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Sex Linked Traits

In humans, sex is determined by the inheritance of two X chromosomes (female) or an X and Y (male). Much information is carried on the X chromosome. This creates a unique situation regarding dominant and recessive genes on this chromosome. In females, the expression of one recessive allele can, of course, be hidden by the expression of a dominant allele on the other X chromosome. But in males, if only one recessive allele is inherited, the trait is expressed. This condition, where only one allele is present and will be expressed, is called the hemizygous condition.


When working out crosses with genes carried on the X chromosome, the X and Y chromosome must both be represented. For example, the trait for color blindness is a recessive X linked trait (we will represent it as Xc). If a heterozygous female (X Xc) marries a normal man (XY), the probability that their children will show the trait can be determined using a Punnett square:















Half of the sons will be colorblind, and half of the daughters will be heterozygotes, or carriers of the condition.
We call traits carried on the X chromosome sex linked traits. Traits not carried on the X or Y chromosome are called autosomal traits. Y linked traits are called Hollandric traits. However, due to the lack of genes on the Y chromosome, this type of inheritance is not well characterized and is disputed by many researchers. 

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