Question

PROBLEM 2 Sopodne Alone ingots aldem Prince Henry Prussian Royal Family - Royal Family dostoso 2 h Felipe Victoria Juan Pablo Miguel Spanish Royal Family do do Prince Prince Peter Zara Princess Princess British Royal Family Figure 6.5 Geen A Conceptual Approach, Seth Edition 2017 W. H. Freeman and Company This pedigree traces Hemophilia B through the interrelated royal families of Europe beginning in the 1800s. Hemophilia B is an X-linked recessive trait that causes excessive bleeding and bruising due to a deficiency in blood clotting factor IX. 1. It is unknown if Edward, Duke of Kent (Queen Victoria's father) had Hemophilia B. What are the possible genotypes of Queen Victoria's parents? 2. Why is Hemophilia B expressed in the male descendants of Queen Victoria, but not her female descendants? 3. Why is Elizabeth II, Queen of England not a carrier for Hemophilia B? 4. Why is Alice of Athlone (in Generation IV) a carrier for Hemophilia B?

Answer 1

1. Queen Victoria (in Generation II) is a carrier of Haemophilia B. Let us indicate the chromosome carrying the recessive gene for X linked Haemophilia B as X_H. Therefore Queen Victoria's genotype would be X_HX.

Therefore the possible genotypes of Queen Victoria's parents could be

a) X_HX and XY OR b) XX and X_HY

2. A female has two X chromosomes, while a male has one X and one Y chromosome. As the mutation is X linked recessive, even if a female descendant gets one of the affected chromosomes, the equivalent dominant allele for blood clotting factor IX on the other chromosome expresses itself and produces clotting factor. On the other hand, a male has only one X chromosome. Therefore, if the male descendant gets affected chromosome from his mother, there is no dominant allele for clotting factor on his corresponding Y chromosome to cancel the effect of recessive allele. Therefore, Haemophilia B is expressed in male descendants of Queen Victoria but not in female descendants. So Haemophilia B is expressed in Queen Victoria's son Leopold (in Generation III).

3.Queen Elizabeth II, the Queen of England (in Generation VI) is not a carrier because her mother (from Generation V) is not a carrier and her father (from Generation V) too is not affected; that is neither of her parents has a chromosome X with the recessive gene.

4. Haemophilia B is expressed in Queen Victoria's son Leopold (in Generation III). Therefore his genotype is X_HY. His wife is not a carrier of Haemophila B. So her genotype is XX. Alice of Athlone (in Generation IV) is their daughter. Therefore her genotype would be X_HX. As one X chromosome carries the recessive allele for Haemophilia, she is not affected by the condition but is a carrier for the condition.