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ID 16315

Animation 13: Mendelian laws apply to human beings.

Description:
Queen Victoria explains pedigrees using the royal family and its inheritance of hemophilia.
Transcript:
Her royal majesty, Queen Victoria of England! We shall tell you about pedigrees; ours is most detailed.We will focus on that part of the family line leading up to the Russian Royal House -- the Romanovs. Pedigrees are most useful for tracing lineage, and since the mid-1800's (during my reign), pedigrees have been adapted for tracking human genetic heredity. Females are represented by circles; males by squares. A horizontal line connects those who marry, and vertical lines lead to children begotten from the marriage. Albert, my dear Prince Consort and I, had nine children for the glory of England. To our sorrow, our line was tainted with the royal bleeding disease, hemophilia. Since the early 1800's, hemophilia was believed to be an inherited disease affecting males. I did not have hemophilia, but it seemed that I carried and thus passed the trait to my descendants. In a genetic pedigree, individuals known to be affected by a trait are filled squares (or circles). Carriers are dotted. I, Queen Victoria, was a hemophilia carrier. Of our nine children, poor Leopold had hemophilia. We know Alice and Beatrice were carriers because hemophilia was seen in their male children. Of my grandchildren from Alice, one had hemophilia, two were unaffected, and two were carriers. 'Twas most unfortunate, my granddaughter, Alexandra, was a carrier. She married Nicholas Romanov II, Tsar of Russia. They had five children. My great grandson, the heir Alexei, had hemophilia. Tragically, the whole family was killed in 1918 by the Bolsheviks during the Russian revolution. You may be more familiar with this story if I told you that one of Alexei's sisters was Anastasia. Of course, since the whole family was killed, it is not known if any of Alexei's sisters were carriers for hemophilia. If you look at our pedigree, only the princes are affected and some of the princesses are carriers. This, we've been told, is a classic example of sex-limited or sex-linked recessive inheritance. A sex-linked trait is on the X chromosome. Females have two X chromosomes; they can inherit/carry the trait without being affected if it acts in a recessive manner. Hemophilia is just such a trait. Males only have one X chromosome so if they inherit the affected X, they will have the disorder -- just like poor little Alexei. Follow my affected X chromosome through our pedigree. I'm Charles Davenport. In 1907, I published the genetics of eye color. Dark eye colors are inherited in a dominant manner. Light eye colors are inherited recessively. Let me show you. We start with a brown-eyed woman whose whole family has had brown eyes for several generations. She is homozygous for brown eyes. She marries a blue-eyed man whose whole family has had blue eyes for several generations. He is homozygous for blue eyes. They will have children with brown eyes. Brown is the dominant eye color. If one of their heterozygous children marries a person also heterozygous for eye color, most of their children will have brown eyes but some will have blue eyes. The proportion of brown to blue eyed children will be 3:1; just like the dominant vs. recessive ratio Mendel observed in his pea plants. In a pedigree, dominantly inherited traits, for example brown eyes, can look like this. Pedigrees of recessive traits, for example blue eyes, can look like this. Notice how the trait skipped a generation. Human genes do follow Mendelian laws. However, humans are not peas. Although I tried to explain eye color inheritance as a single gene system, scientists now believe that two or three genes are involved in eye color determination./dnaftb/concept_13/con13anigene.html Also, the particular shades of an eye color may be a result of the variable expression of these genes in an individual.
Keywords:
queen victoria of england, mendelian laws, nicholas romanov ii, hemophilia, bolsheviks, pedigrees, heredity, royal family, heir
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