Name: ___________________________________
Hour: _____
Pedigree Practice
By examining a pedigree, you can determine whether traits are dominant or recessive, and you can also tell if
traits are sex-linked or autosomal (not on the X or Y chromosome). For example, since there is only one X in
the male sex genotype (XY), a male who has a particular recessive allele on the X chromosome will have the
sex-linked condition. A female who has 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.
Quick review: What do we call a female who has just one copy of the recessive allele?
Hemophilia, a sex-linked recessive trait, is a condition in which the blood does not clot properly. Most people
who have hemophilia are men. View the pedigree below and interpret the diagram to answer the questions.
Part A. Interpreting a human
pedigree.
Use Figure A to answer the questions
below:
1. Number all individuals on the
pedigree at the top of each shape.
2. In a pedigree, a square represents
a male. If it is shaded he has
hemophilia; if it is non-shaded,
he has normal blood clotting.
a. How many males are
there?
__________________
b. How many males have
hemophilia?
____________
3. A circle represents a female. If it is shaded she has hemophilia; if it is non-shaded, she has normal blood
clotting.
a. How many females are there? _________________
b. How many females have hemophilia? ___________
4. On a pedigree chart, a marriage is indicated by a horizontal line connecting a circle to a square.
a. How many marriages are there? _______________
Name: ___________________________________
Hour: _____
5. On a pedigree chart, a line perpendicular to a marriage line indicates the offspring. If the line ends with
either a circle or a square, the couple had only one child. However, if the line is connected to another
horizontal line, then several children were produced. The first child born appears to the left and the last
born to the right.
a. How many children did the first couple (couple in row I) have? ______________
b. How many children did the third couple (couple in row III) have? _____________
6. Level I represent the first generation; level II represents the second generation, and so on.
a. How many generations are there? _______________
b. How many members are there in the fourth generation? _____________
7. The genotypes of the males in a pedigree for sex-linked inheritance 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 non-shaded square will have the genotype XNY and male
represented by a shaded square will have the genotype XnY. Label the following under each individual
on the pedigree:
a. What is the genotype for Individual #1? ____________ Phenotype? _________
b. What is the genotype for Individual #10? ___________ Phenotype? _________
c. What is the genotype for the first born male in generation III? ______________
Phenotype? _________
8. Females with 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 female is
normal for clotting.
a. How many females have the genotype XnXn? _____________
9. Females who do not show the trait for hemophilia may be homozygous dominant (XN XN meaning they
are normal) or heterozygous (XNXn meaning they are carriers). By examining offspring we can often
determine which genotype the parents have. For example, if any child (son or daughter) has hemophilia,
then the mother must be heterozygous or must also have hemophilia, since she had to give the offspring
a Xn allele.
a. If a son has hemophilia, does the father have to also have hemophilia? WHY?
b. If a daughter has hemophilia, does the father have to also have hemophilia? WHY?
Name: ___________________________________
Hour: _____
Part B. Determining the pattern of inheritance and assigning genotypes.
When working through a pedigree, you may not always be told which phenotype is dominant and which is
recessive. If this is the case, you must use what you know about inheritance patterns in order to determine if the
shaded or non-shaded shapes are dominant.
Remember that recessive individuals are always homozygous, so it is easiest to assign these genotypes first
(two lower-case letters; for example: aa). Then look at all of the dominant individuals. Remember, in classic
Mendelian genetics, dominant individuals can be homozygous (example: AA) or heterozygous (example: Aa).
For some individuals, you will only be able to determine one allele of the genotype, so just write the one capital
allele followed by a dash (for example: A_).
Let’s practice determining inheritance and assigning genotypes to individuals on the pedigree chart below:
1. Which phenotype
(white or black)
is dominant?
2. Which phenotype
(white or black)
is recessive?
3. How do you know??
4. Determine the genotypes of all the individuals listed in the pedigree. Write these genotypes below the
shapes. Of the 16 individuals included in the chart, 3 of them will have the genotype A_ (meaning you
cannot determine if that dominant individual is homozygous or heterozygous). For all other individuals,
you should be able to determine both alleles of the genotype.