SEX LINKAGE
Characters which are associate more
with one gender
Sex Determination

What are the odds of
getting a male? A female?

X
Y
X
XX
XY
X
XX
XY
50/50
What is it?

Sex Linkage is the phenotypic expression of an
allele that is related to the chromosomal sex of
an individual

Since there are far more genes on the X
chromosome than the Y, there are far more X
linked traits that Y linked traits
What’s the Comparison Like?
Mode of Inheritance

This mode of inheritance is in contrast to
the inheritance of traits on autosomal
chromosomes

What does autosomal mean?
An autosome is a non-sex chromosome
 It is an ordinarily paired chromosome that is the
same in both sexes of a species

Mode of Inheritance

Non-autosomal chromosomes are usually
referred to as sex chromosomes, allosomes or
heterosomes

In contrast to normal autosomal chromosomes,
both sexes do not have the same probability of
expressing the trait
Why does this happen?


In mammals the female is said to be
homogametic where as the male is said to be
heterogametic.
What does homogametic mean?


It means that there are two of the same
chromosomes (ie., X X)
What does heterogametic mean?

It means that there are two different chromosomes
(ie., X Y)
Why does this happen?

The genes that are present on the X or Y
chromosomes are called sex linked genes

Now, the chromosomes of interest still have
genes and alleles, and these alleles are passed on
to their progeny
Why does this happen?

These traits are also, as we discussed, dominant
and recessive

It can be said that X-linked recessive traits are
expressed in all heterogametics, and in
homogametics that are homozygous for the
recessive allele
Well what does this mean?

Heterogametics only have one X chromosome,
and therefore what ever is there is expressed (ie.,
it can not be masked by the other chromosome)

Homogametics on the other hand must have
both chromosomes affected for the X linked
recessive chromosome to have an effect
Homogametic
Heterogametic
So what do you think?

Which of the two, homogametic or
heterogametic, do you think will have
expression of the genetic disorders more
often?

Think to yourself, share with a friend, and
then lets have a class discussion and make
sure that your answer makes sense
Males get the short end of the Stick

Males, or heterogametics have a higher
incidence of expression because of the fact that
there is no other allele to mask the damaged one

If this is the case, all males that get the gene or
allele that codes for the damaged expression will
express to the fullest extent
So that’s the male, what about the
female?

What are the possibilities for the female?

Is it possible for the female to express the same way
that the male does?
Yes, of course, but there is a stipulation
 The female must contain two damaged alleles
(chromosomes) in order to express


But hold on, what does this mean for the female
with regards to passing on her genes?
What can she be?

She can actually be a carrier

What is a carrier?
A carrier is an individual that has two alleles, one
damaged one and one regular
 Their phenotypes are the same as a regular individual
 But their genes hold a “secret”

Back to men

Can a man be a carrier?
Think about it for a few minutes with a partner, and
really think hard about it
 For X linked alleles, no it is not possible


Why?
What kind of progeny would we
expect?
The progeny

We saw in the last photo that you can get
normal males and females if the mother is a
carrier, and you can also get carrier females and
affected sons

Why can you not get affected females?
There is no affected X that comes from the father
 The only way to get an affected female is if there is
an affected male and a carrier or affected female

An Example



Red/Green Colour blindness
This is a recessive X linked
When passed on to males, they are automatically
colour blind to red and green, females can be
carriers or can be colour blind
Talking about it is easy

How do you think we are going to represent the
flow of genes through a family

The best way is to draw a picture

This picture that we are going to draw has a
special name, its called a Pedigree
What is a pedigree chart?



Pedigree charts show a record of the family of
an individual
They can be used to study the transmission of a
hereditary condition
They are particularly useful when there are large
families and a good family record over several
generations.
Studying human genetics




You cannot make humans of different types
breed together
Pedigree charts offer an ethical way of
studying human genetics
Today genetic engineering has new tools to
offer doctors studying genetic diseases
A genetic counsellor will still use pedigree
charts to help determine the distribution of a
disease in an affected family
Symbols used in pedigree charts




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Normal male
Affected male
Normal female
Affected female
Marriage
A marriage with five children, two
daughters and three sons. The eldest
son is affected by the condition.
Eldest child  Youngest child
Organising the pedigree chart

A pedigree chart of a family showing 20
individuals
Organising the pedigree chart

Generations are identified by Roman numerals
I
II
III
IV
Organising the pedigree chart


Individuals in each generation are identified by Arabic
numerals numbered from the left
Therefore the affected individuals are II3, IV2 and IV3
I
II
III
IV
So now that we have seen that, lets
try some problems