Transcript Sex Linkage - OISE-IS-Chemistry-2011-2012

Sex Linkage
Here Comes the Sun King
 The tale of King Henry VIII and Anne Boleyn
T.H. Morgan’s Flies
 Context: 1908 – before anyone knew the
link between chromosomes and heredity
 Working with the fruit fly, Drosophlia
melanogaster
 A model organism for genetics
 Won the Nobel Prize in 1933 for
discovering that chromosomes carried
heritable material
One day in the lab…
 Was looking at flies, who normally have wild type red
eyes, saw a fly with white eyes
 Wanted to do a breeding analysis to understand about
white eyes
 Did a test cross
Test Cross
 A test cross is used to determine the genotype
of an individual exhibiting a dominant trait
 Determine if this individual is homozygous or
heterozygous (HH or Hh)
 How?
Cross a dominant phenotype
individual to one with the
recessive phenotype.
 By observing the resulting
phenotypes, you can figure
out the genotype
So he crossed them
Maybe white
eyes are
lethal in
females?
So he crossed them
I guess not….but
what does this have
to do with sex?
Morgan’s Results
 The appearance of white eyes
in females shows that this trait
is not lethal in females.
 All possible combinations of
white eyes and sex are
possible.
 The white-eye trait can be
carried over to females when
F1 females are crossed with
white-eyed males.
Did it have to do with
chromosomes?
 The male and females seemed to have slightly different
chromosomes
 Morgan found that the gene for white eyes seemed to
follow the inheritance of sex
 From these and other crosses, he was able to figure
out that genes were carried on chromosomes!
10
Recall: Chromosome Structure
 Each human cell has 46
chromosomes
 23 pairs in total
 22 are autosomes (not
sex chromosomes)
 1 pair of sex
chromosomes
 Each pair is
homologous (similar
but not the same)
Females have two X chromosomes, (XX)
Males have one X and one Y Chromosome (XY)
X and Y on a Karyotype
 A Karyotype is an arrangement of the chromosomes of an
individual that has been sorted according to size and type
1st Law: Law of Segregation
 Mendel’s law of segregation states that every individual
possesses a pair of alleles and passes a randomly
selected copy (one or the other) to its offspring.
The same happens for sex
chromosomes
Sex Linked Inheritance
 Autosomal inheritance: inheritance of alleles located
on autosomal (non-sex) chromosomes
 (This is all the inheritance we have dealt with up until
now)
 Sex-linked: describes an allele that is found on one of
the sex chromosomes (X or Y)
Aa, CDCd
XHXh X HY
Hemizygous
 Just like before, females can be homozygous or
heterozygous for a trait
 XHXh or XHXH
 Males are called hemizygous because they are neither
heterozygous nor homozygous. They only have one
possibility!
 XHY
Sex Linked Problems
 For example, hemophilia A is a blood disease where it
takes a long time for the blood to clot. The gene for
hemophilia is located on the X chromosome and is
recessive.
 If a woman carrying the hemophilia allele marries a
man who does not have hemophilia, what are the odds
their children will have hemophilia?
How does it work
 Let XH represent the normal allele
 Let Xh represent the allele for hemophilia
 (Y is the Y chromosome)
Results
 50% of the males are affected
 0% of the females are affected, although one is a
carrier
Carrier
 Carrier: someone who does not have the phenotype of
a condition but has the allele for the condition.
 This usually applies to recessive genes
 For sex-linked genes, only FEMALES can be carriers.
Sex-Linked Genes
 Male Pattern Baldness
 Located on the X chromosome
 Recessive
 If you are male and your mothers father had it, you will
get it. It is rare in females.
 Why?
Male Pattern Baldness - P
 Let XB represent the normal hair allele, and Xb
represent the baldness allele
 P-
XB
XB
Normal Female
x
Bald Male
XBXB
x
XbY
Xb
XBXb
Carrier Female
Y
XBY
Normal Male
XBXb
Carrier Female
XBY
Normal Male
Both sons are normal, both daughters are carriers!
Male Pattern Baldness = F1
 F1 -
XB
Xb
Carrier Female
x
Normal Male
XBXb
x
XBY
XB
XBXB
Normal Female
XBXb
Carrier Female
Y
XBY
Normal Male
XbY
Affected Male
100% of females are normal, ½ of sons are normal, ½
of sons are affected
Altogether, ¼ of children are affected
What about a bald female?
It could happen, but you’d need
Bald or Carrier Female
XbXb or XBXb
x
Bald Male
x
XbY
There are also Y-linked diseases
 Obviously, only males can get it.
 If your dad has it, you will get it
 Less common because the Y chromosome is smaller and
has less genes