What is hemophilia and how is it passed on?

Queen Victoria’s Legacy

• Queen Victoria (1819-1901) was the longest reigning monarch in Britain’s history. She and Prince Albert had 9 children all healthy except for their 4th son, Leopold.

Hemophilia

• Leopold was born with a serious disorder known as hemophilia.

• Individuals with hemophilia lack a factor in their blood that causes the blood to clot when there is damage to tissues and capillaries.

• These individuals cannot stop bleeding once they start and sometimes bleed to death.

Take a look at her family pedigree starting with Victoria’s parents.

Reminder…

• The circle indicates a __________.• The square indicates a _________.• A circle or square that are filled in

indicates an individual that ________________.

• A half filled in shape indicates that an individual is a _____________ and has a copy of the affected allele but DOES NOT express the trait.

Now looking at your copy of the pedigree…

• Do you notice anything unusual about the inheritance of this trait?

• Is there any trend in the types of individuals that have the trait?

• What do you notice about the carriers?

Hemophilia exhibits sex-linked Inheritance.

• These are traits that appear far more commonly in males.

• Females are carriers of the trait.• These traits are controlled by recessive

genes on the X chromosome.

Your Chromosomes…

• Humans have 23 pairs of chromosomes.• Pairs 1 to 22 are matched pairs that

appear the same under microscopy. These pairs are known as autosomes.

• The 23rd pair are the sex chromosomes and they determine your gender.

• Males have a X chromosome and a Y chromosome.

• Females have two X chromosomes.

So, lets look at how hemophilia was passed down to poor Leopold…Prince Albert was male so he had a X chromosome and and Y chromosome. He did not have hemophilia so his X had the dominant normal allele H. Dad’s Genotype XHY

• Why didn’t we put a H allele on Dad’s Y chromosome? _____________________

If Leopold didn’t get a gene for Hemophilia from Dad he must have gotten it from his mom. This means Queen Victoria must have been a carrier. Queen Victoria was a female so she had two X chromosmes. Because she is a carrier that means 1 of her X’s had a normal H gene and the other X had the recessive hemophilia gene.Mom’s genotype XHXh

Let’s illustrate the cross of Prince Albert and Queen Victoria…

XH Xh

XH

Y

XHXH XHXh

XHY XhY

Now, Let’s determine the gender and condition of the possible offspring.

Normalfemale

Carrierfemale

Normalmale

Hemophiliac male

Why are sex-linked traits so much more common in men?

• Because women have 2 X chromosomes they can have the “bad” allele on one X chromosome balanced out by a normal allele on the other X.

• However, men have only one X chromosome so any “bad” allele that they have cannot be balanced out by the Y chromosome.

Other sex-linked traits…

• Red-green color blindness

• Male pattern baldness

• Duchenne Muscular Dystrophy

Practice

• A woman that is a carrier of male pattern baldness marries a man that has male pattern baldness. Set up the Punnet square to determine the percent chance that the couple could have a daughter with male pattern baldness.

• A man and woman marry and neither of them have Duchenne’s Muscular Dystrophy. They have two sons. One boy is normal and the other exhibits Duchenne’s muscular dystrophy. What is the genotype of mom and Dad?

*

The Pedigree indicates a family with appearances of the genetic disorder Duchenne’s muscular dystrophy. 1 . Indicate the known

carriers in the pedigree by marking half the shape.

2. What is the chance that the female

offspring marked with a star was born carrying the allele for Duchenne’s?