SEX LINKAGESEX LINKAGE

Characters which are Characters which are associate more with one associate more with one

gendergender

Sex DeterminationSex Determination

What are the odds What are the odds of getting a male? of getting a male? A female?A female? 50/5050/50

XX YY

XX XXXX XYXY

XX XXXX XYXY

What is it?What is it?

Sex Linkage is the phenotypic Sex Linkage is the phenotypic expression of an allele that is related expression of an allele that is related to the chromosomal sex of an to the chromosomal sex of an individualindividual

Since there are far more genes on Since there are far more genes on the X chromosome than the Y, there the X chromosome than the Y, there are far more X linked traits that Y are far more X linked traits that Y linked traitslinked traits

What’s the Comparison What’s the Comparison Like?Like?

Mode of InheritanceMode of Inheritance

This mode of inheritance is in This mode of inheritance is in contrast to the inheritance of contrast to the inheritance of traits on traits on autosomal chromosomes chromosomes

What does autosomal mean?What does autosomal mean? An An autosomeautosome is a non-sex is a non-sex chromosome It is an ordinarily paired chromosome that It is an ordinarily paired chromosome that

is the same in both sexes of a speciesis the same in both sexes of a species

Mode of InheritanceMode of Inheritance

Non-autosomal chromosomes are Non-autosomal chromosomes are usually referred to as usually referred to as sex chromosomes, allosomes or , allosomes or heterosomesheterosomes

In contrast to normal autosomal In contrast to normal autosomal chromosomes, both sexes do not chromosomes, both sexes do not have the same probability of have the same probability of expressing the traitexpressing the trait

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 homogametic where as the male is said to be heterogametic.said to be 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 It means that there are two different chromosomes (ie., X Y)chromosomes (ie., X Y)

Why does this happen?Why does this happen?

The genes that are present on the X The genes that are present on the X or Y chromosomes are called sex or Y chromosomes are called sex linked geneslinked genes

Now, the chromosomes of interest Now, the chromosomes of interest still have genes and alleles, and still have genes and alleles, and these alleles are passed on to their these alleles are passed on to their progenyprogeny

Why does this happen?Why does this happen?

These traits are also, as we These traits are also, as we discussed, dominant and recessivediscussed, dominant and recessive

It can be said that X-linked recessive It can be said that X-linked recessive traits are expressed in all traits are expressed in all heterogametics, and in heterogametics, and in homogametics that are homozygous homogametics that are homozygous for the recessive allelefor the recessive allele

Well what does this Well what does this mean?mean?

Heterogametics only have one X Heterogametics only have one X chromosome, and therefore what ever chromosome, and therefore what ever is there is expressed (ie., it can not be is there is expressed (ie., it can not be masked by the other chromosome)masked by the other chromosome)

Homogametics on the other hand Homogametics on the other hand must have both chromosomes affected must have both chromosomes affected for the X linked recessive for the X linked recessive chromosome to have an effectchromosome to have an effect

Heterogametic

Homogametic

So what do you So what do you think?think?

Which of the two, homogametic Which of the two, homogametic or heterogametic, do you think or heterogametic, do you think will have expression of the will have expression of the genetic disorders more often?genetic disorders more often?

Think to yourself, share with a Think to yourself, share with a friend, and then lets have a class friend, and then lets have a class discussion and make sure that discussion and make sure that your answer makes senseyour answer makes sense

Males get the short end of Males get the short end of the Stickthe Stick

Males, or heterogametics have a Males, or heterogametics have a higher incidence of expression higher incidence of expression because of the fact that there is no 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 If this is the case, all males that get the gene or allele that codes for the the gene or allele that codes for the damaged expression will express to damaged expression will express to the fullest extentthe fullest extent

So that’s the male, what So that’s the male, what about the female?about the female?

What are the possibilities for the female?What are the possibilities for the female? Is it possible for the female to express the Is it possible for the female to express the

same way that the male does?same 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 female with regards to passing on her genes?genes?

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 that has two A carrier is an individual that has two

alleles, one damaged one and one alleles, one damaged one and one regularregular

Their phenotypes are the same as a Their phenotypes are the same as a regular individualregular individual

But their genes hold a “secret”But their genes hold a “secret”

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 Think about it for a few minutes with a

partner, and really think hard about itpartner, and really think hard about it For X linked alleles, no it is not possibleFor X linked alleles, no it is not possible

Why?Why?

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

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 normal males and females if the mother is a carrier, and you can also get carrier a carrier, and you can also get carrier females and affected sonsfemales and 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 There is no affected X that comes from the

fatherfather The only way to get an affected female is if The only way to get an affected female is if

there is an affected male and a carrier or there is an affected male and a carrier or affected femaleaffected female

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 When passed on to males, they are automatically automatically colourcolour blind to red and blind to red and green, females can be carriers or green, females can be carriers or can be colour blindcan be colour blind

Talking about it is easyTalking about it is easy

How do you think we are going to How do you think we are going to represent the flow of genes through a represent the flow of genes through a familyfamily

The best way is to draw a pictureThe best way is to draw a picture

This picture that we are going to draw This picture that we are going to draw has a special name, its called a has a special name, its called a PedigreePedigree

What is a pedigree What is a pedigree chart?chart?

Pedigree charts show a record of the Pedigree charts show a record of the family of an individualfamily of an individual

They can be used to study the They can be used to study the transmission of a hereditary conditiontransmission of a hereditary condition

They are particularly useful when They are particularly useful when there are large families and a good there are large families and a good family record over several generations.family record over several generations.

Studying human geneticsStudying human genetics

You cannot make humans of different You cannot make humans of different types breed togethertypes breed together

Pedigree charts offer an ethical way of Pedigree charts offer an ethical way of studying human genetics studying human genetics

Today genetic engineering has new tools Today genetic engineering has new tools to offer doctors studying genetic diseases to offer doctors studying genetic diseases

A genetic counsellor will still use A genetic counsellor will still use pedigree charts to help determine the pedigree charts to help determine the distribution of a disease in an affected distribution of a disease in an affected familyfamily

Symbols used in pedigree Symbols used in pedigree chartscharts

Normal maleNormal male Affected maleAffected male Normal femaleNormal female Affected Affected

femalefemale MarriageMarriage

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 Organising the pedigree chartchart

A pedigree chart of a family showing A pedigree chart of a family showing 20 individuals20 individuals

Organising the pedigree Organising the pedigree chartchart

Generations are identified by Roman Generations are identified by Roman numeralsnumerals I

II

III

IV

Organising the pedigree Organising the pedigree chartchart

Individuals in each generation are identified by Individuals in each generation are identified by Arabic numerals numbered from the leftArabic numerals numbered from the left

Therefore the affected individuals are Therefore the affected individuals are II3II3, , IV2IV2 and and IV3IV3

I

II

III

IV

So now that we have seen So now that we have seen that, lets try some problemsthat, lets try some problems