Download 16 - Sex-Linked Traits and Your Pedigree

SEX-LINKED TRAITS AND YOUR PEDIGREE
Opening question:
1. What difference does it make if a trait such as
hemophilia is sex-linked or not?
2. Would the chances of having hemophiliac children
change if hemophilia was not a gene on the X
chromosome?
Activity: Making Sense of Sex-Linked Traits
Background Information
Sex-linked traits are usually carried on the X sex
chromosome. A sex-linked trait is a recessive trait that
shows up more often in men than in women.
Hemophilia is an inherited disease of the blood. Affected
persons do not have the ability to form blood clots in the
way that normal people do. The letter H represents the
dominant, normal gene. The recessive gene is
represented by the letter h.
How is the trait inherited? Is it a sex-linked genetic disease
or not?
If it is sex-linked, the gene is located on the X
chromosomes. If it is not sex-linked, the gene is located on
a chromosomal pair other than the sex chromosomes.
This activity will guide you through the probability of a
disease occurring when it is considered to be sex-linked
and when it is considered not to be sex-linked.
Materials
 Masking tape
 Two dark chips
 Two light chips
 Pen
Procedure
Part 1: Trait is sex-linked
Genes for sex-linked traits are located on the X
chromosome. A heterozygous female (XHXh) has a 50/50
chance that her egg cells will receive either an XH or an Xh
during meiosis. Normal males have the genotype XHY.
The chances that their sperm cells will receive either XH or
Y during meiosis are 50/50. The offspring of the cross
between a heterozygous female and a normal male can
be determined by coin tossing.
 Put tape on two chips.
 Mark one chip to represent the heterozygous female.
This means mark one side of the chip XH and the
other side of the chip Xh.
 Mark the second penny to represent the normal male.
This means mark one side of the second chip XH and
the other side of the chip Y.
 Toss both chips together 50 times. Record the
combination that results after each toss in the chart
below.
 Total the results of each genotype and record them in
the table.
Results If the Trait is Sex-Linked
Offspring
Offspring
Result of Each Total
Phenotype Genotype
Toss
Observed
Normal
XHXH or XHXh
Female
Female with XhXh
Hemophilia
Normal Male XHY
Male with
XhY
Hemophilia
Part 2: Trait is not sex-linked
If the trait is not sex-linked, the genes for hemophilia are
not located on the sex chromosomes. This means that
there are two sets of chromosomes involved—one set for
determining the sex of the offspring and another set
involved in determining if the offspring will have
hemophilia or not.
Four chips are needed to represent the two pairs of
chromosomes. The following cross will be made XHXh x
XHYh.
Results If the Trait is Not Sex Linked
Offspring
Offspring
Result of Each Total
Phenotype Genotype
Toss
Observed
Normal
XHXH or XHXh
Female
Female with XhXh
Hemophilia
Normal Male XHY
Male with
XhY
Hemophilia