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 sex-linked traits
are recessive. Since there is only one X chromosome in the male, 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 have 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 - from each parent. Her chances of inheriting a sex-linked condition are thus
significantly less than those of a male.
Procedure
Study the pedigree for hemophilia shown in Figure 1 below.
Figure 1: Pedigree of heredity
1. In a pedigree, a square represents a male. If it is darkened, he has hemophilia; if clear, he
has normal blood clotting.
How many males represented by this pedigree 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 this pedigree have hemophilia? How many are normal?
____________________________________________
Adapted from HRW’s Laboratory Experiments: Chapter 6 Investigation 6-2
3. A marriage is indicated by a horizontal line connecting a circle to a square.
How many marriages are indicated in the pedigree?
____________________________________________
4. A line perpendicular to a marriage line indicates the 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. Generation B represents the second generation. Generation C represents the third
generation, and so on.
How many generations are represented in this pedigree?
____________________________________________
6. The genotypes of the males in a pedigree for hemophilia are easy to determine, since
normal blood 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 denoted by XNY. One represented by a darkened square will be XnY. Label the
genotypes of all the squares in the pedigree.
How many males have the genotype XNY?
____________________________________________
Females who have hemophilia have an easy genotype to identify. They are XnXn. Both recessive
alleles must be present for a female to have hemophilia. If one dominant allele (XN) is present,
the individual must be normal for clotting. Label all the females with hemophilia as genotype
XnXn on the pedigree.
How many women have genotype XnXn in this pedigree?
____________________________________________
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 traits among the
offspring can often determine which genotype the parents have. If any child (son or daughter)
has 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 his 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 who have
children with hemophilia.
What is the genotype of the female in the first generation?
____________________________________________
7. A female who has more than four sons, with none exhibiting hemophilia, is likely to have
a genotype XNXN. If she has had four or fewer sons, her genotype is less certain. In such
cases, her genotype is labeled as XNX? Label the rest of the females in the pedigree as
XNX? or XNXN.
How many females in the pedigree have the genotype XNXN?
____________________________________________
Adapted from HRW’s Laboratory Experiments: Chapter 6 Investigation 6-2
Label the genotype of all people represented in the pedigree for Figure 1.
8. Based on the Pedigree in Figure 2, fill in the table below.
Figure 2: Pedigree
Questions about the Pedigree in Figure 2
Number of generations
Number of men with hemophilia
Number of women with normal blood clotting
Number of marriages
Number of men with genotype XNY
Number of women with genotype XnYn
Number of single women
Number of people that never married
Number of women with genotype XNX?
Number of couples with only one child
Answer
Analysis
1. Which sex usually inherits a sex-linked condition? Explain.
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?
Going Farther
Look in the library for a pedigree of Queen Victoria and her descendants showing the
incidence of hemophilia. Copy the pedigree and determine the genotypes of the members
of this royal family.
Adapted from HRW’s Laboratory Experiments: Chapter 6 Investigation 6-2