What is the difference between x-linked dominant traits and x-linked recessive traits? Illustrate with an example. 

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mvcdc | Student, Graduate | (Level 2) Associate Educator

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Dominant traits or characteristics are those that only need one copy of the gene in order to surface as a phenotype - that is, in terms of disorders, a single copy of the gene or allele will cause expression of the disorder. In contrast, recessive traits or characteristics are those that need to be present in both chromosomes in order to be expressed.

X-linked traits means that they are expressed on the x chromosome. We should note here that females have two x chromosomes (XX), while males have one (XY). This difference in the type of chromosome reveals the difference in the genders in the genetic level - and also the difference in how they are affected by x-linked genes. 

Hence, to answer the question, an X-linked dominant trait is a dominant characteristic located in the x chromosome, while an X-linked recessive trait is one that is recessive and located in the same chromosome. X-linked recessive traits usually easily affect males as they only need one of the genes to be present (recall that they are XY) - these are the sex-linked diseases such as red-green color blindness, hemophilia A, and mascular distrophy. This does not mean that females aren't affected by these diseases, but they are less likely to get them since they need both genes to be present. On the other hand, in the case of x-linked dominant traits, the disease does not necessarily affect males more than females, as since it is dominant, both genders only need a single copy of the gene to get the disorder. Examples of this are Vitamin D-resistant rickets, Rett syndrome and Goltz syndrome.

As an illustration, consider an x-linked recessive trait, say b-disease.  Also consider an unaffected father, and carrier mother (unaffected father means `X^o Y` , while carrier mother `X^b X^o` ) The possible offsprings then are: `X^o X^b, X^oX^o, X^bY, X^oY.```

Hence, we have 2 unaffected girls (note that disease is recessive), 1 unaffected boy, and 1 affected boy. If the disease where dominant, we wold have an affected boy and girl.


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