Investigation: A Human Pedigree
Introduction: A sex-linked characteristic is determined by an allele that is carried only on the X
chromosome. The shorter Y chromosome does not carry an allele for a sex-linked trait. Most sexlinked traits are recessive. Since there is only one X in his genotype, XY, a male who carries a
particular recessive allele on the X chromosome will have the sex-linked condition. A female who
carries a recessive allele in one X chromosome will not show the condition if there is a dominant allele
on her other X chromosome. She will express the recessive condition only if she inherits two recessive
alleles—one from each parent. Her chances of inheriting the condition are thus greatly reduced.
One sex-linked trait is hemophilia, a condition in which the blood does not clot properly. Most
people who have hemophilia are men. In this investigation you will determine why this is so.
Procedure:
Figure A
1. Study the pedigree for hemophilia shown in Figure A. In a pedigree, a square represents a male. If it
is darkened, he has hemophilia; if it is clear, he has normal blood clotting. How many males
represented in Figure A have hemophilia? How many males are normal?
2. A circle represents a female. If it is darkened, she has hemophilia; if clear, she is normal. How many
females in Figure A have hemophilia? How many are normal?
3. A marriage is indicated by a horizontal line connecting a circle to a square. How many marriages are
indicated in Figure A?
4. A line perpendicular to a marriage line indicates offspring. If the line ends with either a circle or
square, the couple had only one child. However, if the line is connected to another horizontal line,
then several children were produced, each indicated by a short vertical line connected to the
horizontal line. The first child appears to the left and the last child to the right. How many children
did the first couple have?
5. Level B represents the second generation. Level C represents the third generation, and so on. How
many generations are represented in Figure A?
6. The genotypes of the males in a pedigree for sex-linked inheritance are easy to determine since
normal clotting (N) is dominant and hemophilia is recessive (n). Since these alleles are on the X
chromosome only, a male represented by a clear square will have the genotype XNY. One
represented by a darkened square will be XnY. Label the genotypes of all the squares. How many
males have the genotype XNY?
7. Females who have hemophilia have an easy genotype to identify. They are all XnXn. Both recessive
alleles must be present for a female to have hemophilia. If one dominant allele is present (XN) the
individual would be normal for clotting. Label all the females with hemophilia as genotype XnXn on the
pedigree. How many women have genotype XnXn in Figure A?
8. Females who do not show the trait for hemophilia may be homozygous dominant (XNXN) or
heterozygous (XNXn). A heterozygous female is called a carrier. Examination of offspring can often
determine which genotype the parents have. If any child (son or daughter) had hemophilia, then the
female must be heterozygous (XNXn). If her son has hemophilia, he has genotype XnY. He inherited
the Y from the father, so the other allele in this genotype (Xn) had to come from the mother. If a
daughter has hemophilia (XnXn), she inherited an Xn from each parent, thus making the genotype of
the normal mother XNXn. Label all females XNXn that have children with hemophilia. What is the
genotype of the female in the first generation?
9. Females who have more than four sons, with none exhibiting hemophilia, are likely to be genotype
XNXN. If she has had four of fewer sons her genotype is less certain. In such cases her genotype is
labeled XNX?. Label the rest of the females in the pedigree as XNX? or XNXN. How many females in Figure
A have the genotype XNXN?
10. The genotype of all people represented in the pedigree should now be labeled.
Figure B
11. Label the genotype of the individuals in the pedigree of Figure B.
12. Fill in Table B based on the pedigree in Figure B.
Table B. Pedigree Analysis
Questions about pedigree in Figure B
# of generations
# of men with hemophilia
# of women with normal blood clotting
# of marriages
# of men with genotype XNY
# of women with genotype XnXn
# of single women
# of people that never married
# of women with genotype XNX?
# of couples with only one child
Answer
Analysis & Conclusions
1. Which sex usually inherits a sex-linked condition? WHY? (BE SPECIFIC!)
2. How can you tell whether a female has a genotype XNXN, XNXn, or XNX??
3. How many genotypes are possible in a pedigree of sex-linked traits? What are they?
Kick it up a notch…The most famous example of hemophilia occurred in the family of Victoria, Queen
of England from 1837 to 1901. She was England’s longest reigning monarch.
Victoria married her cousin, Prince Albert and they had nine children—five girls and four boys.
As you can see in the pedigree below, three of her children inherited the allele for hemophilia. Because
it was common for members of European royal families to marry into other royal families, the
defective allele was carried into the royal families of Russia, Germany, and Spain. The disease has been
called the royal disease, or royal hemophilia. Ten of Victoria’s male descendants had hemophilia.
1. Victoria’s youngest child, Beatrice, had one daughter, one normal son, and two sons with
hemophilia. Beatrice’s daughter, Eugenie, married King Alfonso XIII of Spain. They had six children,
one of whom was the father of Juan Carlos, the current king of Spain. What is Juan Carlos’s
phenotype—normal, carrier, or hemophilic? Explain.
2. Victoria’s daughter Alice had a daughter, Alix, who carried the defective allele into the royal Russian
family when she married Tsar Nicholas II. They had four daughters and one son, Alexis. Alexis had
hemophilia. The entire family was murdered during the Russian Revolution. What is the probability
that one of Alexis’s sisters was a carrier? What is the probability that all four of Alexis’s sisters were
carriers? Explain.
3. If Alexis had lived and married a normal woman, what is the probability that he would have had a
daughter with hemophilia? Explain.
4. Is it possible for a female to have hemophilia? Explain why or why not.
5. What is the probability that the next generation of the present royal family—Charles, Andrew,
Edward, and Ann—will have hemophilia? Explain.
Names: _____________________________________________________________ Date: ____________________________ Per. _______
Investigation: A Human Pedigree
Directions: Below are the pedigrees you will need to answer the questions for this lab. Answer the
questions NEATLY on a separate sheet of paper. Label the pedigrees directly on this sheet. Staple it to
the FRONT of your answers.
Figure A
Figure B
Queen Victoria’s Family Pedigree