome is a chromosome that is not concerned with the determination of the gender. A sex chromosome, however, is concerned with the determination of the gender. It is said that the loss of an autosome is almost always lethal but the loss of an X-chromosome may not be. The aneuploidy occurs mainly due to the non-disjunction occurring in the meiosis or mitosis of the gamete and embryo respectively. The non-disjunction creates two cells: one with all chromatids in it to form a trisomic cell and another cell with no chromatids on the specific chromosome. This results in the absence of a chromosome, autosomal and sex. With the absence of a single autosomal chromosome, the possibility of a recessive mutated allele will now be exposed. The absence of the chromosome may cause a disturbance in the protein synthesis due to the alteration in the produced polypeptide chains. Consequently, this produces a non-functional protein which cannot take part in the metabolic pathways. This will cause a build-up in the precursor or intermediate substrate, as a result will cause disease and possibly death. On the other hand, a loss of X-chromosome may not be lethal. In a female embryo, a homologous pair of X-chromosomes is present. If one of the X-chromosomes is lost, the other X-chromosome may code for the same polypeptides and subsequently, genes. However, the proteins produced will not function at its proper function, as in Turner’s Syndrome (XO) where the sexual organs are not fully functional. If the another X0chromosomes contains mutations, non-functional protein(s) may be produced which will cause disturbance in the metabolic pathway(s) present. In a male embryo, the presence of a X-chromosome is extremely important. The Y-chromosome is a lot smaller than the X-chromosome so the X-chromosome will contain more loci and genes. The loss of the X-chromosome, however, will result in the loss of several polypeptide chains, causing many non-functional proteins. This will cause death as many metabolic pathways are disturbed.


Thanks for your contribution. You have clearly linked the loss of a chromosome to disruption in protein synthesis and metabolic pathways - and therefore to changes in the phenotype. Remember that the genes on the X chromosome are unique to that chromosome - the Y Chromosome is not a subset of X - so when you lose X in the male, this is complete loss of both coding and non-coding DNA on that chromosome. It is only because of X inactivation that the XO female can survive. To add value to your answer-you need to explain x-inactivation in females.

R. Bothara Newlands College 2nd of June, 2011

Aneuploidy can occur either in the autosomes or the sex chromosomes. However, if an aneulpoid has a loss of an entire autosome it is lethal but the loss of only the X-chromosome is not lethal. Turner and Klinefelter Syndrome are two types of aneuploidy that are a result of a loss or addition in the sex chromosomes. In Turner Syndrome the X chromosome is either removed or altered. In Klinefelter Syndrome there is an extra X chromosome in the sex cells.

Autosome chromosomes are all the chromosomes in a cell apart from the sex chromosomes. They contain genes that code for specific processes and functions. As well as that, specific genes are dependant on other genes on other autosomal chromosomes that work together to carry out processes. Coding for instructions and processes are distributed all over the autosomal chromosomes, hence they are inter-linked and are dependant on each other for normal functioning.

Sex chromosomes are chromosomes that are responsible for the sex of the offspring. The X chromosomes are always much larger than the Y chromosomes. The X and Y chromosomes are different because the Y chromosome contains genes that when turned on result in a male offspring. It contains genes that bring male characteristics (sperm production, etc.) into active mode. Male and Female karyotype are different because a female has two X chromosomes whereas a male has X chromosome and Y chromosome. It is the Y chromosome in the male that creates the difference between male and females.

In males both the Y and X chromosomes are turned on as both are essential in the normal functioning of a male. However, in females only one X chromosome is active in every cell and the other is inactive and forms a barr body. This means that if an aneuploidy occurs on the X chromosome of a male it will be a non-viable embryo being formed which is lethal due to the loss of the genetic material. Klinefelter Syndrome is where due to an addition of one or more X chromosome imbalances occur. The extra genetic material from the X chromosome will interfere with the sexual development and results in reduction of level of testosterone. However, if an aneuploidy occurs in a female it is not as lethal. Females have two sets of the X chromosomes so a loss of one just means that the other one needs to be turned on. This is called Turner Syndrome; the embryo is still viable but the missing genetic material affects development and causes characteristics such as short stature and infertility. So, although it is a viable embryo it will not be able to function like a normal fertile female.

The loss of an entire autosomal chromosome is lethal and will result in death. A missing autosomal chromosome will carry genes that code for specific instructions, since they are all interlinked when one is missing the others become useless and basic functions cannot be carried out resulting in the non-viable embryo. The loss of a X chromosome however in not lethal in the case of females but in males it is still lethal. X chromosome contains thousand of genes, much more than the Y chromosomes so the loss, alteration or addition of it will still result in imbalances.


Thank you for a comprehensive answer. You use of Klinefelter Syndrome as a comparison helps demonstrate the effect of the X chromosome but also highlights the disruption caused by extra copies. While not directly required in the question, it has added value to your explanation. It is important to note that the loss of a chromosome means the loss of both coding and essential non-coding DNA.