Sex Linked Genetic Disorders – Chromosome Theory and Sex Linkage

by Georgina Cornwall, PhD

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    00:01 That brings us to a little bit of a question and thinking about things that are linked on the X chromosome. We call them X-linked traits or sex-linked traits.

    00:12 You are probably familiar with at least one of these being color blindness. We know color blindness is much more frequent in males. There is red green color blindness at least in males than in females and in general, when you see something that is more common as a phenotype in males, it is probably X-linked or sex-linked. Why do we see this skewed ratio that color blindness is more frequent in males? It is an X-linked recessive case. In this example, we are crossing a carrier female. The female has the allele for color blindness, the recessive B and she can pass that onto her sons but not her daughters. In this case, if the father had actual color blindness, then there is a potential for the daughter to have color blindness, but because the male only has one X. If they get the allele, then they are color blind. There is no possibility of covering it up.

    01:15 Another condition that you may be familiar with is hemophilia. Hemophilia occurs when blood does not clot properly and so someone who has hemophilia, if they get cut they could bleed out. Now we can actually take some clotting factors to make sure that the blood clots properly and so it is not as big of an issue as it has been historically, but hemophilia has been very well mapped throughout the European royal families. You've probably heard to that before too. The point here though is if we have a carrier female and an affected male, you can see how a homozygous recessive form would end up in a female having hemophilia, but that would mean that the mother has to be a carrier and the father actually has to have hemophilia in order to produce that homozygous recessive phenotype or genotype in the female whereas in males, it is pretty easy to pick up. Again only copy is necessary.

    02:19 What is it look like if we have an X-linked dominant situation? We don't see quite the same thing. Most of them seem to be recessive, but if we had X-linked dominance like we see in the condition that causes brown teeth, which is amelogenesis imperfecta. People's teeth are browny. They are not making one of the correct components of tooth enamel.

    02:43 You can see that if the affected female, she could be heterozygous or homozygous, but most likely she would be heterozygous. She is affected. She has browny teeth and if you cross that individual with and unaffected male, then you would find that you have about 50-50 between males and females. You don't see the same impact of mostly males being affected.

    03:11 Again most of these X-linked traits are going to be homozygous recessive type of traits or recessive alleles. So we are dealing with this X, Y, sex-linked business we know that the Y chromosome is tiny and the X chromosome is much larger contains a lot more information. We can ask this question as I do all of my classes. Do women just have too much stuff on the X chromosome as there are too much baggage or do men does not really have enough? It is a unique question because in truth what we were going to see is there is a dosage compensation that takes place in order for us to have an equal amount of information, but the differences could explain some of the changes we see between males and females. The Calico cat is a great example of a genetic mosaic, which is what results from dosage compensation. Basically, one of the XX is going to become inactivated. There are two genes involved in color in the Calico cats. It is some sort of epistatic thing again where we have a biochemical pathway in order to produce color and we have either the allele for no pigment, which would get in the way of expression of pigment or we could have a black fur, orange fur allele. In the areas where the black fur and orange fur allele are expressed, we will see an example of X inactivation. In X inactivation what happens? This is only going to happen in females because we have two X chromosomes and with two X chromosomes in every single cell, one of those X chromosomes will become bound up. The one that becomes bound up is called the Barr body and the other one that is expressed will maintain and that gene is accessible to all machinery of the cell to be expressed as orange in this case and the Barr body is not accessible for expression. The X chromosome on the allele for black fur could be the one that is active and the orange one could be inactivated in which case you would have production of black fur from all of the cells resulting from mitosis of that original cell where the Barr body was created. That is what brings us to the Calico cat. We have an area where the black is inactivated and the orange maintains itself actively. We have the orange fur and contrarily, if the black one is activated and the orange inactivated, we end up with black fur. Pretty neat genetic mosaic. Another example of where we will see genetic mosaics. In females, we tend to have two options of sweat gland expression and not sweat gland expression, you can see mosaics of patches of skin that sweats versus patches of skin that don't whereas males will have an expression of the one X chromosome and sweating all over the skin. Again females could have mosaics in the sweating patterns, but if they are homozygous, you don't have that issue at all. Genetic mosaics result from those compensation and X-inactivation pedicle system.

    About the Lecture

    The lecture Sex Linked Genetic Disorders – Chromosome Theory and Sex Linkage by Georgina Cornwall, PhD is from the course Understanding Genetics.

    Included Quiz Questions

    1. The X-inactivation of one of the female X chromosomes to aid in not having two allelic copies compared to males
    2. The Y-inactivation of one of the female Y chromosomes to aid in not having two allelic copies compared to males
    3. The X-activation of one of the male X chromosomes to determine male sex
    4. The Y-activation of the male Y chromosomes to determine male sex
    5. The autosomal inactivation of recessive alleles so that those alleles do not manifest
    1. Amelogenesis imperfecta
    2. Color blindness
    3. Christmas disease
    4. Hemophilia A
    5. Duchenne muscular dystrophy
    1. Males will have a 50% chance of having the disease.
    2. Females will have a 50% chance of having the disease.
    3. All males will have the disease.
    4. All females will have the disease
    5. All offspring will have the disease
    1. Both female and male progeny have a 50% chance of having the disease.
    2. All of the female progeny will be affected.
    3. All of the male progeny will be affected.
    4. 50% of male progeny will be affected while female progeny will be unaffected.
    5. 50% of female progeny will be affected while male progeny will be unaffected.

    Author of lecture Sex Linked Genetic Disorders – Chromosome Theory and Sex Linkage

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD

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