Sex-linked Crosses-These are special types of monohybrid crosses in which the alleles controlling a certain trait are located on a sex chromosome. In X-linked crosses the determining allele is located on the X chromosome and there is no corresponding allele on the Y chromosome. Therefore, whatever allele a male inherits on the X chromosome is expressed. In females these traits are determined by the interaction between the alleles on the two X chromosomes. More simply stated, males have no second-or backup---allele for insurance. For this reason disorders determined by X-linked genes are much more likely to occur in males. These include hemophilia (1 in 10,000 males born with the disorder versus 1 in 50 million females), color blindness, androgen insensitivity, and Duchenne Muscular Dystrophy. According to the National Institute of Health (NIH September 2009), there are estimated 900 to 1400 genes on the X chromosome and only approximately 70 to 200 genes on the male chromosome (Y). 

Y-linked inheritance is less common, but does occur. That is, there are some genes on the Y chromosome that have no counterpart on the female chromosome. These include the SRY gene necessary for proper male development.

It is vital that you keep track of the chromosomes on which the specified alleles are located when working with sex-linked traits. Therefore instead of just writing a gene pair as AA, Aa, or aa, the alleles should appear as superscripts on the chromosomes they are attached to. For ex XAXA of XAXa or XAYO. Note that a O is used as a superscript in X-linked crosses to indicate that there is no allele present on the chromosome for the specific trait being studied. 

Hemophilia-Hemophilia is an X-linked recessive genetic disorder in which the blood clots slowly or not at all. The failure of the blood to clot is caused by a missing protein (factor) that is needed for blood clotting. Some forms of hemophilia are caused by alleles present on the autosomes while other forms are caused by alleles present on the sex chromosomes. Individuals with hemophilia are in constant danger of bleeding to death. Today, hemophiliacs are supplied with the missing factor by transfusions. These transfusions enable hemophiliacs to live a relatively normal life. Royal Hemophilia is the most famous hemophilia and it is caused by a sex-linked recessive allele. It is called Royal Hemophilia because of its appearance in many of royal lineages. The disorder was apparently inherited by Queen Victoria as a new mutation in one of her parents. There is no previous history of the disease in the British Royal Family. Queen Victoria's husband (Prince Albert) did not have the disorder. Queen Victoria did not have the disorder, but carried it on one of her X chromosomes. Her offspring married into the royal houses of Germany, Prussia, Russia, and Spain and introduced hemophilia into the their royal houses. Her son Edward (King Edward VII) eventually became King. He apparently did not have the disorder since the current royal family does not carry the disorder and they are his direct descendants. Xh = recessive allele on the X chromosome; XH = dominant allele on the X chromosome; YO = no allele present on the Y chromosome.

Use this information to answer the following quesitons. Using a Punnett square and your formulas for probability will help you solve the problems. 

20.
What was Queen Victoria's genotype?
A) XHXH
B) XHXh
C) XHYO
D) XhYO
21.
What was Prince Albert's genotype?

A) XHXH
B) XHXh
C) XHYO
D) XhYO
22.
What was King Edward's probable genotype?

A) XHXH
B) XHXh
C) XHYO
D) XhYO
23.
What was the probability that Queen Victoria would produce a male heir without hemophilia? Note that you have two probabilites here that the heir would be male and that he would not have hemophilia (remember from your PowerPoint presentation the ace and the heart!).
A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
24.
What was the probability that one of Queen Victoria's daughters would have hemophilia?

A) zero
B) 25% (1/4)
C) 50% (1/2)
D) 75% (3/4)
E) 100%
25.
Can a male child ever inherit an X-linked trait from his father?
A) Yes because all males inheirt one X chromosome and one Y chromosome.
B) Yes because males inherit their sex chromosomes from their fathers only.
C) No because males only inheirt a Y chromosome from their father and an X chromosome from their mother.
D) No because males do not carry X chromosomes. They only inherit a Y chromosome.