How is our sex determined?
Pre-assessment Questions

What are the sex chromosomes?

What does it mean when a trait is sexlinked?

What are some examples of sex-linked
traits or genes?
Punnett Squares for Sex Determination

In females both chromosomes are known
as X chromosomes while in males one
chromosome is X while the other is Y.
The Y chromosome

The y chromosome is known as a
degenerate chromosome because it does
not have many genes on it
◦ Only has genes related male anatomy

Unlike the X chromosome which has
many
Sex-linked Inheritance

Sex linked inheritance occurs when
certain genes are located on the X or Y
chromosome (almost always the X).

This means that some traits will come
directly from one parent or another, or
even from one grandparent or another.
Punnet Square for Sex-linked
Inheritance

When illustrating sex linked inheritance
we always use the X and Y letters. If we
are talking about gene “A” on the X
chromosome we will illustrated this way:
XA or Xa.
Why doesn’t
the Y have any
allele on it?
Sex-Linked Inheritance

Imagine if the dominant allele “A” means
normal vision and “a” means colour blind.

Pretend the mother is heterozygous and
the father has normal vision
What percentage of males
will be colour blind? How
about females?
Sex linked diseases

E.g. Haemophilia
Sex linked disease

Muscular Dystrophy
Sex linked disease

Fragile-X syndrome
Sex linked trait

Red-green colour blindness
Sex linked diseases

While there are diseases that exist on the
Y chromosome we will only look at
diseases on the X chromosome
X-linked diseases can come in two forms:
1. Dominant
2. Recessive

Dominant X linked disease

Some X-linked diseases are dominant.
This means they will be expressed
regardless of whether the other X
chromosome contains the disease.
e.g. Disease = XDXD or XDXd or XDY
(males)
Recessive X-linked disease

Other X-linked diseases are recessive.
This means they will only be expressed if
the individual has both alleles with the
recessive diseased allele.
e.g. Disease = XdXd or XdY (males)
Carriers of the disease

Carriers of the disease are those individuals
that have the diseased allele within them but
do not have expression of the disease
because the allele is recessive.

Example of X-linked carriers:
- XDXd (where Xd is the recessive diseased
allele)
Pedigree Charts

Genetic researchers use pedigree charts to track traits from
parents to children
◦ Pedigree charts are similar to family trees
Pedigree Charts
X-Linked Recessive Disease
Example
Assume
d = diseased and
D = normal
P
F1
What is the genotypes
for the F1 generation?
F2
Practice: X-linked Dominant

Here is an example of a pedigree chart for an Xlinked dominant disease. Let us pretend that the Xlinked dominant disease is Fragile X syndrome

XFY (where F is fragile X syndrome dominant allele)
Solve the
genotypes for the
II generation
Homework
#13, 15-18 (p.258)
 #10 – 13 (p.259)
