The document discusses sex linkage and sex determination, explaining the differences between homogametic (XX in females) and heterogametic (XY in males) individuals in terms of genetic expression. It highlights how x-linked traits are more prevalent due to the greater number of genes on the X chromosome, leading to higher expression of genetic disorders in males compared to females, who require two recessive alleles to express the trait. Additionally, it introduces pedigree charts as a method to study inheritance patterns and the distribution of hereditary conditions in families.

1. SEX LINKAGE
SEX LINKAGE
Characters which are associate more
Characters which are associate more
with one gender
with one gender

2. Sex Determination
Sex Determination
 What are the odds of
What are the odds of
getting a male? A female?
getting a male? A female?
 50/50
50/50
X
X Y
Y
X
X XX
XX XY
XY
X
X XX
XX XY
XY

3. What is it?
What is it?
 Sex Linkage is the phenotypic expression of an
Sex Linkage is the phenotypic expression of an
allele that is related to the chromosomal sex of
allele that is related to the chromosomal sex of
an individual
an individual
 Since there are far more genes on the X
Since there are far more genes on the X
chromosome than the Y, there are far more X
chromosome than the Y, there are far more X
linked traits that Y linked traits
linked traits that Y linked traits

4. What’s the Comparison Like?
What’s the Comparison Like?

5. Mode of Inheritance
Mode of Inheritance
 This mode of inheritance is in contrast to
This mode of inheritance is in contrast to
the inheritance of traits on
the inheritance of traits on autosomal
chromosomes
chromosomes
 What does autosomal mean?
What does autosomal mean?
 An
An autosome
autosome is a non-sex
is a non-sex chromosome
 It is an ordinarily paired chromosome that is the
It is an ordinarily paired chromosome that is the
same in both sexes of a species
same in both sexes of a species

6. Mode of Inheritance
Mode of Inheritance
 Non-autosomal chromosomes are usually
Non-autosomal chromosomes are usually
referred to as
referred to as sex chromosomes, allosomes or
, allosomes or
heterosomes
heterosomes
 In contrast to normal autosomal chromosomes,
In contrast to normal autosomal chromosomes,
both sexes do not have the same probability of
both sexes do not have the same probability of
expressing the trait
expressing the trait

7. Why does this happen?
Why does this happen?
 In mammals the female is said to be
In mammals the female is said to be
homogametic where as the male is said to be
homogametic where as the male is said to be
heterogametic.
heterogametic.
 What does homogametic mean?
What does homogametic mean?
 It means that there are two of the same
It means that there are two of the same
chromosomes (ie., X X)
chromosomes (ie., X X)
 What does heterogametic mean?
What does heterogametic mean?
 It means that there are two different chromosomes
It means that there are two different chromosomes
(ie., X Y)
(ie., X Y)

8. Why does this happen?
Why does this happen?
 The genes that are present on the X or Y
The genes that are present on the X or Y
chromosomes are called sex linked genes
chromosomes are called sex linked genes
 Now, the chromosomes of interest still have
Now, the chromosomes of interest still have
genes and alleles, and these alleles are passed on
genes and alleles, and these alleles are passed on
to their progeny
to their progeny

9. Why does this happen?
Why does this happen?
 These traits are also, as we discussed, dominant
These traits are also, as we discussed, dominant
and recessive
and recessive
 It can be said that X-linked recessive traits are
It can be said that X-linked recessive traits are
expressed in all heterogametics, and in
expressed in all heterogametics, and in
homogametics that are homozygous for the
homogametics that are homozygous for the
recessive allele
recessive allele

10. Well what does this mean?
Well what does this mean?
 Heterogametics only have one X chromosome,
Heterogametics only have one X chromosome,
and therefore what ever is there is expressed (ie.,
and therefore what ever is there is expressed (ie.,
it can not be masked by the other chromosome)
it can not be masked by the other chromosome)
 Homogametics on the other hand must have
Homogametics on the other hand must have
both chromosomes affected for the X linked
both chromosomes affected for the X linked
recessive chromosome to have an effect
recessive chromosome to have an effect

11. Heterogametic
Homogametic

12. So what do you think?
So what do you think?
 Which of the two, homogametic or
Which of the two, homogametic or
heterogametic, do you think will have
heterogametic, do you think will have
expression of the genetic disorders more
expression of the genetic disorders more
often?
often?
 Think to yourself, share with a friend, and
Think to yourself, share with a friend, and
then lets have a class discussion and make
then lets have a class discussion and make
sure that your answer makes sense
sure that your answer makes sense

13. Males get the short end of the Stick
Males get the short end of the Stick
 Males, or heterogametics have a higher incidence
Males, or heterogametics have a higher incidence
of expression because of the fact that there is no
of expression because of the fact that there is no
other allele to mask the damaged one
other allele to mask the damaged one
 If this is the case, all males that get the gene or
If this is the case, all males that get the gene or
allele that codes for the damaged expression will
allele that codes for the damaged expression will
express to the fullest extent
express to the fullest extent

