1606735473-sex-linkage and sex linked traits.ppt

a powerpoint presentation of the sex linked traits and the genetic disorders that are related to it.

1. SEX LINKAGESEX LINKAGE
Characters which are Characters which are
associate more with one associate more with one
gendergender

2. What is it?What is it?

Sex Linkage Sex Linkage is the phenotypic expression of an 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 individualan individual

Association or linkage of hereditary traits with Association or linkage of hereditary traits with
sex .sex .

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 traitslinked traits that Y linked traits

3. What is Sex Linkage?What is Sex Linkage?

In addition to their role in determining sex, the sex In addition to their role in determining sex, the sex
chromosomes have genes for many characters.chromosomes have genes for many characters.

Genes located on a sex chromosome are called sex Genes located on a sex chromosome are called sex
linked genes.linked genes.

In humans the term usually refers to X-linked In humans the term usually refers to X-linked
characters: genes located only on X chromosomes.characters: genes located only on X chromosomes.

Fathers can pass X-linked alleles to their daughters, but Fathers can pass X-linked alleles to their daughters, but
not sons.not sons.

Mothers can pass sex-linked alleles to both sons and Mothers can pass sex-linked alleles to both sons and
daughters.daughters.

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

5. Mode of InheritanceMode 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 autosomeautosome is a is a non-sex non-sex chromosome

It is an ordinarily paired chromosome that is It is an ordinarily paired chromosome that is
the same in both sexes of a speciesthe same in both sexes of a species

6. Mode of InheritanceMode of Inheritance

Non-autosomal chromosomes are usually Non-autosomal chromosomes are usually
referred to as referred to as sex chromosomes, allosomes or , allosomes or
heterosomesheterosomes

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 traitexpressing 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 geneschromosomes 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 progenyto 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 recessiveand 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 allelerecessive 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 effectrecessive 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?

13. Males get the short end of the StickMales 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 oneother 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 extentexpress 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 Is it possible for the female to express the same
way that the male does?way that the male does?

Yes, of course, but there is a stipulationYes, 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 But hold on, what does this mean for the
female with regards to passing on her genes?female with regards to passing on her genes?

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

16. Back to menBack 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 itreally think hard about it

For X linked alleles, no it is not possibleFor 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?

18. The progenyThe 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 sonsaffected sons

Why can you not get affected females?Why can you not get affected females?

There is no affected X that comes from the fatherThere 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 femalean affected male and a carrier or affected female

19. Recessive allelesRecessive alleles

If a sex linked trait is due to a recessive allele:If a sex linked trait is due to a recessive allele:

A female will express the phenotype only if she is A female will express the phenotype only if she is
homozygous recessive.homozygous recessive.

If a male receives the recessive allele from his mother he will If a male receives the recessive allele from his mother he will
express the phenotype.express the phenotype.

Far more males have disorders that are inherited as sex Far more males have disorders that are inherited as sex
linked recessives than females.linked recessives than females.

Examples: Examples: Colour blindnessColour blindness
HaemophiliaHaemophilia

20. An ExampleAn Example

Red/Green Red/Green ColourColour 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
colourcolour blind to red and green, females can be blind to red and green, females can be
carriers or can be colour blindcarriers or can be colour blind

21. Red-green colour blindnessRed-green colour blindness

X chromosome has a locus for colour vision with X chromosome has a locus for colour vision with
two alleles:two alleles:

XX
NN
= Normal colour vision = Normal colour vision

XX
nn
= Red-green colour blindness = Red-green colour blindness

Y chromosome does not have a colour vision Y chromosome does not have a colour vision
locus. locus.

If a male receives the XIf a male receives the X
nn
allele he will have allele he will have
impaired colour vision, whereas a female with impaired colour vision, whereas a female with
XX
NN
XX
nn
will not. will not.

22. Red-green colour blindnessRed-green colour blindness
ParentalParental
PhenotypesPhenotypes Carrier Female x Normal Male Carrier Female x Normal Male
Genotypes XGenotypes X
NN
XX
nn
X X
NN
YY
GametesGametes
Offspring 1Offspring 1
GenotypesGenotypes
Phenotypes
Normal Female : Carrier Female : Normal Male : Colour blind Male
1 : 1 : 1 : 1
X
N
X
n
X
N
Y
Female Gametes
X
N
X
n
Male
Gametes
X
N
X
N
X
N
X
N
X
n
Y X
N
Y X
n
Y

24. Sex limited traitsSex limited traits

Sex limited traitsSex limited traits are generally autosomal, are generally autosomal,
meaning that they are not found on the X or Y meaning that they are not found on the X or Y
chromosomes. chromosomes.

The genes for these traits behave exactly the The genes for these traits behave exactly the
same way that any autosomal gene behaves. The same way that any autosomal gene behaves. The
difference here comes in the expression of the difference here comes in the expression of the
genes in the phenotype of the individual. genes in the phenotype of the individual.

25. Contd...Contd...

Sex-limited traits are expressed in only one Sex-limited traits are expressed in only one
gender. gender.

The traits are generally associated with primary The traits are generally associated with primary
or secondary sexual characteristics, and thus are or secondary sexual characteristics, and thus are
expressed only in the gender which utilizes those expressed only in the gender which utilizes those
characteristicscharacteristics

26. Sex influenced traits Sex influenced traits

Sex influenced traitsSex influenced traits are also autosomal, are also autosomal,
meaning that their genes are not carried on the meaning that their genes are not carried on the
sex chromosomes. sex chromosomes.

What makes these traits unusual is the way they What makes these traits unusual is the way they
are expressed phenotypically. In this case, the are expressed phenotypically. In this case, the
difference is in the ways the two genders express difference is in the ways the two genders express
the genes. the genes.

27. Contd...Contd...

One classic example of a sex influenced trait is One classic example of a sex influenced trait is
pattern baldnesspattern baldness in humans in humans

This gene has two alleles, “bald” and “non-This gene has two alleles, “bald” and “non-
bald.” bald.”

The behaviors of the products of these genes are The behaviors of the products of these genes are
highly influenced by the hormones in the highly influenced by the hormones in the
individual, particularly by the hormone individual, particularly by the hormone
testosterone. testosterone.

28. Contd...Contd...

In the presence of high levels of testosterone, In the presence of high levels of testosterone,
the baldness allele has a very powerful influence.the baldness allele has a very powerful influence.

In the presence of low levels of testosterone, In the presence of low levels of testosterone,
this allele is quite ineffectualthis allele is quite ineffectual

29. Holandric inheritanceHolandric inheritance

Y linked inheritance is called "holandric" Y linked inheritance is called "holandric"
inheritance. inheritance.

Only males have these genes and any mutations Only males have these genes and any mutations
would result in all sons of an affected male being would result in all sons of an affected male being
affected and none of his daughters.affected and none of his daughters.

35. Mitochondrial InheritanceMitochondrial Inheritance

Mitochondria are cytoplasmic organelles Mitochondria are cytoplasmic organelles
important in cellular respirationimportant in cellular respiration

Have their own DNAHave their own DNA

Carry 37 genes Carry 37 genes

Transmitted from mother to Transmitted from mother to ALLALL of her of her
offspringoffspring

No recombinationNo recombination

Males and females equally affected Males and females equally affected