Sex chromosomes are the chromosomes that are related to our gender. Human females have sex chromosomes composed of two X chromosomes, and males have sex chromosomes composed of one X chromosome and one Y chromosome. X-linked traits are disorders related to a person's X chromosomes. Both males and females can be affected by X-linked traits; however, males are generally more at risk of being afflicted by an X-linked trait because they only have one X. If one X chromosome is affected in a female, the other X will generally mask the effect.
Using hemophilia as an example, it is possible to look at how these types of disorders can be passed on. A father can pass on a X and a Y allele, while the mother can pass on one of two X chromosomes.
On the Punnett square below, the alleles listed on the top are the sperm a father can supply, and the alleles listed on the left side are the possible eggs a mother could supply. This example shows the mother as a carrier for hemophilia. The X shown in red is the recessive allele that is affected.
Because the mother has a normal X chromosome, any effect the recessive X would have is probably masked. She is only a carrier, and she has a 50% chance of passing this chromosome on to any of her children. The Punnett square shows she has a 25% chance of creating a daughter that is a carrier like her, and a 25% chance of having a son that has hemophilia. She also has a 50% chance of passing on a X that is not affected. She has a 25% chance of having a daughter that is not a carrier and a 25% chance of a son that is not affected.
This illustrates how X-linked traits can be passed on from parents to offspring and why males are generally more susceptible to inheriting these disorders. If they inherit the X chromosome from their mother that is affected, they will automatically get that disease.
Many X-linked disorders follow common patterns of inheritance. There are lots of different patterns of inheritance like this that geneticists can study to understand these diseases and the patterns in which these diseases are passed through generations. Pedigrees are another useful tool in tracking these disorders as well because you can follow the family history of a disease.
EXAMPLEOne type of inheritance pattern is that only daughters can inherit recessive alleles from their affected father because the sons will get the Y chromosome. In other words, if a father is affected, he can only give an affected X chromosome. His daughters, therefore, will either be affected or be carriers.
Source: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND