Answers to hemophilia case study :

1. a) Alice's son could inherit either a defective allele from his mother or a normal one. Thus, the probability that he was a hemophiliac is 1/2.

b) The probability that Alice's daughter is a carrier is 1/2. Assuming no mutations in this trait, the probability that she is a hemophiliac is 0, because unlike her brother, she had to inherit defective alleles from both parents in order to express the trait. As you can see on the pedigree, her father was normal.

c) The probability that the boy was a hemophiliac is 1/2, and the probability that the girl was a carrier is also 1/2, therefore 1/2 x 1/2 = 1/4. (Multiplication rule)

2.   Eugenie had two hemophilic sons; this identifies her as a carrier. Juan Carlos is almost certainly normal. His father was apparently normal, as was his mother. Eugenie was the one who spread the disease to the Spanish royal family.

3.  a) 1/2 x 1/2 x 1/2 x 1/2 = 1/16.

b)  Since Alexis had the male XY chromosome arrangement, his daughters would have necessarily received his defective X chromosome. But in order to be a female hemophiliac, two defective X's must be present. Since Alexis' bride was stated to be normal, none of the daughters could be hemophiliac (probability = zero) but,

c)  all would be carriers.

d)  All of Alexis' sons would have to be normal. Sons have the XY chromosome arrangement. The Y they receive from their father (Alexis); he can't pass on his defective X chromosome to sons - only to daughters. (Notice in the pedigree chart that the same result occurred when Queen Victoria's hemophilic son, Leopold, had a normal son and a carrier daughter.)

4.  a) Eugene's mother, who is supposedly Anastasia's daughter, would be a carrier because Eugene would hardly claim his father was a hemophilic if he wished to claim a royal lineage. Similarly, he would claim his grandmother as a carrier, rather than his grandfather. The chances are very good that he is correct about the maternal ancestry of the trait; hemophilic males seldom survived to reproductive age. But this hardly establishes his credentials for the monarchy. The royal family was not the only Russian family affected by hemophilia (nor is "Romanov" a last name limited to the royal family).

b)  Traditionally, the last name is passed from a male to male, while, genetically, the alleles for hemophilia are passed to a male through a female. Thus, Eugene would inherit hemophilia from his grandmother (whom he believes is Anastasia) and his last name from his father, who, in his turn received it from his father, and so on.

5.  A look at the family tree indicates that the hemophilic trait could not be passed to the children from the father's side of the family. There is no evidence that any of the males in the family - King George VI, Prince Philip, or Prince Charles carried the defective X chromosome. If they had the trait, it would have been displayed; males cannot be normal carriers and hide the trait. There are three other possibilities: (1) Princess Diana might have been a carrier, (2) a mutation might have occurred (as it did before in Queen Victoria), or (3) the child might have been conceived by different parents than Charles and Diana. The latter, of course, is inconceivable.