14. So that’s the male, what about the
So that’s the male, what about the
female?
female?
 What are the possibilities for the female?
What are the possibilities for the female?
 Is it possible for the female to express the same way
Is it possible for the female to express the same way
that the male does?
that the male does?
 Yes, of course, but there is a stipulation
Yes, of course, but there is a stipulation
 The female must contain two damaged alleles
The female must contain two damaged alleles
(chromosomes) in order to express
(chromosomes) in order to express
 But hold on, what does this mean for the female
But hold on, what does this mean for the female
with regards to passing on her genes?
with regards to passing on her genes?

15. What can she be?
What can she be?
 She can actually be a carrier
She can actually be a carrier
 What is a carrier?
What is a carrier?
 A carrier is an individual that has two alleles, one
A carrier is an individual that has two alleles, one
damaged one and one regular
damaged one and one regular
 Their phenotypes are the same as a regular individual
Their phenotypes are the same as a regular individual
 But their genes hold a “secret”
But their genes hold a “secret”

16. Back to men
Back to men
 Can a man be a carrier?
Can a man be a carrier?
 Think about it for a few minutes with a partner, and
Think about it for a few minutes with a partner, and
really think hard about it
really think hard about it
 For X linked alleles, no it is not possible
For X linked alleles, no it is not possible
 Why?
Why?

17. What kind of progeny would we
What kind of progeny would we
expect?
expect?

19. The progeny
The progeny
 We saw in the last photo that you can get
We saw in the last photo that you can get
normal males and females if the mother is a
normal males and females if the mother is a
carrier, and you can also get carrier females and
carrier, and you can also get carrier females and
affected sons
affected sons
 Why can you not get affected females?
Why can you not get affected females?
 There is no affected X that comes from the father
There is no affected X that comes from the father
 The only way to get an affected female is if there is
The only way to get an affected female is if there is
an affected male and a carrier or affected female
an affected male and a carrier or affected female

20. An Example
An Example
 Red/Green
Red/Green Colour
Colour blindness
blindness
 This is a recessive X linked
This is a recessive X linked
 When passed on to males, they are automatically
When passed on to males, they are automatically
colour
colour blind to red and green, females can be
blind to red and green, females can be
carriers or can be colour blind
carriers or can be colour blind

21. Talking about it is easy
Talking about it is easy
 How do you think we are going to represent the
How do you think we are going to represent the
flow of genes through a family
flow of genes through a family
 The best way is to draw a picture
The best way is to draw a picture
 This picture that we are going to draw has a
This picture that we are going to draw has a
special name, its called a Pedigree
special name, its called a Pedigree

22. What is a pedigree chart?
What is a pedigree chart?
 Pedigree charts show a record of the family of an
Pedigree charts show a record of the family of an
individual
individual
 They can be used to study the transmission of a
They can be used to study the transmission of a
hereditary condition
hereditary condition
 They are particularly useful when there are large
They are particularly useful when there are large
families and a good family record over several
families and a good family record over several
generations.
generations.

23. Studying human genetics
Studying human genetics
 You cannot make humans of different types
You cannot make humans of different types
breed together
breed together
 Pedigree charts offer an ethical way of studying
Pedigree charts offer an ethical way of studying
human genetics
human genetics
 Today genetic engineering has new tools to
Today genetic engineering has new tools to
offer doctors studying genetic diseases
offer doctors studying genetic diseases
 A genetic counsellor will still use pedigree
A genetic counsellor will still use pedigree
charts to help determine the distribution of a
charts to help determine the distribution of a
disease in an affected family
disease in an affected family

24. Symbols used in pedigree charts
Symbols used in pedigree charts
 Normal male
Normal male
 Affected male
Affected male
 Normal female
Normal female
 Affected female
Affected female
 Marriage
Marriage
A marriage with five children, two
daughters and three sons. The eldest
son is affected by the condition.
Eldest child  Youngest child

25. Organising the pedigree chart
Organising the pedigree chart
 A pedigree chart of a family showing 20
A pedigree chart of a family showing 20
individuals
individuals

26. Organising the pedigree chart
Organising the pedigree chart
 Generations are identified by Roman numerals
Generations are identified by Roman numerals
I
II
III
IV

27. Organising the pedigree chart
Organising the pedigree chart
 Individuals in each generation are identified by Arabic
Individuals in each generation are identified by Arabic
numerals numbered from the left
numerals numbered from the left
 Therefore the affected individuals are
Therefore the affected individuals are II3
II3,
, IV2
IV2 and
and IV3
IV3
I
II
III
IV

28. So now that we have seen that, lets
So now that we have seen that, lets
try some problems
try some problems